- It doesn't matter
what kind of grand, creative ideas you have,
if the part of your image
that should be in focus ain't in focus.
Now you might say,
"Isn't it also like that with exposure and composition?"
Not necessarily,
because exposure and composition
can be altered in post-production.
If an image is out of focus, you can't fix it.
Now, before you say,
"But there are sharpening tools in my image editor,"
no, sharpening tools don't restore focus.
This is why I've been so adamant
about keeping track of shutter speed
and auto focus while you're shooting.
A slow shutter speed puts you at risk
of blurring camera shake
and if you're not keeping track
of what your camera's auto focus is doing,
you won't necessarily get what you want in focus.
So having said all of that,
we're now going to look at the concept of depth of field,
which lets you ensure
that some parts of your image will be out of focus.
Why would you want that?
When you're creating an image,
your job as a photographer can be boiled down
to a single task:
to control the viewer's eye when they look at your photo.
It's a visually complicated world out there
and you are trying to capture a representation
of part of it.
Within that representation, you are trying to make certain
that the viewer looks at specific things
and sees them in a particular way.
Every technical and creative decision
that you make serves that one goal.
Now, you have a lot of tools for doing that.
You can compose in a particular way.
You can expose in a particular way.
And you can focus in a particular way.
When you're exposing, you can, for example,
plunge shadows into complete black
so that there are no distracting details.
Depth of field gives you control over focus
and the simplest example is one that we've all seen a lot:
a portrait.
Here is a portrait with deep depth of field.
That is everything in the background is sharp
and in focus as is the subject.
So you might think that this is a successful portrait
because everything is in focus and as he said,
focus is the most important thing.
Well, not necessarily.
Here's the same portrait shot with shallow depth of field.
The background is blurrier,
but my subject is still in focus
and that defocused background
is a way that I've grabbed the viewer's eye
and made sure that they look at the subject
rather than the background.
Depth of field is the final piece
of the answer to the question
of why there are two physical mechanisms
for controlling light.
Shutter speed and aperture both allow you
to control the brightness of your image.
And as you've seen,
shutter speed also allows you to control motion stopping.
Aperture, in addition to controlling brightness,
gives you control of depth of field,
and I will argue that depth of field control
is something you'll think about a lot more often
than motion stopping.
Unless you're a sports or wildlife shooter,
motion control is not something
you'll need to think about very often,
but depth of field is something you'll consider
every time you go shoot.
Why?
Because for example, in a landscape shoot,
you usually want to have very deep depth of field.
Portraits, as we've just seen,
usually have shallow depth of field.
Product shots usually want deep depth of field.
Still lives might want either.
In street shooting,
your depth of field might vary a lot.
Like shutter speed,
depth of field has its own corresponding set of numbers.
This may seem intimidating at first,
but as with shutter speed, depth of field is very simple
once you understand some fundamental concepts.
Apertures are measured in F numbers,
which have this odd progression:
f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16.
You should know
that each one of these numbers represents the size
of the circular opening,
that is the aperture in your lens.
You should also know that, from left to right,
each one of these numbers represents an aperture
that is half the size as the previous number.
In other words, each one of these apertures represents
a one-stop difference in exposure.
The tricky bit here is that larger numbers correspond
to a smaller opening,
so f/16 is half as big as f/11 or one stop smaller.
For example, if your camera chooses at 30th of a second
as a shutter speed,
a speed that might be too slow For handheld shooting,
you know that you can increase the shutter speed
by one stop to a 60th of a second,
but that's a change
that will darken your exposure by one stop.
So to compensate for that,
you can open your aperture by one stop
to let in the same amount of light
that your shutter speed change eliminated.
Those equal and opposite changes give you the same level
of overall brightness in your final exposure,
but with the advantage of us faster shutter speed
in that case.
However, changing the size of your aperture does more
than simply change the brightness of your exposure.
As shutter speed changes affect brightness
and motion stopping power,
aperture changes affect brightness and depth of field.
As your aperture gets smaller, depth of field increases.
For example, I'm going to look in this camera over here.
Notice the background.
We're on a tighter shot here,
and the background is about as sharp
as what you've been seeing in this camera over here.
But right now, we're affecting a change in that other camera
so that when I look back to it,
depth of field is much more shallow than it was before.
And what I mean by that is the background back there
is a little bit softer.
The change that we affected
is that we opened up the aperture of this camera.
Before, we had a very small aperture,
which gave us deep depth of field.
Now, we have a larger aperture,
which is getting us a much more shallow depth of field.
Again, one is not necessarily better than the other
in any sort of objective sense,
but they do have very different fields.
Let's take a look at some stills side by side.
So in the left one,
you can see I've got my smaller aperture,
which is giving me deeper depth of field.
In the right-hand image, I've got a wider aperture,
which is giving me more shallow depth of field.
You can argue that the shallower depth of field
kind of isolates foreground more
by making the background less distracting.
Here's a landscape shot with very deep depth of field.
Shooting the scene with shallow depth of field
doesn't really work in this case.
We want to see all of the details in the shot
so going shallow doesn't work as well, in my opinion.
But again, depth of field is subjective
and very often your subject matter
will drive your depth of field choices.
So now you have another factor to work with
in your exposure balancing act.
Perhaps you're shooting in a dark room
and your meter is reporting very slow shutter speeds.
You can of course increase the ISO,
but you can only take that so far
before your image becomes too noisy
so you can open up your aperture.
This will allow you to use faster shutter speed,
but it will also reduce the depth of field in your shot.
Depending on what you're shooting,
that may or may not be a problem.
If you want to shoot in low light,
have very deep depth of field, and freeze motion,
that may not be possible.
In that situation, some cameras will do better than others
because some cameras can have their ISOs pushed higher
before they produce a lot of noise,
but still, there will be situations
where, for reasons of practicality and physics,
you simply cannot get the balance that you want.
Aperture then can be summed up as follows:
An aperture is a mechanical circular opening
usually contained in your camera's lens.
It can be made bigger and smaller.
And as you make it smaller,
the depth of field in your scene increases.
Smaller apertures are represented by larger numbers.
You just need to memorize all of that.
Won't take long and soon,
the particulars of aperture will be second nature.
However, these aren't the only things
that you need to know to take control of depth of field.
Wider apertures make for shallower depth of field,
but aperture is not the only thing
that affects the depth of field in your image.
Sensor size, the distance to your subject,
and the size of the objects in the background of your image
all have a bearing on how soft the background will go.
Smaller sensors have inherently deeper depth of field
than larger sensors.
This has nothing to do with sensor technology.
It's just a fact of the physics of optics.
Smaller pieces of film
also have inherently deeper depth of field
than larger pieces of film.
Here's a portrait shot with two different-sized sensors.
I shot this with an iPhone.
It's got a tiny smartphone camera-sized sensor in it.
It has a fixed aperture
and it has a very particular focal length.
That is it has a lens of a particular length.
Here's a portrait shot from the same position
with a Canon 5D Mark IV.
It has a sensor that's the size
of a 35 millimeter piece of film that's dramatically larger
than the sensor that's inside the iPhone.
I set it to the same aperture
and I used a lens of equivalent length.
In other words, pretty much everything is equal
in these shots except for the sensor size.
And as you can see,
the iPhone produces much deeper depth of field
than the SLR does.
If you want the option to shoot
with the shallowest depth of field possible,
then you'll want to choose a camera with a bigger sensor.
In general, though, any SLR or mirrorless camera,
and even some high important shoots,
will be able to produce a nice defocused background
if you know how to properly use them.
Because of their tiny sensors,
smartphone cameras will never be able
to get that blurry background.
These days, some cell phones have a special portrait mode
which fakes the look of shallow depth of field
by digitally blurring the background.
Sometimes that works.
Sometimes it doesn't work so well.
As you get physically closer to something,
depth of field decreases.
This is true for your eyes as well as your camera.
It's also why macro lenses
have very, very, very shallow depth of field.
When you shoot with a macro lens,
you're usually extremely close to your subject.
Next to aperture, the single biggest factor
in achieving a shallow depth of field
is the size of the objects in the background of your image.
Look at these two pictures.
The backgrounds are equally defocused,
but you can't tell because in this image,
the background details are so small
that you can't see how much they've defocused.
Because of the way that you use a telephoto lens,
such a lens usually results in the background
of your image being larger in the frame,
that makes shallow depth of field more visible.
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