Originally posted by raider Say I am going to take a portrait of a person but instead of focus locking on her eyes, I am focus locking on say another object of better contrast so that the AF can lock on. Would this technique not produce a shot whereby the person's eyes are not tact sharp, something which we all wanted to achieve in a portrait?
Yes, if the thing you are focusing on is a different distance from the camera than the eyes, you will potentially have issues. This idea of substituting a different focus target is fine in situations where the *exact* focus point isn't too critical. If you re shooting at a relatively small aperture - say, f/8 - then the depth of field (DOF) is enough that if a person's shirt is focus, their eyes will be too. It's only when shooting at really wide aperture - say, f/2.8 and bigger - that the couple of inches difference in distance might matter. So I wouldb't be using this technique on portraits. Or if I did, I might expect to need to fine tune the focus manually after the AF did its job.
The technique you describe is really intended for less demanding applications that just happen to involve low-contrast subjects.