confusion

[d4170n]

I was reading an article on f stops. it said that aperture f stops double and halve just like stops in iso and shutter, but they double and halve in area.
this all makes since until i thought about zoom lenses and stuff.
here is the article.

http://www.uscoles.com/fstop.htm

can someone explain this to me any better?

its about the 6th paragraph down in the article

 

[oftheherd]

Just what is it that doesn't make sense? You have to understand that first of all, zooms are a specialty lens. However, by whatever means they change focal length, that happens. Then if there isn't special compensation, the aperture will change as the focal length changes. Actually, I am not saying that right. If the size of the hole doesn't change but the focal length does, then the aperture will be different for different focal lengths.

By the way, I think I remember reading that the first changable apertures were pieces of metal with holes in them, that therefore "stopped" the light. The different sizes "stopped" different amounts of light. As in Waterhouse stops. Stops were not at first standarized, but later were.

Am I wrong in that?

 

[Keith]

I like the way this guy explains things ... Fred's site and information not to mention his exposure charts were a revelation for me a couple of years ago. 🙂

http://www.fredparker.com/ultexp1.htm

 

[d4170n]

the only thing i dont understand is if the focal length changes, and the aperture doesnt, then does that make the light meter completely messed up in a manual camera that does not know when the focal length changes?
if it does, is it a significant amount? like a full stop or so?

 

[pvdhaar]

the only thing i dont understand is if the focal length changes, and the aperture doesnt, then does that make the light meter completely messed up in a manual camera that does not know when the focal length changes?
if it does, is it a significant amount? like a full stop or so?

How much the aperture changes when focal length changes depends on the design of the zoom lens. It's often marked either on the lens barrel, and listed in the manual for a number of focal length-aperture combinations. Some zooms go from f3.5 to f6.7, which is almost 2 stops! So you constantly need to look up what happens.

If you're also shooting with flash, this variable aperture behaviour becomes a nightmare, because you not only have to adjust the shutter speed to compensate for ambient exposure, you also have to adjust the settings on the flash to compensate for fill.

This is the very reason why I like constant aperture zooms. You may have to pay a price premium over variable aperture zooms and their zoom range is not as large (typically 2x-3x, and not 5x-10x or so), but it frees you of so many headaches..

 

[drewbarb]

There are a few different issues at work here. First is the concept of variable maximum aperture, which some zooms have, and some don't. Then there's the fact that when changing focal lengths, or zooming, the aperture will change in one of two ways; either the actual opening changes size, to match a given f-stop with the changing focal length, or the opening doesn't change, and you get a different effective f-stop from the same opening, based on the FL you are zoomed to. With auto-exposure of any kind, the camera should compensate to provide correct exposure. With manual gear, you may or may not need to re-meter after you have zoomed, depending upon the design of the lens. Most lenses these days will stop down to the indicated f-stop, regardless of focal length selected.

With variable maximum aperture lenses, as you zoom to longer lengths, the wide open aperture remains physically the same size, but becomes a relatively smaller opening and f-stop. For instance, a 24-85 f3.5-5.6 zoom gives f3.5 wide open at 24mm FL; when you zoom to 85mm, without touching the aperture ring, the same wide-open setting now gives f5.6. If you set this lens to 50mm, the wide open aperture is probably around f4. Since aperture is relative to focal length, as you change length and leave the physical size of the opening the same, the effective aperture changes.

Now, if you are using manual flash and working wide open with a variable aperture zoom, you may have some trouble. But if you set this same lens to, say, f8, if should be the same f8 at all focal lengths- that is, different physical size, giving f8 at that length. The mechanism in the lens will adjust to the correct opening to give f8 at the selected focal length. Manual flash exposure becomes easy.

Constant mazimum aperture zooms are really nice, since you don't lose any speed as you zoom, but they are very expensive, and larger and heavier. By and large, these are one of the major differences between most "professional" zooms and many "consumer" zooms. The good news is that (except for camera/lens combinations that only offer stop-down metering) any TTL meter- on auto settings or manual setting- will give correct exposure when using zooms, regardless of how they are set. So don't worry.

 

[Chris101]

the only thing i dont understand is if the focal length changes, and the aperture doesnt, then does that make the light meter completely messed up in a manual camera that does not know when the focal length changes?
if it does, is it a significant amount? like a full stop or so?

No, the meter does not get messed up. When you zoom out, and the aperture changes to a smaller f-stop, less light will get through the lens to the meter and film. Therefore, in a fully manual camera, the meter will (correctly) indicate that you need to add more time to the exposure to compensate for less light.

 

[d4170n]

ok, i understand now. thanks a lot.
and thanks for that site link. i'm not done reading it all, but i really like it so far.
i'll just stick to primes.

 

[shadowfox]

i'll just stick to primes.

Good decision! 😉