I always see the reflection on the camera sensor having a little chamfer:
But the plane in front of the sensor does not have this chamfer, the reflection should be a right angle if it's a mirror:
Where does this chamfer come from?
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The camera lens projects an image of the outside world on the surface of the light sensitive sensor (or film). For aesthetics, we want this image recorded with a perfectly straight boarder with perfect corners that meet as right angles. Same is true for negative carriers used in an enlarger. From a machining standpoint, we need a knife edge to make this happen. The campher is a neat way to make a knife edge.