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

Started Apr 17, 2011 | Discussions thread
edwardaneal Veteran Member • Posts: 9,101
tell me Steve

So tell me - why does the test I posted above show that AF is more accurate between F/2.8 and F/4 and why does Canon specifically state the following

"The final advantage concerns the focusing. More light can be gathered by these lenses because of the larger maximum aperture making it easier for the autofocus sensors within the camera to find contrast even in low light or low contrast situations. Lenses with smaller maximum apertures may struggle to focus in such low light conditions as they cannot allow enough light into the autofocus sensors."

are they all lying?

also as a side note if a person were using live view where the AF system is actually using the sensor wouldnt faster glass now have an advantage?

SteveL54 wrote:

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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