Originally posted by alvarossorio Why does the camera allow to shoot in several positions when using focus trap? Is there any way to reduce the margin of error?
With "focus trap"/"catch-in focus" your focus accuracy is only as good as the underlying AF system. The precision of the focus ring is not part of the equation, though the speed with which you move the ring (moving through the zone of focus) might be.
Simply put, even the best AF systems will miss critical focus for many subjects. This is because the AF "patch" used to establish focus may or may not correspond with the actual desired subject or may be too wide for adequate precision. Factor in limited DOF with faster lenses and/or higher magnification and your result will definitely vary. A few classic cases where AF failure is higher:
- Receding plane with limited DOF
- Semi-profile portrait (focus should be on eye, AF chooses bridge of nose or near edge of orbit)
- Any natural light macro, particularly of low contrast subject
If critical focus with manual focus lenses is essential, the best solution is to use an aftermarket focus screen. The focus aids (microprism and/or split image) make all the difference in the world. For macro work, a fine matte field is preferred and works best (may also be preferable for portrait work as well).
Steve