Q:

Are points off-center from the lens axis out of focus?

If I take a picture of a flat surface perpendicular to the direction of the lens (i.e. the surface is facing directly at the camera), and I focus on the center, are the points off center in perfect focus, since they are further from the lens than the center point?
Alternatively, do camera lenses have a way to account for this?
If in fact the points off center are slightly out of focus, couldn't this have a significant effect on achieving focus across the entire surface, especially if the camera were close to the object?
Thanks to anyone who can shed some light on this.

A:

Theoretically, the focusing plane should be flat, so the focusing distance should be the same and all points should be in focus. This is not always true due to aberration called field curvature. This causes that the field is not always perfectly flat, but curved, some lenses with more complex corrections can even have wave shaped curvature.

