Today we're doing the depth of field video,
the one that I've alluded to a couple times before,
and the one that I hope will be
a once-and-for-all explanation that removes any confusion
and debate on the subject.
Let's get Undone.
[offbeat music]
♪ Gerald Undone ♪
♪ He's crazy ♪
What is happening, everybody? I'm Gerald Undone,
and today we're talking about depth of field.
Now, I'm making this video for the same reasons that I made
the previous video on f-stop and apertures,
because there's a lot of misinformation
about this topic and because I know you guys like this stuff.
But more importantly with this topic, I think that this whole
depth of field conversation and the misunderstandings
that go along with it have been
negatively impacting many people's opinions
on their own gear and needs as a photographer and videographer.
And ultimately the most important reason to learn
anything about creative technology is to use it
to your advantage when it comes time to create.
So, first up, what is depth of field, or DoF?
Well, depth of field is a range of distances around
a specific plane of focus that are also acceptably in focus.
Now, we say acceptably in focus because only that specific plane
is truly in focus, but the range around it that is sharp enough
to seem reasonably in focus or where you can't really tell
the difference is what comprises your depth of field.
And this range is formed in front and behind of the subject.
And if you have a lot of it, your field is deep,
and if you don't have a lot of it, your field is shallow.
Also, I'm gonna be talking about the entrance pupil
and the diameter of the entrance pupil a lot in this video.
And when I do, I'm referring to the aperture
as it's seen when looking through the front of the lens.
If this is confusing or doesn't mean much to you,
I highly recommend you go and watch my previous video before
continuing because it thoroughly explains how the diameter
of the entrance pupil is more important than the diameter
of the aperture when it comes to calculations like these.
So, if you wanna pause this video here
and go check that one out, it's no problem at all.
I'll just entertain the rest of the viewers
with magic tricks while we wait for you to get back.
[lively jazz music]
♪
Okay, so now that we have our footing with the terms,
let's move on to the misconceptions
about depth of field. So, in short, despite anything else
you might have heard, there are only two things
that directly affect your depth of field.
The first is your distance to the subject,
and the second is the diameter of your entrance pupil.
Now, I'm putting the emphasis on "directly" here
because that's where most of the confusion comes from.
I mean, you might even have first-hand experience
that tells you that when you change focal lengths
your depth of field appears to be different.
And in fact, in the past, I've even included that
in previous videos in order to keep the explanation simple
because it's not exactly incorrect if you say that your
focal length is "indirectly" affecting your depth of field.
And this also goes for sensor size,
which is another huge item of debate that people are always
saying that your sensor size affects your depth of field.
But again, it doesn't directly affect
your depth of field, only indirectly.
So, by that admission, yes, there are also three other
indirect factors that affect depth of field, which of course
are your f-stop, your focal length, and your sensor size.
There's also some bizarre tertiary factors
that can affect depth of field, but there's very little point
in spending any time on them in this video
because they are functionally unimportant.
Okay, so let's give a quick explanation as to why
the two direct factors affect depth of field
so that the other indirect reasons can fall into place.
We're talking about distance to subject
and the diameter of the entrance pupil.
Now, the illustration that I like to use to explain
how this works is a classic of optics, the crossing lines.
Light rays need to be converged in order
to be brought into focus, and the point where they meet
is where the point of true focus is that
we were talking about earlier, that plane of focus.
And the zone where the lines aren't too far apart,
that's our range of acceptable focus, or our depth of field.
And this is a perfect time to answer Steve's question
from the comments of the last video where he asked
what exactly focal length is, because it's basically
the distance that it takes for this process to happen.
So, if a lens takes 50mm for parallel rays to be converged
into a single point, and you measure the distance from
when the rays are first bent until they actually converge,
and that distance is 50mm, you have a 50mm lens.
That's why people say a 50mm lens is a 50mm lens
whether it's on full frame, Micro Four Thirds,
or APS-C, because it doesn't change based on
the sensor size of the camera, it's based on how long
it takes that lens to converge those rays.
What's actually changing between those lenses
is their angle of view, not their converging power.
And if you want to know more about angle of view
and crop factor, check out my video on that, which I'll put up
at the end screen of this video
and also in the description below.
Anyway, good question, Steve.
Alright, so let's use this illustration
to explain how depth of field can change.
Now, we're gonna oversimplify the optics here
and just focus on the convergence of rays.
But if you picture the rays coming into our lens
after perfectly converging on our subject,
we'll be able to establish our depth of field.
Now, this illustration is a bit wonky because
this convergence actually occurs inside the lens,
but I assure you the practicality of this explanation
is the same. Now, let's say that for this imaginary lens
and distance combination that our depth of field is here.
This means that any time the rays are this close together,
they'll be acceptably in focus because we can't tell
the difference in sharpness between the rays
that are this close together and this close together,
or this close together.
They all look equally sharp to us.
But if they're this far apart, we can tell the difference
and they start to look blurry.
So, the distance between these two cut-off points
of where we can and can't tell the difference,
that's our depth of field.
Well, now we can manipulate this illustration to show
how the distance to the subject affects our depth of field,
because if we get closer to the subject,
the rays converge more sharply, giving us a shorter range
where the rays are that same closeness together.
So, remember, when they're that close together, that's when
they're acceptably in focus, and when they're this far apart,
it's blurry and we can tell the difference.
So, when the rays converge more sharply,
they're closer together for less distance.
That range-- the cut-off points are now closer together.
That's why when the subject is closer to the lens or the lens
closer to the subject, you have a more shallow depth of field.
And the same is true if we increase or decrease
the size of our entrance pupil.
When the size of our entrance pupil gets wider, then the rays
again will converge more sharply, which is gonna give us
a shorter range where they're actually close together,
and so our depth of field is gonna be more shallow.
But if our entrance pupil is much smaller,
then the rays are gonna converge much more easily,
giving us a much greater distance between
our cut-off points and a much deeper depth of field.
Now, these two methods are the only true ways
of affecting your depth of field.
Everything else is just a secondary method
of affecting those two primary factors.
An easy example of one of these secondary factors would be
your aperture or your f-stop, because like I explained
in the previous video, when your aperture gets larger,
the diameter of your entrance pupil gets larger.
And when your entrance pupil is larger, the rays converge
more sharply, thus giving you a more shallow depth of field.
But now, what about sensor size?
Well, sensor size really has no impact on your depth of field.
I mean, how could it? It's just sort of the thing
that captures the information. It doesn't change
your distance to the subject or anything.
But it may indirectly cause you to get closer or farther away
from your subject because of a thing called crop factor.
In order to achieve the same framing on a Micro Four Thirds
camera as you would on a full frame camera, if we're using
the same focal length, you'll have to get twice as far away,
because a Micro Four Thirds Camera has a two times crop.
And as we know from the illustration, the further
you are from your subject, the more easily the rays
converge, and thus the deeper your depth of field.
So, the only way to compensate and keep the framing
the same is to use a lens with half the focal length.
But if you do that, you'll still have a deeper depth of field.
But if you think about the rest of the compensation that takes
place-- because in order to achieve the same depth of field,
now that we have our framing the same using half of the focal
length, we also have to divide our f-stop by two.
So, if you were using a 50mm lens at 2.8 on full frame,
you'd have to use a 25mm lens at f/1.4 on Micro Four Thirds.
So, you're dividing both by two, the 50mm and the 2.8.
And if you remember from the previous video on f-stops
and apertures, you'll know that the f-number is actually just
a ratio of focal length to diameter of the entrance pupil.
So, with that, we can quickly see that all
we're really trying to do is make equal size entrance pupils
between the full frame and the Micro Four Thirds systems.
Let's do some quick math and I'll show you what I mean.
So, let's say that we were using a 50mm lens, f/4, on full frame.
Well, that means 50mm divided by four is gonna give us
the diameter of our entrance pupil,
which is 12.5, 50 divided by 4.
And as we were just talking about, if we want to achieve
the same angle of view on a Micro Four Thirds system,
we're gonna have to use a 25mm lens.
But if the 25mm lens was also set to f/4, then the diameter
of the entrance pupil is gonna be 25 divided by 4.
It's only gonna be 6.25mm, which is half the size of
the entrance pupil on that full frame lens we just described.
Which is why it's gonna be a deeper depth of field,
because that smaller entrance pupil means that the rays
are gonna converge more easily, giving us a greater distance
where the rays are close enough together that we can't tell
the difference and it's in acceptable focus.
So, in order to create the same depth of field, what do we do?
Well, we have to affect our f-stop by the same crop factor,
which means dividing it by 2 as well.
So, we go from f/4 down to f/2.
And then if we apply the same formula, 25mm this time,
divided by 2 for the f-stop of 2, we're gonna have an entrance
pupil diameter of 12.5mm, the same diameter as it was on
the 50mm f/4 lens that we were using on the full frame camera.
So, really all these settings are trying to do
is create equal diameters of entrance pupil.
So, it's not the aperture, or the focal length,
or the sensor size that affects depth of field,
but it's the combination of these things that require you
to compensate in order to either create
an equal entrance pupil diameter, or in order to get
the same distance from the subject
to achieve that depth of field.
Now, the last two details I want to cover when it comes
to focal length is the bad methodology out there
that leads to silly results.
In example one, you'll see the tester
standing in a fixed position, taking a photo of a subject
that's also in a fixed position, and then they'll switch
to a wider lens and compare, and they'll say that the depth
of field is shallower on their longer lens.
Without changing your aperture, you can actually create
that shot with the same depth of field
just by framing the subject the same way.
But that's why those tests are silly, because the tester
always stands in the same position and switches lenses.
But that's-- that doesn't make any sense.
The-- the shot isn't the same, so to achieve the same shot,
all they would have to do is get closer.
If they get close enough that the subject occupies
the same amount of the frame as they did when they were
standing further away and using a longer lens,
the depth of field would be exactly the same.
And these equivalences is what proves that
it's not focal length that affects your depth of field,
but how focal length indirectly either affects
the size of your entrance pupil, or makes you have to get closer
or further away from your subject:
the only two true things that affect your depth of field.
Now, the second focal length myth is about the background.
Whenever you use a lens with a longer focal length,
those background bits always seem larger and more creamier,
which makes people think that their focal length
is what's affecting their depth of field.
But again, this is just an illusion
because longer lenses make things appear closer,
including the blurry bits in the background.
Now, I'm not suggesting there's no difference between focal
lengths, because longer focal lengths can definitely give you
a more pleasing bokeh or better separation from the background,
which can be sought after when it comes to portraits.
But if we took that same photo with a wider lens and then just
zoomed in on the background, we would see that those
background bits are actually equally sharp or equally
out of focus regardless of what focal length you were using.
Which means that the depth of field wasn't altered,
just the magnification of the out-of-focus parts.
Now, there's a great resource you can use
both in the browser and also as an app for your phone
or your tablet, and it's called DoF Simulator.
And basically what it lets you do is put in your sensor size,
your focal length, your f-stop,
and your distance to the subject,
and it'll calculate your depth of field for you.
Now, this is great for figuring out what lens and aperture
combination you might want to use and at what distance
to avoid those unfortunate situations where you've got,
like, one eye in focus and the other eye out of focus, etc.
But you can also use it to test the things
that I was telling you about in this video.
So, for example, I said that a wider lens
would have an equal depth of field if the subject took up
the same size in the frame. So, let's test that.
So, here we've got a full frame sensor, and let's set it
to 50mm, because I think that's what I said in the test.
And we'll set it to an aperture of 2.8,
and let's set our subject to be 2m away, or 200cm.
And then we can see over here-- and we can also change,
uh, the background and stuff like that. So, if I wanted
to change it from Paris to a tree, there we go.
So, now we can get a rough idea of what
our image would look like.
This is our subject, and then this is our background.
And then we can see down here what our depth of field is.
It's 25.4cm, and then over here it says "In front
of the subject is 11.9cm, and behind the subject is 13.5."
So, it's a total amount that's gonna be acceptably in focus--
is gonna be 25cm, with a certain amount--
so, say you were focusing on the eye.
Then you're gonna be able to get 13.5cm behind the eye in focus,
and 11.9cm in front of the eye in focus.
That's your depth of field.
So, now let's change it to a 25mm lens.
So, we can just click "Add" down here,
which is gonna save the settings we just had.
And then we'll change this to 25mm,
but we'll leave the f-stop the same, but we can see clearly
that the subject is much further away now.
Now, all I said was that if we make the subject the same size,
which is gonna be that-- we're gonna have to go down to 1m,
because if the focal length is half the size,
we're gonna have to get twice as close, right?
So, at 1m away, 25mm, still at f/2.8, we can take
a look at our depth of field, which is coming in at 25.7cm.
Now, we'll add this one as well so that we can switch
back and forth quickly, and we can see that
the subject is taking up the same size in the frame.
We do notice, however, that the tree in the background
is getting closer and farther away, because like--
like I was saying before, that's a magnification.
The-- everything's getting closer
when you use a longer lens. But if we look
at the depth of field, it's 25.7, and 25.4.
Now, I realize that it's not precisely the same,
but like I said, there's small tertiary
differences that can also affect depth of field,
but for practical applications, that's the same depth of field.
And all we had to do was get a little bit closer.
Now, the other thing that I said was that if we would've just
zoomed in on the background, that the out-of-focus parts
would actually have been equally sharp, just some are closer.
Now, if we look at the 25mm here and we see how blurry
if you wanna call it, the tree bits are,
and then we look at those same tree bits here,
they're actually the same amount out of focus.
The tree is just closer in the 50mm lens.
So, in summary, does aperture, focal length,
and sensor size affect your depth of field?
Yes. But only because they either change the size
of the entrance pupil, or require you to change
your distance to your subject, the only true two factors
that actually change your depth of field.
Anyway, that's gonna be it for me. I hope you found
this video helpful or at least entertaining.
And if you did, make sure you leave it the old thumbs-up
and consider subscribing if you haven't already.
But if you did not find this video helpful or entertaining,
feel free to hit the dislike button twice.
Alright, I'm done.
♪