how inaccurate/unreliable is phase detect AF?

Started Jun 8, 2012 | Discussions thread
Roland Karlsson
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Re: In all, I think hybrid is the way to go
In reply to awaldram, Jun 17, 2012

awaldram wrote:

How can you define an edge with less than 2 pixels.??

The contrast detection does not need to use edges. Moreover .. even if the did ... (more about that below).

If you can I'll have to chuck all my Digital theories out the windows, Which will be interesting

Push ... and out i go

2x the highest frequency required was given by Nyquist as the maximum number of code elements per second that could be unambiguously resolved

A contrast detection algorithm does not have to reconstruct the image. So - Nyquist criteria is not relevant.

In the case of defining an edge this has to be 2 pixels.

See above.

I have to confess - I dont know how they do it. Maybe they use edges. Maybe you are right that the algorithms they are using are so simplistic as you assume.

I know some about image analysis though.

Lets take the edge detection algorithm first. The edge is blurry if it is out of focus. If you do focus - the edge get sharper. If you have an AA filter, it will never get 100% sharp. But the contrast will be higher and higher if the image get sharper. And at some focussing the contrast has a maximum. There is nothing I can think of that do limit that to 2x the pixel size. The measure will be noisy, so the limit might be much more than 2x pixel size.

But .... there are other ways to measure contrast. You can e.g. measure on a slanted edge. That will guarantee that you can measure sub pixel accuracy.

And you can be even more advanced. You can measure a total contrast index for e.g. 32x32 pixels. And then optimize that value. Not looking at edges at all. I see no reason why this contrast index should have any limit at 2x pixel size. The AA filter and the Bayer pattern will make it difficult. But - no theoretical limit.

One problem with all AF implementations is that you need an area to get an accurate focus lock. You cant just use e.g. 3x3 pixels. You might need at least 8x8. This might make it hard to choose focus point.

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