Originally Posted by DHDSP
Ok , I see I forgtot to mention a couple of things:My camera is a Canon EOS 350D , the lens was the Canon 70-200L IS, the crop factor of the camera is 1,6x, resulting in +- 300 mm.
The setup was a studio setup with 4 flash heads, so I put my shutter to 1/250s to be sure. The wireless trigger could just handle this speed.
As I wanted to shoot verticals I already put the AF (yes, shooting auto focus) field to the right, so after turning the camera to a vertical position, the AF field is at the top. This way I tried to reduce the movement needed to recompose after half pressing (and so fixing the focus on the eyes) the shutter.I know I could have turned the IS off, as it probably is redundant shooting flash.
Now I still ask myself if moving the camera by , lets say 1 inch to the front or the back, would only move the plane of things being in focus also by one inch?
(of course fixing the focus, not letting the AF correct the shift of the camera)
My guess is that this depends on the lens magnification.
Right ? I mean , wouldn't the shifting (after shifting the camera) of the plane of things beeing in focus be bigger if I use a 500mm lens than using a 50 mm lens ?
I think you're overthinking the whole thing. But a few points.
1. Turn the IS off. It will actually give you poorer results (if on) under the circumstances you mention. Keep in mind that the flash will freeze the action (the shutter speed in this case is not relevant for freezing action).
2. The most sensitive AF sensor is the one in the middle. If at all possible you should use it.
3. IF you are shooting at f2.8 and you move the sensor plane 1 inch after focus lock, you have just set yourself up to have the point you locked on out of focus. What you want, is the area that you locked focus on to be the same distance from the sensor/film plane at the time you shoot. If it is less or greater than this distance, then it will no longer be in sharp focus. Period.
4. When using flash heads you are always better off to be 1 stop below the maximum sync speed. So for Canon at 1/250th, you will usually be much safer to use 1/125th. Keep in mind that in a normal studio with strobe lighting the ambient light is almost never of any concern and would not alter the photograph unless you got the shutter speed down below 1/30th in most cases. The exposure is solely determined by the fstop and the power of the flashes. Here is an example (from an outtake) of where I wanted the background lights exposed properly. I dragged the shutter (1/15th), but the fstop was based on the strobes that lit the model (Candace):
5. When shooting with studio strobe, you will usually get better results if you are in the f8 to f11 range. This will also give you better DOF and less problems with focus issues. Also, the sweet spot of most lens is more likely in that area than at 2.8.
Have you done the tests I mention earlier to determine if you have a lens or camera problem rather than just a problem in technique? Once you have verified that the lens and camera are working properly, then you can address the other issues.