Sigma Lens

[DSLR noob]

No the lens can't open up to 3.5 at 300mm so the aperture value of a fully opened up lens at that time is 6.3 and AF won't work. I know I contradicted myself, I thought you meant at any given aperture focal length doesn't change AF performance, which I agree with. But the aperture DOES NOT equal 3.5 at 300mm and physically can not equal 3.5, even if the same lens is able to do that at 28mm. The lens is fully open at 300mm, it doesn't stop down to 6.3 when you take a picture, therefore fully open it is at 6.3, so the camera sees 6.3 and does not autofocus. It might work in REALLY bright light but the lens, under normal conditions, won't be able to AF.

 

[Garbz]

DSLRNoob When you look through a variable aperture lens, Zoom it, Does the image get any darker? The AF module sees what comes through the pentaprism, as such AF performance will not change over the zoom range.

Even assuming that f/6.3 is the only light that comes through that still doesn't affect much other than low-light focusing where the camera's builtin light will not be able to illuminate a dark scene enough. In the daylight AF will still work just fine.

 

[DSLR noob]

DSLRNoob When you look through a variable aperture lens, Zoom it, Does the image get any darker? The AF module sees what comes through the pentaprism, as such AF performance will not change over the zoom range.

Even assuming that f/6.3 is the only light that comes through that still doesn't affect much other than low-light focusing where the camera's builtin light will not be able to illuminate a dark scene enough. In the daylight AF will still work just fine.

Well how come the camera can't let the lens operate wide open when zoomed in? what makes variable aperture....... variate? I'm confused. Sorry if I've been given false info (the point he made about zooming not making anything darker sealed the deal that I was wrong). I appologize to you EOS JD.

 

[EOS_JD]

Well how come the camera can't let the lens operate wide open when zoomed in? what makes variable aperture....... variate? I'm confused. Sorry if I've been given false info (the point he made about zooming not making anything darker sealed the deal that I was wrong). I appologize to you EOS JD.

No worries I'm not really sure of the technical aspects of a variable aperture lens ( I think I know but can't describe it too well) but so long as the max aperture is greater than f5.6 for a non 1 series camera (f8 for a 1 series camera) the AF will function properly - if slowly for such a small aperture lens. The max aperture of this lens is f3.5. Even though it's f6.3 at the long end it's still the max aperture of the lens that counts as you now know).

Do a google search on variable aperture and I'm sure that will explain all.