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Anonymous
>>129381 mostly wrong but has a grain of truth.
Any and all lenses create an image on a spherical plane. Think of it this way: if a lens is focused to a certain distance, then the "plane" that it is focused on will actually be a sphere of radius = focal distance. Understand?
Lenses mostly appear to focus on a plane because (a) the narrow angles of view and long subject-distance:image-distance ratio make the plane you see a small segment of a very large sphere, and (b) lenses are corrected to some extent to try and minimize this.
Fact remains that the plane of focus is more like a bubble of focus. This is the curved field.
Of course, since what the lens is doing is creating a mirrored but identical image of what it sees, the image will also be created on a curved plane. Same reasons as above, a flat plane can approximate this. However, it's not quite true.
As a result, the center of the lens (where the focal plane is parallel to the camera back) is the sharpest part. As the film/sensor picks up the corners of the image, they will be in less ideal focus and so will be a little blurry.
SOME cameras actually do use curved film planes to account for this. The most famous example is the early Minox subminiature cameras (James Bond, CIA, KGB -- the classic open-close wind spy camera). I have one, and the film plane is visibly curved and has a little pressure plate to push the film into that shape for maximum sharpness with such a small negative.
There are also some issues with chromatic aberration that show up because of the higher distortion, but that's more advanced.
If that doesn't explain it well, let me know. I'll try to do better.
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