Today, I'm going to attempt to dispel some misconceptions
about what exactly an f-number is when it comes to lens speed,
and show you what it's actually measuring.
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 f-stops.
Now, I decided to make this video for two reasons.
One, because every single day I see a source from what should be
an authority on photography get this equation wrong, and two,
I know from the previous videos I've made like this that
you guys seem to like learning about how things really work.
So, let's quickly run through the evolution
of aperture knowledge that most photographers experience.
Usually, it starts with you wanting to know more
about how the elements of exposure works
and also wondering why some lenses are better than others
and you keep hearing this information on f-stops
and f-numbers. Then, you find out that the smaller
the f-number, the more light that the lens lets in,
which gives you more latitude with your exposure settings
and also gives you better focusing results.
Side note there on another misconception,
although faster lenses do often focus faster,
that's not why they're called faster lenses.
They're called fast lenses because they let more light in,
which allows you to set a faster shutter speed.
Anyway, this is where a lot of photographers stop
when it comes to learning about aperture.
They know what they need to know in order to make
gear decisions and to nail their exposure and that's enough.
They also might know at this point
that these settings also affect their depth of field,
but I'm going to make a video on depth of field later on,
so we're not going to talk about it much in this video.
Now, for those of you that are like me, this probably
wasn't enough information and you wanted to know more,
so you probably looked into, "Why is it that the light
increases when the f-number decreases, or gets smaller?"
or "What is the actual formula for this?"
and this is where a lot of sources will lead you
to an answer that, although is functionally useful,
is technically incorrect.
You'll usually hear that the f in the f-number
is for focal length, and you divide that
by the second number to get
the physical diameter of your aperture.
They'll usually provide you with an example like a 50mm lens
with an f-stop of two will have an aperture diameter of 25mm,
but if you close down the aperture to f/4,
then the diameter will be 12.5mm,
which is why it's darker
because less light is getting in the smaller hole
of 12.5mm versus 25mm.
Now, like I said, this is functionally useful
when it comes to figuring out equivalencies between lenses, or
compensating for exposure, but technically, it's not correct.
So, let's tear that explanation apart
and give you the real info.
So, first of all, an aperture doesn't open or close,
and by that I mean, an aperture is not a physical
or mechanical device. Everyone always addresses it like that,
but the aperture is just a hole,
it's the opening that the light travels through.
Now, you can make it bigger or smaller,
but the hole doesn't actually do anything, it's just air.
What actually does the work here is the diaphragm.
That's that multi-bladed iris that opens and closes to create
a larger and smaller opening, or aperture.
It's just like your eye.
Your pupil is the aperture and your iris is what
closes down and makes the pupil smaller when you shine
a bright light in there, and I know this is a bit nitpicky,
but hey, that's what this whole video is about.
Alright, next up, the entire designation,
when written out, is not the f-number.
Only the number that follows the divide sign
is the f-number, that's, like, 1.4, 2.8, 5.6,
that number is the f-number, and it can also be expressed
by the letter N, and the 'f' is for focal length.
So, the entire formula for figuring out the f-number, N,
is N equals f, for focal length, over D,
which is the diameter of the entrance pupil.
Now, you'll notice I didn't say diameter of the aperture,
because f-stop is not a measurement of aperture,
it's a measurement of the entrance pupil,
and although these numbers can be similar in some lenses,
they can be very different in others, and the same goes
for the relative aperture that's commonly written on lenses.
When it says something like 1:2.8, what that means is
for every 1mm of entrance pupil, there's 2.8mm of focal length.
Now, the entrance pupil is an easy concept,
and again, it's just like your eye.
In order to see it, all you have to do is look down
the front of the lens like this and take the lens caps off
and make sure there's enough light passing through,
and you'll be able to see the aperture down inside,
and if the diaphragm is closed enough,
you'll probably see the blades of the iris as well.
Now this image of the aperture, as seen through
the front of the lens, is the entrance pupil.
And if you turn it around and look through the other side,
you're going to be looking at the exit pupil,
when you're looking through the back of the lens,
and you might notice that they're not the same size,
even though the aperture hasn't changed.
Now, it's difficult to measure because of the front element
and because it's more of an image being displayed
to you than it is like a flat object in your hand
that you can really measure, but if you held up a ruler
and you tried to line it up, that measurement of the aperture
as you see it would be the D in our formula from before,
the diameter of the entrance pupil.
So, it's not technically the aperture, because the aperture,
deeper down in the lens, can be a different size completely.
Now, with a lens like this that's not too big,
often the sizes are very similar, and a lot of this
might be academic, but the formula really starts
to fall apart the way that the other sources teach it
when you talk about big, super telephoto lenses.
If you imagine a lens that's 300 or 400mm at f/2.8,
and you use that incorrect formula that suggests that
it's the diameter of the aperture, that would mean that
a 400mm lens at 2.8 would have an aperture of 143mm.
That's 14.3cm or 5.6 inches.
And then you have to ask, "How the hell can you fit
a 5.6 inch hole at the back of my lens?
It's bigger than my fricking camera!"
That's exactly right, and it makes no sense.
And this is why it led people to believe that maybe
that formula is for figuring out the size of the front element,
because on those big super-telephoto lenses,
the front elements are usually quite big,
so you think, "Ah, that must be what it's measuring",
but that's also not true because usually
the front element is up to 10% bigger than the entrance pupil.
So, really, all it is is the size of the aperture,
which will still be quite big on a 400mm 2.8, but magnified,
because you're looking through all that glass
when you look down the front of the lens and then you can see
a truly 143mm aperture, but it's as it's displayed
to you through the telephoto lens.
And this can easily be seen with the zoom lens,
if you have a 24-70, or a 70-200mm lens
with a constant aperture of, say, f/2.8 or f/4, and you turn
the zoom ring on the lens, you're going to see
the entrance pupil when you look through the front of the lens,
get bigger and smaller as you zoom in and out, even though the
aperture remains the same size, because it's a constant aperture
and it's been mechanically set by the diaphragm,
so it's just the magnification of the aperture that's changing
when you look down the lens and zoom in and out.
So, why is this important?
Well, it's because the size of that entrance pupil
is what determines how much light is going to be gathered
at that focal length and aperture combination,
and so at this point you might be wondering, "Well,
then shouldn't a 200mm at f/2.8 be brighter than a 70mm at 2.8,
if the pupil is bigger?" and you're right.
It actually is, but the reason why it doesn't
appear any brighter in your image or on the back
of the LCD screen is because of the cone
of targeted light decreases when the focal length increases.
Just imagine them like triangles,
and the area within the triangles is the light.
A more narrow triangle will have less area
than a wider triangle if the reach is the same, so,
in order to compensate for that, the pupil needs to be bigger
to let in an equivalent amount of additional light.
The other reason why understanding entrance pupil
is so important is because of the huge role that it plays
in depth of field, because despite
what you've been told, focal length doesn't affect
depth of field the way that you might think it does,
but we're going to get into that in the next video.
And so for now, that's going to be it for me,
but I hope you found this video helpful or at least entertaining
and if you did, make sure you leave it the old thumbs up
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feel free to hit the dislike button twice.
Alright... I'm done!