Lenses....some Focus faster than others...

[Daniel Browning]

Why occlude the center at all?

The purpose is to get two or more different views of the same subject. My understanding is that it's more than just a central occlusion: each of the paired AF sensors gets light from an aerial image of a virtual subaperture at opposite sides of the f/2.8 annular ring. If it were designed to see more of the center, the accuracy would decrease in proportion with the decrease in baseline. If it were designed to see more of the outer aperture (e.g. f/1.4 lens), then it wouldn't work at all with f/2 or f/2.8 lenses (just as f/2.8 sensors don't work at all with f/4 or f/5.6 lenses). It only sees just that part of the lens that it's designed to (within certain tolerances... f/5.6 AF sensors will sometimes see a little of the f/8 ring, which is why they can kinda-sorta autofocus, though with a smaller baseline.)

[MikeWhy]

My understanding is that it's more than just a central occlusion: each of the paired AF sensors gets light from an aerial image of a virtual subaperture at opposite sides of the f/2.8 annular ring.
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I think I get it now. Instead of an actual ring, the paired sensors make up a portion of a virtual ring, the spacing being the same or at least analogous to the central occlusion. I'm quite sure I still don't understand why the ring or baseline distance improves accuracy (couldn't I just read adjacent pixels on the main sensor for contrast, for example?), but I already have a much clearer picture now of how the AF works. Thanks for the lucid explanations.

[neuroanatomist]

couldn't I just read adjacent pixels on the main sensor for contrast, for example?

Sure - in fact, that's exactly how contrast-detection AF works in LiveView on the newer bodies, and also how typical point-and-shoot cameras focus. It can be quite accurate - in fact, focusing a Canon dSLR in LiveView with a 10x magnification on the LCD is just about the most accurate way to focus a Canon camera. But if you've tried that, or used a P&amp;S camera, you know that relatively speaking, contrast detection AF is very slow, sometimes taking nearly a full second to achieve focus. That's one reason that drives P&amp;S users to dSLRs - it certainly was for me, too many missed shots of my fast-moving toddler!

[Daniel Browning]

You're welcome.

I still don't understand why the ring or baseline distance improves accuracy

Because the degree to which each paired AF sensor differs is directly proportional to the baseline distance. The further you get from the center of the aperture, the more that the light rays diverge. The f/5.6 AF sensors have a short baseline distance; their accuracy is 6 times worse than the f/2.8 AF sensors.

[Feanor]

I have to say that I don't understandhow the focus speeds of similar f/5.6 lenses can be so great.


My 70-200 f/2.8L IS lens focuses almost instantly. Add a 2x extender in to turn it into a 140-400 lens thoughand focusing becomes painfully slow. People have said that this is because of the reduced aperture, but my prime 400mm f/5.6 lens also focuses almost instantly - even in poor light it's very quick.


This makes no sense to me. Since people have found that the tape-on-connectors trick to prevent lens reporting to the cameraincreases focus speed, Ican only assume that Canon have slowed down this when using the extender on purpose -perhaps to encourage people to buy the proper lenses.

[neuroanatomist]

Since people have found that the tape-on-connectors trick to prevent lens reporting to the cameraincreases focus speed, Ican only assume that Canon have slowed down this when using the extender on purpose -perhaps to encourage people to buy the proper lenses.

I don't know the reason for it (I would hope there's more to it than trying to 'force' people o buy more expensive lenses), but yes, it's by design. When used with the 1.4&times; extender, autofocus speed is reduced to a half. When used with the 2&times; extender, autofocus speed is reduced to a quarter.

[neuroanatomist]

Since people have found that the tape-on-connectors trick to prevent lens reporting to the cameraincreases focus speed, Ican only assume that Canon have slowed down this when using the extender on purpose -perhaps to encourage people to buy the proper lenses.

I don't know the reason for it (I would hope there's more to it than trying to 'force' people o buy more expensive lenses), but yes, it's by design. When used with the 1.4&times; extender, autofocus speed is reduced to a half. When used with the 2&times; extender, autofocus speed is reduced to a quarter.