If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
SteveL54 Senior Member • Posts: 2,701
Re: And Wikipedia is wrong in this regard

edwardaneal wrote:

you are acting as if opening up the apreture only effects the brightnes in some outer band outside some mythical "f/5.6 ring" as you call it and your theory assumes that the phase detect sensors cant see the area outside this "ring"

but as you well know opening up the apreture increases the brightness equally across the entire frame.

when the projected image is brighter it is easier to detect contrast and easier for the AF system to do its job. And yes, even phase detect AF systems rely on being able to see contrast to focus properly

You assume the AF module captures light from the main lens like the camera's main imaging sensor. It does not.

What you describe happens only at the lens focal plane where the main sensor would be. Look at the Wikipedia diagram. The AF apertures and condenser lenses are well beyond the focal plane. Most of the rays from that lens have already spread out and missed the AF lenses which are restricted by their own internal aperture.


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