Originally Posted by clavo
When you use the lock focus, recompose technique you often have the results that you are seeing. That is because you have changed the distance from the sensor/film plane between when you focused and locked and when you shot. If you are going to use the focus lock/recompose method, then you must compensate for the change in distance before you shoot.
So how do i compensate for the difference?
First I would run some tests. Shoot the shot with the model's eye right in the center of your frame. Focus on that eye and shoot without recomposing. Now shoot again but start with your model off center. Lock focus on the eye and then recompose toward the center. If the eye in the first shot is sharp (after post processing) but not sharp in the second (after post processing), then you know for sure it is your technique and not the hardware.
Ways to compensate:
1. Try a different fstop. For example if you're now shooting at f5.6 try f11 or f16. This gives you more DOF.
2. Compensate by moving the camera slightly forward or backward. You can only learn from experience how much movement is needed. Obviously, your goal is to get the sensor plane of the camera the same distance from the eye you focused on. So you lock focus and you recompose. You should be able to tell just from "feel" about how much the distance has increased.
3. Make sure when you do the recompose that it is not so radical a movement that the new distance is excessive. So you can often combine 1 and 3 to get sharper eyes.
4. An even easier method is often just to use one of the other focal points that the camera offers for focus lock. For example my camera has 9 focus points. Just compose the photo, and then lock focus on the eye with the point closest to the eye (slight movement of composition may be needed).
Finally, it is sort of about the feel of the shot. I find my self just changing distance by a slight lean forward or backward. If on a tripod, then just a tiny scoot forward is all that is needed.