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

Started Apr 17, 2011 | Discussions thread
noirdesir Forum Pro • Posts: 13,521
Re: noirdesir

edwardaneal wrote:

what about live view?

doesnt the AF system use the actual sensor when the camera is in live view?

Yes, it does. And it uses a completely different technique for autofocusing called Contrast-Detect. Essentially is just moves through the whole focussing range and when the image shows the highest contrast (at the point in the image selected to focus on) it stops. It must initially guess a direction into which it starts moving the focussing but usually relatively quickly notices whether the contrast increases or decreases (and reverses direction in the latter case).

Contrast-Detect AF is very accurate but the lower the DOF (fast lenses, macro, tele) the slower it is since if the target is out of focus it might not be able to find the right focussing direction right away and since it is limited by the speed the main image sensor can be sampled at, it cannot move the focussing too fast, otherwise it might miss the high-contrast, in-focus position and starts hunting.

if it does wouldn't faster glass have an advantage in that it would allow the sensor to get more light and also let it "see" contrast better?

Yes, more light helps, as it reduces the noise in the sensor signal but fast lenses at the same time mean lower DOF which means the contrast-detect AF has to move slower (as not to miss the focus position) and it has more trouble finding the correct focussing directing initially.

This also means that as sensor sampling speed increases with newer cameras, contrast-detect AF speed increases as well.

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