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Based on a quick perusal of contrast detection descriptions on the internet, I understand that the camera computer measures the gradient of the intensity in that region and then changes the focus, in order to reach at a local maxima.

Now all this is good for a point source of light at the centerline of the lens and that's what is used in most of the descriptions. But natural scenes are never like that - therefore I would like to know how this method works most of the time? And if it doesn't, then what are those cases?

My concern is that I would assume, since there's light usually being reflected into the lens from a large number of places, that even an unfocused image, could cause a peak in the intensity values just because, maybe the unfocus caused light from another region to land on the region of interest and took in intensity beyond the focused value? Is this not possible?

Thank you.

  • I think you are correct that sometimes contrast detection autofocus might not work. The same is true for phase detection autofocus. Then again, manual focus doesn't always work either. Just as manual focus sometimes means manually misfocusing, autofocus also means automatically misfocusing sometimes. All automatic camera features don't work 100%. – user50888 Jan 4 '17 at 17:28
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Contrast detection does not work based on a "...peak in the intensity values..." That is, it doesn't operate by finding the focus distance that makes the brightest things in the area of sensitivity as bright as they can be made. It works based on maximizing the intensity difference between areas very close to one another.

My concern is that I would assume, since there's light usually being reflected into the lens from a large number of places, that even an unfocused image, could cause a peak in the intensity values..."

When light sources are unfocused it causes the light to be spread more evenly over a greater area. This reduces contrast, not increases it. Essentially this is what veiling flare is - unfocused light spread over most or all of the frame.

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Contrast Detect works most of the time with great accuracy and incredible sensitivity. It actually does pretty much what you describe but the amount of contrast needed is incredibly small. Many mirrorless cameras use Contrast-Detect AF down to -4 EV of light which is comparable to the performance of Phase-Detect AF.

All the camera needs to know is if it has maximized contrast around at least one AF point. Contrast is measured and the lens is moved. The process repeats until the cameras has found the point at which contrast is maximized. Remember that scenes almost always have details and all it takes is a one bit difference to detect contrast. Shooing a blank wall or cloudless sky, any system will fail but if you are to shoot nature, you will find contrast not only in the edges of leaves for also in details within them.

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