DOF Depth Of Field - Photography & Filmmaking Tutorial 15


Uploaded by polcan99 on 29.12.2010

Transcript:
Hi, my name is Tom Antos and today I'm going to talk about the depth of field
and also how the size of the image sensor doesn't change the depth of field,
as a lot of you out there seem to think. Also, a lot of times I get asked
how can I achieve a shallow depth or field, or how can I have everything
that's in my shot be in focus. Really there are only three things
that affect your depth of field. Those are the aperture size or setting;
the focal length of your lens; and also the distance to the subject.
Before I explain each one, let me talk about the depth of field itself.
What is it? It's basically the portion of your scene that appears sharp or in focus.
Also, different people use various terms to describe the depth of field.
So, a shallow or narrow, or, as some call it, a small depth of field,
all that really means is that there is very little in focus.
Like we see here...
where only our main subject is sharp and everything else behind him is out of focus.
Now, a deep or wide, or a large depth of field means that all or most of your scene is in focus.
Like we see in this example here... So if you want to get a narrow depth of field
then the three factors that I mentioned before should be as follow:
the aperture should be opened as wide as is possible.
You should basically use the largest focal length of your lens.
So, if you have a zoom lens, then you should zoom in,
or if you're using different primes lenses, let's say on your SLR,
then you should use the narrowest lens or the one with the biggest number that you have,
like a 200 or 300mm telephoto lens.
And finaly, you should be as close to the subject as possible.
So, those are the three things that you need to get a very narrow depth of field.
Now, if you want to get a wide depth of field, then you should have the aperture closed
as much as you can. You should also use a small focal length lens
and zoom out as much as you can,
or if you're using prime lenses, then use the widest available lens
that you have, like a 16mm or a 24mm,
which are... they're nice wide-angle lenses.
You should be also as far away from the subject as possible.
It doesn't mean that you have to do all three of those things to get
the kind of look that you want.
Sometimes doing just one or two of these changes is enough.
Here is a few examples.
Let's say that we want to get a shot of this subject
and have only him be in focus. Now, let's see how it looks
when we change one of the factors that affect the depth of field.
First, we'll use the biggest zoom lens that I have with me,
which is this 300mm lens. Or if you're using a video camera,
we zoom in as much as we can. Then I will open the aperture
as much as I can. A lof of you don't know what that means,
so let me explain it. Every lens has an aperture inside it...
which is the same thing as the iris inside your eyes.
It's an opening that controls the amount of light that enters the lens and the camera.
The aperture is measured in f-stops. The bigger the f-stop number,
then the more the aperture is closed, which results in less light coming in
and also in a wider depth of field. Now, in this example, we want to get
a narrow depth of field, so we'll in fact open the aperture
all the way up. Or another way you can phrase it is that
we'll set the f-stop to the smallest number, which on this lens happens to be f4.
If you're using an automatic lens or a video camera
then you'll have to change your f-stop using the menu buttons on your camera,
provided that your camera allows you manual control.
A lot of times I'll hear people say that they've got a good fast lens.
What that simply means is that their lens can be opened
even more then this lens that I'm using. You know, like a lens that, for example,
can open to f1.4 or f1.8,
which is a great option to have, because it means that you can let in
a lot of light. Also, it comes in really handy when
you're shooting in low light situations. Plus, you can get a narrow depth of field
if you ever needed to do that. Okay, so getting back to our example,
once I open the aperture all the way up, then I will have to adjust my exposure
to make sure the image isn't overexposed. What a lot of people do when shooting outside
is that they will either leave their camera
in the automatic mode, which usually causes the camera to
close the aperture down for you since there is so much light outside,
or they themselves will close the aperture to get the proper exposure.
That's why often people have problems getting a very narrow depth of field
when shooting in bright conditions such as this.
You gotta remember that the aperture affects the depth of field,
but it also changes the amount of light that enters the camera.
And since in this case we care more about having a narrow depth of field,
we'll have to open the aperture all the way and adjust our exposure using another method.
Another quick way that you can compensate for an overexposed image is to set
your shutter speed to a really high number,
like I did here. I switched it to 1/320th of a second.
Which works great when shooting in a static
or really slow moving shot like this. But the second your camera or your subject
in the shot move, then you'll notice this strobing effect.
That's because the shutter speed is set so high
that there is very little motion blur in your shot.
So, every frame is so sharp that it gives the motion
this kind of a strobing effect. For our example, it's okay since we're not
moving around. But if you were, then you should instead
darken the overexposed image by putting a neutral-density filter
over your lens. Or if you're using a video camera that has
a built-in ND filter like this camera that I'm also using,
which is a Canon XH-A1... then you would just simply turn on
the ND filter by switching this button to the desired strength.
Now, the ND filters come in different strengths, or you can get a variable ND filter
such as this, which is great since you can easily darken
your scene exactly as you need just by turning the filter.
Now, this is how the motion blur looks once we use an ND filter to darken the image
and then slower the shutter speed.
In this case, we slow it down to a 1/40th of a seceond.
You can see the motion looks a lot smoother then when it was shot at 1/320th of a second.
Also, when looking at the still frames, the images have a lot more motion blur
when shot at the slower shutter speed then when they're shot at the higher speeds.
Anyways, once the aperture is opened all the way up
or set to the lowest f-stop number, and we've adjusted our exposure using
the shutter or the ND filter, then the last thing left for us to do is
to adjust the distance to the subject. This is often very limited because
many times we can't move the camera closer or farther away
since it really changes our framing.
But, you know, for this example, let's just say
we don't care how close we are to the subject. We just want to see a bit of his face and
have everything behind him be out of focus. So, since we know that if we want
to create a narrow depth of field we have to be as close to the subject as possible,
we'll move to about eight feet away. Before that, we were twenty feet away,
or twenty-five feet away. Something around there.
This is how the shot looks at this distance. If we keep all the settings the same
but we move in EVEN closer to only about three feet away
from the subject's face, then this is the final effect that we get.
As you can see, the depth of field is so narrow that we can't even keep
his whole face in focus. Now, let's say that we do want the whole face
to be in focus, but not the background...
we can close the aperture a bit more since we know
that the more closed the aperture is, or the higher the f-stop number,
then the wider the depth of field. So, set it to f16, for example,
which looks like this now. It's a bit more in focus now.
We can see now pretty much the whole face in focus
and even a part of his jacket now is sharper. We can also do this another way.
We can, for example, leave the aperture the same as before,
which was f4 and simply move the camera farther away from our subject,
to about twelve feet away. Then, as you can see, the depth of field
is wide enough where the whole face is in focus.
If we wanted to get even more of our scene in focus,
then we'd simply close the aperture as much as we can.
On this lens that is f32, which looks like this.
Right away we notice that the background is a lot sharper than it was before.
Now, let's say that we want this scene to have everything in focus,
then we know that the last thing we can do is to use the smallest focal length lens or
the widest lens that we have available. So, we'll use this 16mm lens and set it
at f22, which looks like this.
In fact, this lens is so wide that even if we were to move in closer to the subject,
everything is still pretty much in focus. Now, let's look at the same example
using a video camera. I'll be using the Canon XH-A1,
which has a 1/3 inch image sensor. For the widest depth of field,
we'll use the lowest focal length or we'll zoom out as much as possible
and we'll set our f-stop number to f9.5, which happens to be the higest
f-stop setting on this camera. Now, to get a narrow depth of field,
we'll move the camera as close as... we can focus this lens,
which is about ten feet away, once we zoom all the way in.
Also, we'll open the aperture as much as we can,
which happens to be f3.4. And, as you can see, it's a pretty shallow
looking depth of field. In fact, it looks very similar to
what we got using our DSLR on the 300mm lens. Even once we move the video camera
a little bit farther away from the subject to about fifteen feet away.
As you can see, you can also get both a narrow and a wide depth of field
on a video camera too... You just gotta know how to control that.
So, remember that as long as you're shooting with a camera that allows you manual control
over the aperture or the f-stop and also lets you either change out the lenses
or zoom in or zoom out, then you can control how much
depth of field you have in your scene. This stuff doesn't just happen by magic.
It all has to be decided by you. Now, one common myth that I hear is that the way
to control the depth of field is by having a camera with a larger image sensor,
which is completely false. Image sensors don't change the actual depth of field.
I know a lot of people are gonna get really angry when they hear me say that,
cuz they maybe spent money on a newer camera with a bigger image sensor.
But what you gotta understand is that the only thing that changes when using a camera
with a bigger image sensor versus a camera with a smaller sensor,
is just the framing. Simply put, with a larger image sensor,
it is easier to get a shallow depth of field because you're not cropping in as much
on your image. For example, let's look at this scene
that's now shot using the Canon 5D Mark 2 and a 50mm lens that's open all the way
to f1.8. This is how it looks.
And here is the same scene, but shot using the 7D and the exact same 50mm lens.
Both examples were shot from the same distance,
but because the 7D has a smaller sensor, the image is basicly cropped in a lot more.
Now if we were to take that image that we got using the Canon 5D
and crop in digitaly to about the same framing, then you'll see that the depth of field
in both images is exactly the same. So, really the only thing that changed
between the two cameras is just the framing. We're in a bit closer on the subject
simply because the smaller image sensor results in a cropped image.
So now, if we wanted to get the same framing as we got with the 5D
but using the 7D, we in fact have to move farther away from the subject,
which then results in this image. Now, here we can see a slight difference
in the depth of field between the 7D and the 5D...
but still you gotta remember that it's not the camera or the image sensor that made that
difference, but only our distance to the actual subject,
which, like I said before, is one of the three factors that affects
the depth of field. So, in reality, the image sensor size
doesn't change the depth of field, but in practical terms,
it's just easier to get a shallow depth of field
when using a camera with a larger sensor simply because you're not cropping so much
of your image, so, in fact, you don't have to move farther away
from your subject to get the same framing.
Alright, that's it. I hope this tutorial didn't confuse you too much.
But if you have any questions,
then just leave them below in the comment box
and I'll try to answer as many as I can. Also, stay tuned for the next tutorial where
I'll be showing you how to setup up this cool little scene.
Later!