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Why do some cameras limit aperture size based on zoom?

T

TommyVercetti

Diamond Member
Ok I just forgot what the name of it is, it's the aperture size, which controls the amount of light that can enter. Well anyways, I was looking at a Canon S50 today. If you zoom in, you can only select certain sizes, it limits you to 4 choices. But if you are not zooming, then you can select a whole lot more sizes. Why would the zoom control that? I thought the aperture part of the camera body, not the lens.
 
aperature is part of the lens. the maximum aperature size is constant no matter how long the lens is. the aperature is the result of the focal length divided by the f-stop. if your focal length is increasing and your aperature is constant, your f-stop also must increase
 
Originally posted by: ElFenix
aperature is part of the lens. the maximum aperature size is constant no matter how long the lens is. the aperature is the result of the focal length divided by the f-stop. if your focal length is increasing and your aperature is constant, your f-stop also must increase
Click to expand...

He meant shutter speed, not aperture.

 
??? My understanding correctly of the matter, isn't aperture is a measurement of how wide the shutter opens up to allow lights in?
 
Originally posted by: ElFenix
aperature is part of the lens. the maximum aperature size is constant no matter how long the lens is. the aperature is the result of the focal length divided by the f-stop. if your focal length is increasing and your aperature is constant, your f-stop also must increase
Click to expand...
Actually, F-stop is focal length divided by the diameter of the aperture. You have the right idea, you just mixed up the equation.

ZV
 
Originally posted by: TommyVercetti
Ok let me clear it up. This is what I am talking about

Aperture priority ? Wide: F2.8, F3.2, F3.5, F4.0, F4.5, F5.0, F5.6, F6.3, F7.1, F8.0
? Tele: F4.9, F5.6, F6.3, F7.1, F8.0

which I got from http://www.dpreview.com/reviews/canons50/page2.asp

See the 2 different sets of aperture numbers?
Click to expand...
ElFenix covered it in the first post, although he got the math mixed up. F-stop is the ratio of the focal length to the diameter of the aperture. If the aperture is a constant diameter and the focal length doubles, then the F-stop number doubles too. The aperture diameter that gives and F-stop of 2 on a 55 millimeter lens will give an F-stop of 4 on a 110 millimeter lens.

ZV

EDIT: It looks to me as though the camera's sensors cannot function properly with an F-stop greater than 8, which is why that's the upper bound, and the iris cannot close any further than a diameter that gives an F-stop of 2.8 at wide angle and an F-stop of 4.9 at telephoto.

Digitals give us some problems though since focal length is different. Many digital cameras list the lens' focal length as "35mm equivalent", or "105mm equivalent". The actual focal length of the lens is not 105mm, but it gives the same zoom as a 105 mm lens with a 35mm film camera. Make sense?
 
If you take a lens that has an opening of one inch and focuses the image at 4 inches, that lens has an F value of 4.0

If you take the same diameter lens an have it focus at 8 inches it has a focal ratio of 8.0

As you see, the longer the lens, the higher the number (like 6.3 would be for example)

Lenses have an iris like your eye which effectively can change the diameter of the lens. At a short focal length, it can be set to say 2.8. It can maintain that as you zoom out, but at a certain point, the iris cannot open further. The lens doesnt grow bigger either. You wind up with a larger F value just like in my example of the F 8.0 lens.
 
I wish I knew what you all were talking about 🙁

<- just bought a Fuji FinePix 3800 and hasn't left "Auto" mode 😱
 
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