Why do cameras go in and out of focus? Why can’t everything just be in focus?
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The camera is seeing the world through a circular lens and any light reaching that circle helps form the image. The right side of the circle sees the world from a slightly different point of view than the left side. The image you get is an average from every point of view on the lens.
Focus allows you to correct for the different points of view but it only works for a small range of distances. Objects within that range will be in sharp focus.
Big lenses have the worst problem because the range of points of view is greater. Lenses can be “stopped down” to let in less light and this makes focusing less critical; you have a greater “depth of field”. Stopping down will happen automatically in bright lighting conditions, so outdoor shots on a sunny day tend to be all in focus but, in a dark scene, focus is critical and more of the image tends to be blurry.
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