FStop Question! (I can teach some things, but I'm still learning most things)

[Dubious Drewski]

Let's say you're looking at a lens, say my kit 18-55. It's a f/3.5-5.6 lens. What is the clearest, most succinct way to describe to someone what that f/n number means?

I would describe it like so:

"f3.5 to f5.6 is a measurement of how much light is reduced before it reaches your camera. When the lens is at 18mm, the fStops are 3.5, and the physical amount of light is halved 3.5 times. At full focal length, f5.6, light is halved 5.6 times. (The reduction of light is nonlinear)"

Is this accurate?

Does the f/n number ever directly relate to the physical diameter of the aperture then? It doesn't sound like it necessarily does. Is it a combination of the focal length and the aperture then? If that's the case, then how is it possible to have a zoom lens with a fixed f/n value, like those fancy 2.8 lenses?

 

[Battou]

Yes it does directly relate to the physical diameter of the aperture, The smaller the number the larger the opening.

http://en.wikipedia.org/wiki/F-number

The most succinct discription is "The smaller the number the larger the opening.
The larger the opening the more light gets to the film plane."

 

[Mystwalker]

Are fractions in some way used to calculate f-stops? I know - 'go read that Peterson's book' I'm going to remember to order it one of these days.

I remember it as "1/1.8", "1/2.0" ... "1/1.8" is a bigger hole then "1/2.0" ... so it lets in more light. But what you said about the light reduction makes more sense.

 

[Yahoozy]

and just in case this was a matter of confusion
the f numbers listed on lenses arent fixed apertures theyre the lowest aperture the lens has
for an 18-55mm 3.5 is the lowest aperture on the 18 length and 5.6 is the lowest aperture on the 55
those fancy 2.8 lenses have their lowest aperture setting at 2.8 but thats not the only aperture
if you already knew this never mind hahah =P

 

[Helen B]

The f-number is the focal length divided by the diameter of the entrance pupil. The entrance pupil is the image if the iris (aperture) as seen from the front of the lens, it is not the iris itself*.

The image of the iris (ie the entrance pupil) is formed by the lens elements in front of the iris. As you change the focal length of a zoom you are altering the positio of the elements in front of the iris, so the size and position of the entrance pupil changes. You can see this happening if you look in the front of your lens and change the focal length.

By clever design the lens makers can arrange it so that the diameter of the entrance pupil gets altered in the same proportion as the focal length, thus holding the f-number constant.

Even with zoom lenses that don't have constant aperture the diameter of the entrance pupil changes. If it didn't the change in f-number would be in the same proportion as the change in focal length.

*Unless there are no elements in front of the iris, when the entrance pupil and the iris are the same.

Best,
Helen

PS The entrance pupil is also the 'centre of perspective' of the lens - ie the place it sees the outside world from, or the place your eye's iris would have to be to see exactly the same perspective. When you make a stitched panorama, you can rotate the camera about the entrance pupil to eliminate parallax error. This 'no-parallax' point is often referred to as the lens' "nodal point", but that is incorrect. The position of the entrance pupil is easily found by simply looking into the front of the lens.

PPS There's a similar thread, in which I go into a little more detail about constant aperture zooms, here: http://www.thephotoforum.com/forum/showthread.php?t=112784

PPPS If you want to consider the reduction in light you need to bear in mind that the f-number needs to be squared to represent the change in area of the hole in the lens. You halve the area, and the light, when you go from f/2 to f/2.8, for example. Square those numbers: you go from 4 to 8 (approximately, the f-numbers like 2.8 and 5.6 are rounded off).

 

[Mav]

Lenses with larger apertures (smaller f/number) let in more light which lets you shoot at faster speeds (to stop action), lower ISO (to reduce noise), and allows you to isolate your subject better with selective focus.

 

[RockDawg]

What's the proper way to say it? For example, if I adjust my lens from f/2.4 to f/5.6, do you say I went to a larger aperture in refernece to the number becoming larger or do you say I went to a smaller aperture because the opening became smaller?

 

[Mav]

larger number = smaller aperture. Wouldn't want to make things simple. :mrgreen:

 

[Stranger]

What's the proper way to say it? For example, if I adjust my lens from f/2.4 to f/5.6, do you say I went to a larger aperture in refernece to the number becoming larger or do you say I went to a smaller aperture because the opening became smaller?

I believe you say "i stopped down" although i could possibly have been mistaken this whole time getting them confused.

I (although i have no photography friends) would say, "im stopping down to f/11 (coming from f/2.8)

and if the opposite, i would say "im opening it up to 2.8" (coming from f/11)

 

[Helen B]

What's the proper way to say it? For example, if I adjust my lens from f/2.4 to f/5.6, do you say I went to a larger aperture in refernece to the number becoming larger or do you say I went to a smaller aperture because the opening became smaller?

Yes, it's perfectly correct to say 'smaller aperture' in that case. The aperture is smaller. The ways that aperture is written usually hint at the fact that it is a ratio:

f/2.8

also

1:2.8


1/5.6 is smaller than 1/2.8, of course.

It's also correct to say that going from an f-number of 2.8 to an f-number of 5.6 is going to a higher f-number.

Best,
Helen

 

[Dubious Drewski]

Thanks everyone, and thanks Helen! Wow, you know your stuff.

So just for clarification, with those fancy 2.8 zooms, the aperture physically expands in proportion to the varying focal length, but when I zoom my kit 18-55 it also does this...but not as well? Is that it?

So what I don't understand then is why the aperture closes at the greater focal length. The designers do this on purpose? Why can't the aperture remain at full diameter even when I zoom all the way to 55mm?

(Also, Helen, that info about the center-perspective at the iris is invaluable! Thanks for bringing that to attention. I never thought about that.)

 

[Mav]

So what I don't understand then is why the aperture closes at the greater focal length. The designers do this on purpose? Why can't the aperture remain at full diameter even when I zoom all the way to 55mm?

Because you need a whole lot more heavy and expensive precision optical/photographic grade glass to maintain a large aperture at all focal lengths.

Nikkor 18-55 f/3.5-5.6 ($100):

Nikkor 17-55 f/2.8 ($1200) not to the same scale as the 18-55:

The 17-55 has a lot more glass in it, especially considering that the 17-55 is actually probably 3 times the size/length of the little 18-55.

 

[Dubious Drewski]

Oh, fascinating. Thank you. That's good to know.

 

[Big Mike]

So what I don't understand then is why the aperture closes at the greater focal length. The designers do this on purpose? Why can't the aperture remain at full diameter even when I zoom all the way to 55mm?

Remember that the F number is a ratio between the size of the aperture and the focal length. So if you zoom out but keep the same size aperture...the F number will change.
I would think that in lenses like yours, the aperture is at it's maximum physical size at any point in the zoom...but because the focal length changes, the ratio (F number) changes.
What some people call 'Constant aperture lenses' like the F2.8 lens above...actually have an aperture that gets bigger as it zooms out (to keep the same F number/ratio).

(I guess, to be accurate, replace my 'aperture' with "diameter of the entrance pupil")

 

[Tiberius47]

The aperture is written as a fraction of the focal length of the lens.

So an f8 lens tells us that the aperture diameter is the focal length of the lens divided by 4. If it's a 100mm lens, therefore, the aperture is 25mm across.

In fact, the F in f stop means focal length. So to find the size of the aperture, divide the focal length of the lens by the f number.

Bear in mind that you can have different lenses at the same f stop with different sized aperture, because the lenses gather different amounts of light through their front elements. A wide abgle lens gathers light from more area than a telephoto, so the wide angle has more light to play with. So it's easier to make them with faster apertures. This is why you never see a 500mm f1.8. or, why they;'d be tremendously expensive.