how does a camera know when to refocus?
In: Technology
There are two main ways, very well explained in this video by [Computerphile](https://www.youtube.com/watch?v=B-TOUPXytw4)
The cheap method is by slightly varying the focus and seeing how sharp the image is. The sharper the image, the more pixels will be different from their neighbors. Then it’s a matter of adjusting the focus until that difference is maximized.
The more expensive method uses phase detection. You check two different paths of light along a strip of pixels. If the camera is focused, the two paths produce images that are identical. If the camera isn’t in focus, the images are offset. The camera can measure the difference between those two images and tell both which way and how much to refocus, so it’s a lot faster than the first method.
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