Theoretically, if you "focus and recompose" to keep a specific point of interest in focus - the eyes in a portrait, for example - only a curved field of focus will keep the focus where you intended. If the field of focus is flat, recomposing will cause the focus point to move behind the point of interest. The further away from the initial point of focus the camera axis moves - the greater the focus error.

Whether this effect would be discernible in an image would depend on distance to subject, depth of field, focal length, lens quality, etc., etc.

In this example the camera is initially pointed at the point of interest (along the red axis) and focused. The camera is then recomposed along the green axis. At this point, with a curved focal plane (blue), the focus at the point of interest would be "correct". With a flat focal plane (grey), the focus point would move behind the point of interest by the distance marked in yellow.

Theoretically, if you "focus and recompose" to keep a specific point of interest in focus - the eyes in a portrait, for example - only a spherical field of focus will keep the focus where you intended. If the field of focus is flat, recomposing will cause the focus point to move behind the point of interest. The further away from the initial point of focus the camera axis moves - the greater the focus error.

Whether this effect would be discernible in an image would depend on distance to subject, depth of field, focal length, lens quality, etc., etc.

In this example the camera is initially pointed at the point of interest (along the red axis) and focused. The camera is then recomposed along the green axis. At this point, with a spherical focal plane (blue), the focus at the point of interest would be "correct". With a flat focal plane (grey), the focus point would move behind the point of interest by the distance marked in yellow.

Theoretically, if you "focus and recompose" to keep a specific point of interest in focus - the eyes in a portrait, for example - only a curved field of focus will keep the focus where you intended. If the field of focus is flat, recomposing will cause the focus point to move behind the point of interest. The further away from the initial point of focus the camera axis moves - the greater the focus error.

Whether this effect would be discernible in an image would depend on distance to subject, depth of field, focal length, lens quality, etc., etc.

In this example the camera is initially pointed at the point of interest (along the red axis) and focused. The camera is then recomposed along the green axis. At this point, with a curved focal plane (blue), the focus at the point of interest would be "correct". With a flat focal plane (grey), the focus point would move behind the point of interest by the distance marked in yellow.

Theoretically, if you "focus and recompose" to keep a specific point of interest in focus - the eyes in a portrait, for example - only a curved field of focus will keep the focus where you intended. If the field of focus is flat, recomposing will cause the focus point to move behind the point of interest. The further away from the initial point of focus the camera axis moves - the greater the focus error.

Whether this effect would be discernible in an image would depend on distance to subject, depth of field, focal length, lens quality, etc., etc.

In this example the camera is initially pointed at the point of interest (along the red axis) and focused. The camera is then recomposed along the green axis. At this point, with a curved focal plane (blue), the focus at the point of interest would be "correct". With a flat focal plane (grey), the focus point would move behind the point of interest by the distance marked in yellow.