On Fri, Aug 16, 2013 at 10:19 AM, Aahz Maruch <[email protected]> wrote:
> That doesn't make sense. Can you explain further? If you pick your > focus point, half-press shutter (or lock focus/exposure), move the lens, > shoot -- where does the defocus come from? I've certainly shot plenty > that way. Camera lenses are designed with a flat plane of focus. Imagine you're shooting a brick wall, straight on, at a distance of 10 feet. You autofocus on the wall straight in front of you, and the camera sets the lens to 10 feet. The entire wall is now in focus, from the center to the edge of the frame. Now, suppose you turn to the left, focus on the left part of the wall, and recompose back to the same straight-on composition as above. The part of the wall you focused on is more than 10 feet away (you could use trigonometry to figure out how much). The camera sets the lens to some distance more than 10 feet (maybe 12 feet), so that the left part of the wall is in focus at the time you focus. You recompose, and the lens is set to 12 feet, but the wall is still 10 feet away at the center. No part of the wall is in focus. The problem is worst for wide-angle lenses (because you swing over a larger angle to recompose), and wide apertures (because the DoF is narrow). -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
