aperture/Fstop question

Naftoli

it is my understanding that aperture size is determined by focal length divided by Fstop, which means for example a 70-200 f/2.8 lens set at 2.8 will have a larger diaphragm/aperture at 200mm than at 70mm and allow in the same amount of light, this can explain why a typical kit lens has a variable aperture of 3.5-5.6 b/c when zoomed in the aperture cant expand larger to accumulate equal amount of light from its zoomed out position.
If this is so than why isnt a 70-200 f/2.8 a 1.8 - 2.8 lens since at 70mm its aperture is not its widest at 2.8?

Nikonguy

I don't really understand your question(s) but I'll see if I can help: A constant aperture lens like the 70-200 f/2.8 is able to maintain its largest f stop (in this example f/2.8) across the entire zoom range. However, that doesn't mean the effective hole size is the same through the range. The hole gets larger as you zoom from 70 to 200. It needs to so it can maintain f/2.8. Similarly, for a variable aperture zoom like the 18-105 f/3.5-5.6, the hole size also increases as you zoom from 18 to 105. However, while the hole size does increase as you zoom from 18-105, it does not increase enough to maintain f/3.5 through the range.

Naftoli

ill try an explain my question a bit better, a 70-200s aperture gets larger when zoomed to 200mm, so why cant it stay that large when zoomed out to 70?

Allen

Naftoli, there's a good explanation on pages 1 & 2 of the following article:

http://www.outdoorphotographer.com/gear/lenses/get-the-most-out-of-variable-aperture-lenses.html