Does my aperture setting affect autofocus performance?

[sereze]

Regarding the same camera, I know for low light conditions AF can be less accurate. Is it wise to get my aperture wide open first, focus and then change the aperture value to desired one. Or have camera manufacturers already thought about that, and the camera automatically does that for me?

[McQ]

An SLR camera performs its autofocus using the maximum lens aperture -- regardless of the aperture setting you have set for the exposure. The aperture setting will therefore not affect your camera's autofocus performance. Similarly, when looking through the viewfinder, you are always seeing the image at its maximum aperture (unless you use the depth of field preview button).

On the other hand, with "Live View" cameras that show the image on the back LCD screen, you can certainly achieve more accurate manual focusing if you perform this when the lens is set to its maximum aperture. This is just because it will exaggerate the differences and make it easier for you to distinguish where the plane of focus is located.

[William W]

You mentioned on another thread you have a 450D and the kit lens . . . as already mentioned, this camera will auto focus with the lens at the maximum attainable aperture: the designers thought of that, quite a while ago, with SLR (film) cameras.

(Generally) with all SLR cameras and lenses made for them, the aperture does not close down until the shutter button is executed, then, after the aperture is stopped down (lens's iris is closed down, to be precise), the shutter curtains, open to allow exposure.

So when you look through the viewfinder you are always "seeing" the aperture of the lens wide open - that is, unless you have the DoF preview depressed (Depth of Field). I have forgotten whether the EOS450 has a DoF Preview.

As you mentioned AF accuracy . . . this is another reason why fast lenses are sought by many Photographers – because of superior AF accuracy.

Also note that whilst your statement is correct:

“I know for low light conditions AF can be less accurate.”

It is because contrast differences are less severe in low light, that the AF is likely to be less efficient, not because of the low light, per se.

AF can be problematic in a bright scene, if there is very little contrast.

Whilst we are on apertures, (and focus), note that when you at 17mm with your lens you have a max aperture of F3.5, but at 55mm you have only F5.6

Roughly the max. Av you have, across the zoom range are:

17 – F3.5

24 – F4.0

28 – F4.5

32 – F5.0

38 – F5.6

WW

[Colin Southern]

On the other hand, with "Live View" cameras that show the image on the back LCD screen, you can certainly achieve more accurate manual focusing if you perform this when the lens is set to its maximum aperture. This is just because it will exaggerate the differences and make it easier for you to distinguish where the plane of focus is located.

When live view SLRs started to appear from Canon and Nikon many "traditionalists" weren't very receptive to the idea of using it - however, as a low-light / manual focus shooter I find it absolutely heaven sent - especially when one can magnify a portion of the scene 5x of 10x. It's especially effective where lights are involved (focus until they're at their smallest, and "job done") (for that kind of shot).

For what it's worth, I messed up the most dramatic cloud formation at sunrise that I'd ever seen due to relying on AF - I used to rely on the AF to focus the lens initially at which point I'd switch it to manual and leave it set; AF got it wrong and thus every single shot was OOF.