DAN ARMENDARIZ: Hello, everyone.
I'm Dan Armendariz, preceptor in computer science for [? Cs ?]
and today I'm going to be talking to you about digital photography.
Now, in particular we're going to do a crash course in just 60 minutes
on a number of topics in digital photography.
Unfortunately, we have a packed house here
to sort of like choose your own adventure,
and we will try to get through as much as possible.
>> So without further delay-- unless you happen
to be hiding under a rock-- humanity has for the very first time
put a lander on a comet, which is a pretty cool thing.
Phi-lay or Phil-y or some way of actually pronouncing
this-- I've heard it pronounced a variety of ways,
but of course this lander and the associated satellite
that actually brought the lender to the comet each
have some digital cameras attached and associated with them.
So this is the view of Philae from Rosetta's OSIRIS narrow angle camera,
so Rosetta is the machine that actually brought Philae over to the comet.
>> Philae is the lander itself and as Philae was its way landing on a comet,
it snapped some photos.
And so there's something interesting about this that I want to point out,
and first of all, this is just the lander,
of course, but if you notice surrounding that there seems to be no stars.
So I added a little bit extra black just sort of design of the slide,
but the very center, the very corner of this slide
is in fact original, the original image that came from Rosetta's OSIRIS camera.
So just sort of give that some consideration--
why, if this is in fact in deep space, is it
the case that there are no stars in this photograph.
>> So just a couple of other things to take a look at-- this
was a photo that came back from Philae, this was yesterday I think,
after it had actually landed.
And unfortunately, it was the case where the very first that Philae landed
it bounced a couple times, and so it's not actually the proper position
that they expected, but still it has this kind
of neat look of the comet itself.
And one of things that's really neat about this is that you realize that
Rosetta has been travelling for about 10 years through space,
so this means that the digital camera technology that's contained within
Philae and Rosetta is least 10 years old,
but if you go back through the records there's actually a scientific paper
that was published back in 1998 that talked about the specifics
of the specifications of the cameras on each of these satellites.
>> And this is 1988, that's a long time ago.
Do you have any idea what kind of digital camera technology
was available back then?
There happen to be a digital camera called the Canon EOS d2000
and it was really the first digital camera
that came out that people considered to be serious and usable digital cameras,
so was it the case that back in 1998 when
there were creating the specifications that they simply
duct taped one of these Canon EOS d2000s to this lander?
Well, of course not.
>> This is meant to be a scientific instrument
and so there's a lot of detail that actually went into this,
but just to give you some context, this top of the line d2000 camera
had two megapixel sensor and could take photos at about 3.5 frames per second.
So two megapixels is pretty abysmal, if you
have a modern smartphone such as an iPhone or Android phone it might
be that the camera on the front of your device
actually has one or two megapixels, about the same number of pixels
as the Rosetta camera itself-- that's sort of the high quality one.
The Philae lander actually has other cameras
that are only one megapixels each.
I think there's an array of six for panoramas
and then there's another for some scientific studies
and so basically the photo that we were just looking at
was taken essentially with a one megapixel camera.
>> Now of course, this is kind of not a very fair comparison,
because when we're talking about the scientific aspect
of digital photography then there's a lot to of additional work that
has to go into making sure that it's actually going to be correct
and that they can actually get some usable data out of this.
And there's some interesting things about the Rosetta camera
that we can actually learn from the paper that was published back in '98.
In particular, it had a four megapixel camera, which was pretty impressive.
It actually had a very large sensor size--
we'll talk more about sensor size.
That was pretty well equivalent to a standard 35 millimeter frame.
We'll talk more about that in just a little bit, hopefully
if we actually get to it.
>> And the maximum shutter speed, so in other words,
the maximum amount of time that, rather than the fastest amount of time that
the sensor was actually able to capture data and to capture the lights
for the exposure was one 1/100 of a second,
which is frankly pretty abysmal compared to this digital camera that actually
that came out in 1998, which operated about 1/4,000 or maybe 1/8,000
of a second.
So let's take a look at another image from space.
>> This came out of JAXA, which is Japan's space agency
and this is a picture of they released a satellite that went around the moon
and took some photographs, and this was I believe a moon rise that
came over that, and it's a fantastic image,
but again you have to wonder what is going on.
Why are there no stars in this scene?
So realize that we we're talking about digital photography, one
of the most important aspects of it is to consider the exposure.
And of course, exposure is not something that we actually
deal with solely in digital photography, this
applies to film photography as well and also videography
and a variety of other fields where we're actually capturing images,
but there's really four major things that impact the exposure.
>> One of the most important things is the amount of available light.
Now sometimes you can control this, if you're in a studio,
for example, or in this room we can control the amount of light
by turning some lights on, turning the lights off,
but in the case of the satellites they really
don't have any control over this.
It is the amount of sunlight that exist in the sky
or rather in space that reflects off of each of these objects
and can be collected by this sensor.
So the amount available light, we may or may not
have control over depending on the circumstance,
but notice that we also have three other settings
as well-- shutter speed, ISO, an aperture through which any camera
actually uses to manipulate to try to capture the amount of available light
that exist in the environment.
So another way to think about this is that you
have a sensor in a digital camera, it can collect a certain amount of light,
there's a range of amount of light that it can actually collect,
too little light and it won't register, so it'll look totally dark.
Too much light and it will actually overwhelm the sensor
and it will look totally white.
So we have these settings to try to compensate
for the amount available light that exists in the scene
and fit that amount of light in the scene to the range
that our sensor can actually capture.
>> So let's take a step back and talk a little bit about light.
So you might recall from high school physics,
light is of course is photons that has properties of both the wave and matter,
and because of its properties of a wave it
operates in various wavelengths and we as humans can only
interpret and understand and receive through our eyes
a small spectrum of the electromagnetic spectrum, which
represents the color that we're able to see.
Now, it's interesting to note of course that our visual system
is a rather complex system that is made up of a wide variety of parts, not only
just our eyes, but even all of the sub parts within the eyes,
including the lens, the iris and the retina
in the very back with all the cells associated with that,
but also the pathway to the brain and the visual cortex itself.
>> And this can lead to some very interesting phenomenon that actually
impact us as photographers, and perhaps more
correctly impact the design of cameras and digital cameras.
So this you may or may not have seen if you've
been trolling around on the internet for long enough.
It's just an optical illusion where there
are two tiles that are labeled-- tile A at the top of this illusion and tile B
at the center, and it just so happens that they
are in fact exactly the same color.
So even if you know this fact, you look at it
and it still doesn't look quite right.
This is in fact a very strong visual perception
that our brain is playing on us.
Just to try to prove this to you a little bit,
>> I'm going to bring up the same image in Photoshop
and I'm going to bring up the eyedropper tool, select the color in the A tile,
and I'm going to draw a little color bridge between A and B
and hopefully now you can sort of see what is going on,
or you can at least convince yourself that this color is
in fact the same in these two tiles.
So let me digress a little bit, because I really am showing you this just
to make clear the fact that we have a visual system that complicates matters.
Our eyes do not operate scientifically like the Philae lander would
and like a digital camera would, and this
causes some problems that actually impact us as digital photographers.
So if we take a look at the structure of the eye
we don't have to really worry about too much of it,
but there is of course the iris and the lens that actually focuses
the light into the back of the eye, which has the retina.
The retina has a variety of cells, and in the very center of our vision
there exists a structure called the fovea where
we have a very high concentration of detail cells that
allow us to see color vision and a variety of other things.
Now the retina is made up of a variety of types of cells.
There's two major types that we're really concerned with.
There's rods and cones, and each of these
have different properties, so the rods for example
are primarily associated with night vision,
whereas cones give us our day vision.
What this means is that rod cells are more sensitive to light.
They're the ones that are activated and that
are in use when you're outside in the middle of the night, for example.
And cones tend to be in use when you have high detailed vision
or when you're actually in daylight.
So just like we were saying, rods have more light sensitivity,
cones have less.
In the fovea, which was that structure that I mentioned
that's in the very middle of the retina in the center of your field of view
you have a high concentration of cones and a low concentration of rods.
In fact, the relative presence of rods overall in your entire retina
is very high.
You have far more rods than you have cones, which is pretty interesting
and sort of eludes a little bit to the fact that the greatest amount of detail
that we have and the greatest amount of day vision
that we have is in the very center of our vision.
>> When we go outside at night if you've been to a planetarium for example,
you might have heard the host actually say
that when you want to look at something up in the sky
actually look at it in the corner of your eye.
The reason for that is you have more rods in your periphery
than you do in the center, and this means
that you can perhaps see that detail a little bit better
with that more sensitive cell.
>> Now, the primary stimulus for cones is trichomatic,
that means that the cones are really the ones that provide to us
our color vision, so among other reasons this in combination
is why in broad daylight we can actually perceive far more colors
than we can in the middle of the night.
You might have noticed if you go outside in the middle of the night
the colors don't seem to be as bright.
One of the reasons for that is that the cones
are the ones that provide to us our color vision,
and the cones are what become inactive at night.
>> Now similarly, rods actually detect motion
and this is another reason why it's very useful in the periphery
and why we can detect motion more in the periphery than when we're actually
looking directly at something.
Now, the reason that we are able to actually have trichromatic vision out
of these cones cells is because we have different types of cones
that respond to different wavelengths of light, and it's not an exact science.
We don't say that one specific type of cone cell
responds precisely to some specific wavelengths of light,
know there's a response curve that's associated with these.
And that implies that some of them there's some overlap in this element,
so we might actually have sort of a non-linear stimulus
to various types of colors.
>> And in fact, this is precisely what happens, if we take a look at this
we have three different types of cells-- The s-type cell, which
is for short wavelengths, the MDL types, which are absolutely
the most prevalent types of cones within our eye,
and you notice that those are very high up in this spectrum,
much closer to the green spectrum.
And this actually is very, very important to us
as digital photographers and in the construction of digital cameras
because this is one of the primary reasons why-- well, there's
a lot of things that this impacts and hopefully we'll
get a chance to get to them.
But the result of this is that we actually
respond better to green wavelengths than we do to red or to blue,
and in fact our response curve is very different for that.
>> And if you sort of close your eyes for just a minute
and imagine that you have three similar rooms that are all
totally dark except in the very center there is a light bulb.
And in one room, you have a green light bulb,
in one room you have a red light bulb, in another you have a blue light bulb,
and that's all you have in this room for illumination.
And if you imagine the relative brightness of these rooms based
purely on this single light source, try to imagine
which one might feel brighter, and the correct answer is green.
Generally what happens is that because we respond, because our cone cells are
stimulated much more by the green wavelengths than by any others,
we respond much more to that light, and so that is actually
very important for our perception of brightness and luminous,
as opposed to some of these other colors.
>> Now, if we take a look again at this, the eye structure that we had,
we had course light that comes in on the left side of this diagram
through the iris, focused by the lens and onto this so-called "censor,"
our retina at the very back of the eye, and this is very similar
to the structure of a digital camera as well in some ways.
We have a lens, which is actually used the focus the light.
And that light is then focused onto the very back
of the camera, which has the sensor.
>> Now this is a diagram of a digital SLR-- a single lens reflex camera, which
for those of you that are unfamiliar are sort
of the more professional looking ones.
They're the ones that allow you to change lenses,
they're the ones that have a hump on the top of the camera where
the prism and the viewfinder is so you can actually look through it.
The reason that it works that way that it does
is that the pentaprism actually reflects the light that
has come in through the lens and reflected off
a mirror that operates that sits at a 45 degree angle.
It goes up through the pentaprism and then out through the viewfinder
where you are able to see the image.
>> When you actually take the exposure, the mirror moves up and out of the way,
the shutter is opened, and that allows the light to pass all the way back
through and directly hit the sensor, which causes the exposure to happen.
So in the typical configuration you cannot actually see the image through
the viewfinder in a proper digital SLR, you cannot actually see the image
through the viewfinder and also capture the image.
If you happen to have one of these cameras
you might say well I have a preview mode,
but what that essentially does it lifts the mirror out of the way.
It turns off, essentially disables, the optical viewfinder and it
uses the screen on the back of the camera based on the light
that the sensor is receiving.
>> Now there's an important aspect of light to recognize beyond the fact
that it is made up of wavelengths, that it is made up of colors, that
as a result of the various wavelengths, and that
is that the individual photons that make up the light
have a direct correlation to the relative brightness,
or to the intensity of that light.
So every time that we double the number of photons
at any particular wavelength of that light then
we're essentially doubling the intensity,
we're doubling the brightness of that light,
and this has a very important name in photography.
It's called stops.
So when we're talking about exposure, we talk about stops in this way.
We generally want to try to manipulate this is quantized notion of photons
that are actually entering into our camera
by either having or doubling the amount of light that is allowed in.
So it's very, very frequent that you'll see
numbers related to this idea of stops.
So for example, the idea of exposure compensation,
which we'll talk more about in just a minute,
operates in this notion of stops where a single stop
is a doubling or halving depending on the direction
you're going of the amount of light that's being entered.
>> Now of course, when we're talking about a number of stops, so for example,
let's say we're talking about a change of two stops as opposed to one stop.
This means we are not just doubling it, but we're doubling it again,
so a changing two stops results in a four times
difference in the intensity of the light.
Likewise, a three stop differences is eight,
four stops is 16, so on and so forth.
>> So even a low number of stops can represent
a wide variety of different intensities in light.
And in fact, when we're talking about daylight versus the brightest
day versus the darkest night we're really talking about 20 stops perhaps
at the absolute most.
That's probably something closer to 15 stops or so,
but that'll be important in just a minute as we
keep talking about exposure.
>> So we talked a little bit about light and so let's talk about some
of these other exposure settings that actually
allow us to capture the light that exists in a scene.
There's shutter speed, there's ISO and aperture,
and we alluded a little bit to shutter speed before,
but I do have a video that sort of shows the anatomy of a camera
and also will illuminate this idea of the shutter itself.
So I have here this high speed photo which
I happened to find on the internet, and what you will see
is this action of actually capturing an exposure
on this particular digital SLR.
>> So as I'm talking I want you to pay attention to a couple of things.
First, notice that the mirror moves up out of the way,
recall that we talked about this in a digital SLR.
Now notice that the thing that we're seeing it behind that
is not the raw sensor itself, but it is in fact a piece of plastic
or Kevlar depending on the quality of the camera that
operates as the shutter.
It's a mechanical shutter actually move out of the way
and exposes the sensor underneath.
So let's take a look at this one more time
so you can sort of watch the action of the shutter.
The mirror moves up by the way, notice shutter opens
and then very quickly there is another curtain that closes behind it.
This is a very typical set up for digital SLRs with mechanical shutters.
We'll have two curtains that operates either horizontally
or vertically depending on the particular camera
and it will move across the entire plane.
First the first curtain will open, exposing the sensor underneath,
and the second curtain will close thereby stopping the exposure.
>> Now there are other types of shutters as well, and really for our purposes
we don't have to worry about them too much except for the electronic shutter.
So this is a mechanical shutter, and you'll typically
find this on digital SLR.
And the whole combination of these movements,
including the mirror moving up, out of the way,
the shutter opening, and then the second curtain closing behind it,
results in that characteristic click that we hear in cameras.
But for cameras that don't actually make that physical noise,
such as camera phones and compact cameras and smart phones
and a variety of others is that they have an electronic shutter.
An electronic shattered doesn't operate in the same way,
but rather it starts to read data off the sensor and then immediately stops,
or rather it allows the sensor to accumulate the data of the changes
in the voltage caused by photons hitting the sensor
and then it will actually clear it once the exposure is actually complete.
>> So this is sort of the most rigid definition of shutter speed,
but what ultimately this means is that this is defining how much light we
are actually receiving on the sensor plane,
and ultimately this means that we can change the shutter
speed in terms of stops.
We might have the shutter open for a single second,
for example, and so we would say that our shutter speed is then one second.
And what that means in the mechanical terms is that the first curtain opens,
the sensor is then exposed to light for one second,
and then the second curtain closes behind it.
>> Then of course, we can change this by a stop
if we go a stop brighter this means that we then
have to keep the shutter open for longer,
so that we can collect more photons.
So a stop brighter would result in two second shutter speed.
Likewise, a stop darker, which would mean that we have to have the shutter
open for less amount of time so we would have half a second of a shutter speed.
We can keep going in either direction, but if you play around
with the settings on your camera, you probably
will notice that it seems to approximately double
or halve depending on the direction of your tuning.
>> Now, the shutter speed because we can have it open for some arbitrary
amount of time does have some impact on our image.
In particular, imagine what happens if you're
capturing all of the photons in a particular scene
over a couple of seconds.
You might imagine if there's some movement within this scene,
so for example there's a ball that moves across the scene,
or in the case of this photograph there's
a wave that moves across the scene.
>> I'm capturing the photons from that entire movement,
so this is causing a motion blur that becomes
very visible within the photograph and sometimes this is intentional.
Sometimes you actually want to get some motion blur so you can smooth out
the motion of the waves, for example, or perhaps you
want to actually capture movement of a fast moving
car, you want to actually capture the movement of fireworks, for example.
By the way, many people love to go outside and take pictures of fireworks
and have very high, fast shutter speeds, which just looks abysmal,
because it's just the brief moment of explosion or a couple of seconds after
and then they're all chimping.
>> Do you know what chimping is?
It's like you take a picture, right, and then you're hunched over your camera,
and you show your friends and you're like, " oh,oh,oh."
Chimping, right?
OK.
>> So come back, so you have this idea of fireworks where it's really
the movements of these fireworks that's really interesting, so
try experimenting with your shutter speed
and capturing the movement using a very long shutter speed,
rather than a very short one.
Of course, this means that you can get motion
blur due to a wide variety of factors.
It might not just be the object in this scene that's moving quickly,
as is the case in the fireworks here, or the other car or the environment
in this photo on the left, but instead imagine
if you're trying to hold of the phone or your camera for that long.
No matter how much you actually brace yourself,
you will have a small amount of movement that translates to some motion
blur within your camera.
>> So if you're try to counteract that, you either
have to increase the shutter speed so that it decreases the amount of time
that the shutters actually open and thereby freezing that motion,
or you need to stabilize the camera in some way.
In which, case you might want to use a tripod
or to set the camera down on some stable table or something along those lines
to actually freeze that particular motion.
So this is an artistic question that you have
to ask yourself is in which direction do I actually want to take this,
do I want to try to capture the motion by having this intentional motion blur,
or do I want to freeze the motion, and sometimes
freezing the motion is precisely what you want, in the example of sports
photography for example.
>> You really want to capture that precise moment that something is happening,
or perhaps rather than get this smooth motion of the entirety of some ways
you really want to capture the sort of instant moment
that a wave crashes or breaks against the rock
and you want to capture that moment.
You certainly will want to capture this.
By the way, this is what it looks like, my camera got soaked, I got soaked,
it was totally fine.
Don't worry about it, a lot of cameras are
a lot stronger than you would imagine.
The buttons on the camera were a little gritty
from the sand stuff-- ended up being fine.
>> Now sometimes you actually want to mix both motion and still in one camera.
So imagine what happens if you have a moving object
and you pan your camera with that object keeping some part of that object still
totally still relative to some portion on your sensor,
if you're able to have a long shutter speed that actually captures movement
of the environment but you keep that one part of the object
still relative to some portion on your sensor you can mix both and get
a sort of neat effect where you are able to get something in sharp focus
and without any motion blur, but sort of blur
everything else in the environment.
And sometimes this is actually what you want also for sports,
sometimes you do you want to convey this motion of the motion itself
or the idea of speed.
So for example, in a car race you might not
want to totally freeze the motion of the car and the wheels,
because then it will look like it's not going anywhere.
It's just standing on the track, providing
some of that can actually give some amount of drama to the scene.
So let's take a step back from the shutter speed a little bit
and talk about some of these other settings as well.
One of them is ISO, and you might have heard
of the term in the context of sensitivity,
but that's not really an accurate way of thinking about it, at least
in terms of digital cameras.
We're not actually changing the sensitivity of the camera,
there's actually some other electronic trickery that's
happening underneath the hood, but for our purposes for now,
thinking of it as sensitivity is an OK way
to think about it, especially In terms of exposure value.
>> So ISO starts generally at a round value of 100.
It's just sort of an arbitrary value, and if we
are to think of it in our simplified terms as sensitivity,
increasing the ISO means that the sensor become slightly more
sensitive to light, which would then allow
us to change the shutter speed to be faster.
So, in other words because we're trying to get the amount of light
in our scene to match the specific range of our camera
we have to play with these settings, so these two settings
that we've mentioned and also aperture that we'll talk about in just a moment,
in order to really get that precise range of photons within our sensor.
So one of the ways that we're able to do this one, and one of the ways
that we're able to alter our shutter speed
is to also change the ISO for a given scene.
So by increasing the ISO we increase the so-called sensitivity,
which allows us to make the shutter speed faster,
or likewise perhaps we actually want to make the shutter speed longer.
Perhaps we actually want to have a lower ISO
and increase the time that the shutter is open to capture our motion
or to capture that motion blur for some artistic purpose.
>> Now the downside to ISO of course, is that we actually
get a fair amount of noise as a result.
And these are some examples from relatively old cameras,
but generally this shows an interesting general trend
that larger cameras tend to do slightly better in combating issues of noise.
And it's not really the case that larger cameras are doing it,
there's a lot of factors that play into this-- the age of the sensor
is one important distinction, but also the size of the pixel,
so it's not really the size of the camera,
but the size of the pixels itself can make a huge difference because larger
pixels can capture more light, there's more area through which you
can actually capture more photons.
And also the electronics are a little bit bigger
and they cant hold more voltage, perhaps,
and be able to give us a better signal to noise ratio.
So there's a variety of reasons why, but generally speaking, bigger sensors
or bigger pixels more specifically allow us to get better quality out
of our higher ISO settings.
If you're really struggling with getting a lot of noise out of your images,
perhaps you are using, for example, a smartphone that
has a sensor that's really, really small and because it
has a very high megapixel count, the pixels also
have to be very small, which results in a relatively noisy image at high ISOs.
>> So one of things that we've noticed is that ISO noise improvements has just
been enormous, especially in recent years.
The sensors essentially a technology very similar to that of our computers
and over time it's really, really improved,
and nowadays the noise that we see in digital cameras really greatly
exceeds the noise capabilities of film.
So in other words, the digital cameras with digital cameras
we can take images that are far less grainy, far cleaner than film,
and this is maybe good or bad depending on how you look at it.
Sometimes you like having that additional texture for that,
but you can of course add that later in software.
>> So let's take these two into combination in these two ideas
and combine them to realize how we can alter one to impact the other.
So in the context of ISO and shutter speed,
imagine that I'm taking this photograph, which
I did many years ago back in 2007 in New Hampshire.
I was on a dock at the edge of Lake Winnipesaukee
and there was some cool stars whose trails I wanted to capture.
So I set my camera outside, changed the modes
so that I could have several minutes worth of exposure time,
and just waited outside in the cold for 15 minutes and got this picture.
>> And so there's a variety of stars here, it's an OK photograph,
but at the very center I've highlighted one particular star, which
I think I asked an astronomer friend and they said it was big at the time.
One of the interesting things to notice is
that you can of course see the Earth's rotation in the star trails,
but notice that the radius of the circle seems
to get smaller as you get to the upper right portion.
That's because I was pointing the camera towards the north,
and this appeared just of the slide just little bit
was the North star through which the Earth was rotating.
OK.
So anyway, we have this star that I want to point out.
Vega, it has a specific length, and realized
that if I wanted to make the star trail longer the thing
that I would need to do is to change the shutter speed.
I would have to have the shutter open for a longer amount of time,
but the amount of light in this scene is fixed,
I can't actually change the shutter speed without changing something
else so that the amount of light that enters into my camera
continues to be correct, and I continue to get a properly exposed photograph.
>> So I can of course change the sensitivity,
and if you're able to look at this relatively small text below each
of these images you'll see the change that
happened is that I changed the ISO by one stop, so changing it from ISO 800
to ISO 400, which then allowed me to increase the shutter
speed approximately by a value of 2.
And that is how we were able to get precisely
this star trail that was twice as long.
>> All right, so then let's talk about this third idea of aperture.
Now aperture, unlike shutter speed and ISO,
doesn't have a very nice doubling or halving
to represent a single stop change in exposure.
The reason for that is that aperture or f-number is really
a ratio of some things that are related to a lens.
Now this icon is actually from the now defunct apple Aperture
software, which is too bad.
It was a fantastic software, but one of the things that this icon has which
is representative of a lot of lenses that you have on cameras
is the data on the lower right of this lens.
You notice that it says 50 millimeters, which is the focal length of the lens,
and it also has this 1:1.4, I know it's upside down, but you can read it,
it's 1:1.4 and that is actually this aperture.
This is actually the f-number, the maximum possible aperture of this lens.
And this is important because this tells us
quite a few properties about this particular lens-- the focal length
tells us how zoomed in or zoomed out it is, 50 millimeters on a typical camera
is a very stand sort of field of view, it's not too zoomed out,
it's not too zoomed in, it's perhaps somewhat
equal to how it would look to our eye, but there's definitely
some changes in the field of view.
>> Let's take a look now at this aperture.
The ratio here is precisely the ratio of the focal length divided
by the apertures effective diameter, so what does this actually mean?
So let's keep in mind this division for just a minute.
The f-number from this previous slide was in fact this 1.4 value,
the 1 colon just represents the fact that this is a ratio,
and the focal length is this 50 millimeters.
So this is important and we'll be able to find out why in just a second.
>> So here's an oversimplified view of a lens, it's a side view of the lens.
On the very far right of this image we have an imaginary sensor plane.
Notice this symbol here, there's a vertical line with a circle.
That represents a sensor plane, and if you
happen to have a digital SLR or some sort of other advanced camera
take a look on the body of that camera, you might actually find the symbol
and that represents the plane through which your sensor actually
exist somewhere within that camera, but anyway we
can measure the focal length from the nodal point of the lens, which
in this oversimplified thing just happens
to be in a single lens element, all the way to the focal plane itself.
And there's an effective diameter of that lens.
>> The diameter is the maximum aperture through which
the photons enter and are focused on to the sensor.
But imagine what might happen for just a minute
if we had this amount of light that was actually
able to enter through our lens, but we actually restricted this,
so we have some sort of device that actually reduced the amount of light
on the outside from coming into this lens--
very similar to the iris in our eyes.
When you go outside, for example, and it's
bright daylight you might actually notice that your iris constricts
to let in less light, likewise when you go inside into a very darkroom,
your iris expands to allow more light.
It's precisely am analogous situation to what we have here.
>> And so what this actually means is that the f-number has
some indication of precisely how much light this lens is actually
able to accumulate through this diameter and the focal length,
because as we actually increase the focal length,
the diameter would need to increase to allow the same amount of photons
to enter into the lens and fall onto the sensor.
So there's some math that we can do to actually figure out
precisely what a stop difference is between the various f-numbers.
So I'll hopefully be able to post a handout
next to the slides that will actually show you that math.
>> That goes through this and takes all this into account,
but you can also sort of figure it out yourself
through this ratio that we were just talking about
and imagine that the way that we are able to restrict light
through this mechanism is to have different amounts of areas
through which the light is able to flow.
So if we have a circular lens that has an aperture
that's this large that means that photons are flowing through that area,
but imagine how this might change if we actually restrict that area.
So because we're actually talking about a difference in area
rather than some sort of linear change, such as shutter speed,
this is actually what causes the very strange numbers
that we see out of f-numbers.
>> So there's an easy way to remember the differences
in one stop between all of the f-numbers.
First remember two numbers-- f1 and f1.2 and double each one to get a subsequent
number.
So for example, you would double f1, we get f2,
so now the string of aperture values that we have are f1, f1.4, f2.
Now we take that second number, 1.4 and double that.
So now we have 2 and 2.8, and we can continue along in this fashion.
4, 5.6, 8 and so on and so forth.
This breaks down after about the 32 or something like that,
but it's close enough approximation for our purposes.
>> So just like shutter speed and ISO, the aperture
does have an impact on our images, and one of the biggest impacts
that it actually has beyond the fact that it's
allowing more or less light depending on whether we have constricted
our aperture or increased it's size, the biggest change perhaps that it has
is the amount of background blur that you might actually
have within your image.
The larger the aperture, the more background blur
you'll actually have in your image.
So you can reduce the size of the aperture, thereby letting in lets light
and get more of your scene in focus, or you
can try to increase the size of the aperture by decreasing the f-number
and you will get less of the scene in proper focus.
And this can be an effective tool as well
if you want to isolate your subject from the background, for example, or perhaps
you actually have a landscape shot and you want to do the opposite.
You want to try to get as much of that as possible in focus,
and so what you might actually do is then decrease the size of the aperture
by increasing your f-number and altering the other shutter values,
or the other exposure values as appropriate to actually capture as much
of your scene and focus as you might like.
>> So this is the big four.
We talked about the amount of available light, the shutter speed
that's actually there, ISO, and aperture and how the amount of available light
is we're sort of at the mercy of the scene that we happen to be capturing,
unless we happen to have an indoor setup or some other way
that we can impact that amount of light, and how
we can use the three values-- shutter speed, ISO, and aperture,
to vary the amount of light that enters to our sensor
and captures our exposure.
And so there's this discussion of stops and how
I mentioned earlier on how there's this distinction.
>> There's about 20 stops difference perhaps
between the brightest bright day and the darkest dark night without any moon
shining or anything like that, and cameras
tend to operate in a dynamic range, so the possible range
of light that they can actually capture tends to be much lower.
Perhaps along the lines of about 10 stops, or maybe at a maximum 12 stops,
and we're talking about some really high end cameras here.
You might recall from our discussion earlier of the Philae lander
that had some phenomenal technology-- well,
the Rosetta camera had some phenomenal technology for the time period, 1998,
and that actually has possible 14 stops of dynamic range.
>> But this really implies something about this
that if we have some object, such as the moon or a comet that's
illuminated full on by sunlight with any atmosphere
especially to reflect some of that light, then anything in the background
is just going to be so completely dark that we're not
going to be able to see it.
So this is sort of the primary reason why a lot of these photographs have
such harsh lighting is that there's no atmosphere to reflect it and sort
of fill in the gaps in the crevices of the moon, for example,
or the crevices of the comet, but also because the stars that are actually
within the night sky are so dark relative to the ground that's being
illuminated by the sun that they fall away in exposure and we cannot actually
see them whatsoever.
>> So some terminology here, there's underexposure,
overexposure, sometimes there's both, underexposure
is when something's a little bit too dark,
you actually need to increase the exposure
to actually get all the details.
Underexposure-- the hallmarks of it is everything just looks way too dark,
the shadow areas have absolutely no detail.
This one isn't horrendously underexposed, but it's pretty bad.
>> Overexposure is the opposite.
You have overexposed portions of your image
and you've lost detail because it's simply too bright for your sensor.
You might need to change your exposure values to compensate for that.
And if you have both, we'll you're just sort of out of luck.
>> So one way to overcome these issues, because frequently you
will come into a compromise between the capabilities of your camera
and the amount that you can actually vary these three exposure
values and the amount of light that exists in the scene so one of the best
powers that you have, especially if you're taking photographs outside
is to just wait a little while for better light.
Generally midday light is really harsh, it casts very harsh shadows,
there's less atmosphere to actually reflect and scatter some of the light
and so it just tends to be not a very good situation.
If you're able to wait even just a few hours,
wait until dusk or if you're able to do so, get up at dawn
and you'll be rewarded with wonderfully soft light
that has a lot of color-- warm colors and tone
that results from the light passing through more of the atmosphere.
>> Now very quickly, there's this concept of metering,
which is what the camera actually does on our behalf
to alter each of these three exposure values
and try to capture an appropriate image.
And generally what the camera does is it tries to take the entire scene
and look at it in the sort of middle gray.
It tries to figure out what is the middle tones, the middle brightness
of the scene, and it will try to expose your photograph for it.
>> And typically there's some additional fantastic goes into this,
it will divide it into a variety of zones
and it will try to figure out in which zone you've actually focused,
and say OK that's probably a very important zone
and so it will apply some extra weighting or priority to that zone
and all that stuff is fine, but this will still
have the problem that even though you might have some images that
are being exposed to this middle gray, the scene may not actually
be appropriate for that.
And so unless you're using the absolute most manual mode
available on your camera, you're probably relying on your cameras meter
to some degree to try to help you pick these exposure values.
And this means that occasionally you need
to do something called exposure compensation to notify
the camera that the scene is actually a little bit
different than its assumption.
So in particular, if you have a scene where there's a lot of snow,
or a lot of white sand as in the case of this image
or it has a lot of dark areas, it's a very shadowy, very dark alleyway
or something like that, dark at night and you actually
need to notify the camera that it needs to not
expose for the very middle you can apply some exposure
compensation to overcome this issue.
>> So in this example, the original exposure that the camera wanted
was on the left.
Notice how it looks sort of dull gray, it's not precisely what you want
and I would suggest that this is actually one of the best things
that you can do to improve your photography
is to pay more attention to the exposure compensation setting on your camera
because most likely if you are taking a scene in the snow, which is especially
relevant for those of us here in Cambridge,
very soon it's going to start to snow, or if you're outside
and it's dark at night then you actually have
to apply some exposure compensation.
>> So you apply exposure compensation in stops
and what you do is you tell the camera to either increase or decrease
the exposure compensation based on its assumption of middle gray,
in this case, I know that because the scene was going to be brighter
than the camera was expecting it I needed
to actually tell it to increase the exposure compensation,
so by adding a positive 1 stop of exposure value of exposure compensation
I told the camera that it's actually brighter than it was anticipating
and would then take a properly exposed photograph.
Likewise, we might have a scene that was too dark.
For example, if you are trying to take an image of someone who's
wearing a dark coat for example then it might actually confuse the camera
into making everything little bit too bright,
you might need to dial in some negative exposure compensation
to overcome this issue.
>> Now many cameras have a wide variety of metering modes.
In fact, what you will find is that the simpler the camera,
the cheaper the camera the more modes it has
and this is just ridiculous what they've gone through.
I've seen cameras now of course there is like a self portrait mode,
but they have a party mode, candlelight mode, a sunset mode, fireworks mode,
beach mode, snow mode.
I saw one camera that had a beach mode and the beach two mode,
so I have no idea what the difference between those two was,
but it doesn't matter.
You don't really need any of those modes,
because the vast majority of the time they do nothing special to the camera,
to the settings in the camera, other than altering these three exposure
values.
So if you just sort of think about what you might want out
of that particular image, you could overcome those issues
and use one of the simpler, one of the more raw metering modes
so that you can actually take photos with a great deal more control.
So for example, in a portrait you might actually
want to isolate your subject from the background, which
would mean decreasing the f-number or having a very large aperture,
so you get very nice background blur from them or within that shot,
and so that would be your priority.
And that's precisely what the portrait modes in these cameras do,
is it tries to make the apertures as large as possible
and alters the other settings as a result.
>> OK.
So let's go into a completely different direction and talk a little bit more
about the digital aspect of digital cameras
and just talk very quickly about sensors and some
of the different technologies and some of the things
that actually impact us as photographers.
I'd alluded to dynamic range before and we can think of sensors
as being an array of bucket that capture light in the form of raindrops.
>> So imagine we set out an array of buckets outside
and they're going to capture rain, and we can then measure the amount of rain
in each of those buckets and that is our image,
so-called, and we can take this analogy quite far
and it's actually a relatively good analogy
because it alludes to a number of things within the digital camera.
Imagine a couple of scenarios.
First of all, imagine what might happen if we allow rain or photons to actually
fall into our bucket and not a lot to actually falls there.
Now imagine that we have some sort of way of measuring this,
if we have some measurement that's not accurate enough
to measure the small amount of water that we've actually collected then
it's indistinguishable from noise, we're not actually
going to be able to measure that as any sort of signal.
>> And so we'll perhaps guess as to the value that is actually
appropriate for that small amount of white.
This alludes to this problem of sensors that do not collect enough photons
and it's just too dark and so there's noise
in these dark regions of the image.
Likewise, if we allow too much to collect into this bucket it might fill
up and actually overflow and so beyond that point
we have no way of measuring or knowing how much rain has precisely
fallen within this bucket, we just know that it's beyond the maximum.
That's precisely what happens in these buckets as well, or in these pixels
as well, is that once we've gotten to their maximum of voltage
then it's not actually possible to get any more detail out of that
and we would get an overexposure.
>> We can actually take this analogy just a little bit further
if you imagine again this array of buckets
that are sitting next to each other.
One of these buckets fills up with water.
You can imagine it might spill over into neighboring buckets,
and this concept is known as blooming within a digital camera
and we actually see this in a wide variety of circumstances where
a very, very bright section of the scene that is extremely overexposed
will actually bleed some of its data over to the neighboring pixels as well
and cause those to become overexposed as well, which
is kind of an interesting phenomenon.
>> Now imagine that we're actually able to take
a division between the maximum amount of volume
that we're actually able to measure here,
our full well capacity, our full bucket capacity,
divided by the smallest possible signal.
This would be our dynamic range and one of the ways,
there's variety of ways that we can improve the dynamic range for a camera
and what this essentially says is the possible range, this range that we were
alluding to before, that allows us to specify how much or how little light
we can actually capture with our camera.
So there's a variety of ways to improve this dynamic range
as you might imagine.
One of them is to have a bigger bucket-- actually
allow us to capture a fuller signal.
Another way to do this is to minimize the detectable signal,
to actually decrease the amount of noise that we get out
of the electronics of this particular sensor,
and some of the advancements in recent years
have, in fact, been to decrease the smallest
detectable signal within the sensor and then
we are able to improve our dynamic range and get improvements
within our photographs.
>> Now one of the other really important things to realize with digital cameras
is that they come in a variety of sensor sizes
and so there's a wide variety of sizes.
One of the great things of modern digital cameras
is that we are seeing larger and larger sensors in smaller and smaller cameras,
but there's a wide variety of things that this actually impacts,
not the least of which is the way that focal length will actually
change the field of view depending on the size of the sensor.
So imagine, just for minutes, and sort of a teaser for what you should look
into after this seminar is actually over--
imagine that we have a lens that because it's circular projects
this circular image on to some location and imagine
we have a sensor that's relatively large and captures as much
of this area as possible, in this case our red sensor here.
>> Now imagine we have a smaller sensor, this blue sensor that
captures the center portion of this image.
If you blow both up to be approximately the same size you'll
notice at the blue sensor seems to be a crop,
it seems to be this center portion and it
makes it look like you're using a larger focal length
lens than you actually are.
So for this reason, as we shrink the size of sensors
we also have to shrink the size and the focal length of our lenses
in order to compensate for that change in the field of view.
And as you might recall from our discussion about aperture
just a few minutes ago, this means that we also
have to change the diameter of our aperture to maintain the same f-number.
>> So we can go on and on to a wide variety of topics in sensor sizes and all
these things, but this is really just a teaser for some of the things
that you might actually start looking into.
When we start talking about this a little bit more
we start talking about 35 millimeter equivalency.
We might have some sort of reference size of a digital sensor
that we're able to compare other sensors to in order
to discuss our focal lengths in a more meaningful way
and so I certainly suggest that you start doing your research in that area
if you're interested in doing that, but for now it
seems like I've run out of time and we'll have to sign off.
>> So I want to thank you all very much for viewing.
I'll post the slides that we have here online and also
that handout that allows you to understand a little bit
more the mathematics behind the wacky f-numbers,
and I do encourage you to take a look at that.
And so thank you very much for watching and I hope to see you soon.
Oh.
Thank you, thank you.
The illustrious audiences enjoys it.