how can the NEX auto-focus miss these shots?

Started 10 months ago | Discussions thread
mbietenholz
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Re: how can the NEX auto-focus miss these shots?
In reply to OpticsEngineer, 10 months ago

OpticsEngineer wrote:

In autofocus modes, I have figured out that my NEX7 focuses on the areas that are the brightest.

People always say the AF picks the areas with the most contrast. But in my experience, that just is not correct.

In the picture with the blurry face, if you look in the left hand side you see a brighter area of foliage that is in focus. Just as an example.

I don't think that is what is happening here - the face itself is probably at least as bright as that area of foliage.  In this picture, the camera clearly focussed on the background, and I suspect it is because the background (including that area of bright foliage) is higher contrast, and because there is more of it, than the face.

You can only measure contrast over some area in the image - ie. you need many pixels, there's no contrast in a single pixel .  The most obvious choice would be just to maximize the contrast over the whole picture, which would probably get you a
 a result close to the blurry-face picture, background in focus because there is much more of it and it is higher contrast.

The NEX cameras likely do something a little more sophisticated than just maximizing the contrast over the whole image, and try to divide the image into different areas, and then decide which one to maximize the contrast on.

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