Aperture (lens) question

Giphsub

This may have an obvious answer, but can someone explain to me why the maximum usable aperture gets smaller (not the number, the actual opening size) when you zoom on something? The obvious logic is that more light is necessary, right? Am I off the mark there? Is there some kind of physical/engineering reason for this necessity to close it down a stop or two?

pathfinder

As you zoom out to longer focal lengths, the physical diameter of the front lens element must be much larger for the same fstop ( aperture). So if your lens zooms out longer, but does not increase in diameter ( and it doesn't of course) then the effective aperture is going to get smaller.

Maximum Aperture is DIRECTLY related to the diameter of the front element of a lens.

I wrote about the Sigma 120-300 here

My Canon 300mm f2.8 IS L measures EXACTLY 107mm with a metal ruler across the front lens element.

300mm/ 107mm = 2.803 or an fstop of 2.8

As I said, there is a direct relation between the aperture, the focal length, and the diameter of the front lens element......

Giphsub

Ah, that makes perfect sense! Thanks Pathfinder!

Scott_Quier

And, of course, the spanner in the works is when you look at constant aperture zooms. There you have a constant objective lens, but the focal length gets longer. So, there's got to be something going on in the lens as you zoom from from focal length to another that has an impact on the effective aperture.

ziggy53

Scott_Quier wrote:

And, of course, the spanner in the works is when you look at constant aperture zooms. There you have a constant objective lens, but the focal length gets longer. So, there's got to be something going on in the lens as you zoom from from focal length to another that has an impact on the effective aperture.

http://www.dgrin.com/showpost.php?p=523730&postcount=2

Giphsub

I dunno if I want to even think about that! :lol

Joves

Ah yes but constants are nice. A tad pricey but, nice.