Lens Aperture and auto focus

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Stan Kaslusky
Stan Kaslusky's picture
Lens Aperture and auto focus

I've always thought that a very bright lens, like a 1.4 didn't gain anything when you shoot in f8 or f16 etc. I recently heard that this is not true - Your camera's auto focus ,focuses at the lens' wide open aperture and then immediately responds with the aperture setting (i.e. F8) when you click the shutter. Is this correct?

Anton Largiader
Anton Largiader's picture

That is how I understand Canon does it. Obviously there are benefits even when shooting at smaller apertures, but it gets more complicated when you are using teleconverters. I remember reading a forum post when the 1Dx came out, basically stating that the previous 1D model was basically the ultimate birding rig and the justification involved crop factor, teleconverters and how certain focus grid points only work at f/4 (?) or less and using teleconverters to get the 1Dx to the same 'zoom' as before made those points no longer active. QED the previous 1D had better focusing for the same framing. Or something like that.

Anyway, the Canon teleconverter reports to the body and the corrected aperture is what is used to determine the active AF points... as I remember it.

Another complication is that some lenses have a small focus shift as the aperture changes. I think the EF50 1.0 was known for this problem. So it focuses at 1.0 and then shoots at whatever else you have set. But of course the only reason to have that lens is to shoot it at 1.0 anyway, so it should never actually be a problem. :)