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I have come across a couple lens manufacturers that mention that when a lens is focused such that the center of the FOV is well focused, the extreme ends of the FOV will be slightly defocused.
What is the reason for this? I was of the opinion that lenses are focused for a certain distance and if the entire FOV is filled with a target at that distance, it would be equally well focused all over.
edit: after going through some more I came across the page where they measure resolution of lens. It seems like they focus at the center (which reduces the resolution at the sides since it is in relative defocus). And then they turn back the focus some so that it defocus the center a bit for larger gains in focus at the ends. Is this common practice?
edit: for instance, this is an excerpt from this page on Edmund Optics
This also can cause issues depending on whether the system is only focused in the center of the FOV or across the entire FOV; because of the resolution varying based on location, it can be difficult to determine when the entire FOV is in best focus. Some lenses obtain very high resolution in the center of the FOV, but very low resolution in the corners when the lens and camera system is focused on the center of the image. A slight defocusing of the lens can balance the resolution across the field, although usually to the detriment of the center resolution.