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Let use this SLRGEAR 24mm f/1.4 usm II MTF model: http://slrgear.com/reviews/zproducts/canon24f14l2/ff/tloader.htm
to illustrate the aperture light bending effects of your image plane to be shifted after your focal plane has been set:
1) AF sensor(s) read the default optical plane for AF. Using the above GUI to see your f1.4 optical plane at wideopen and remember your focus plane position.
2) Let say you want to shoot at f2 and use off center AF for your shot. Using the above GUI to see your f2 optical plane position. This optical plane position is your final image
3) Compare your focus point from last two optical planes (f1.4 and f2) to see why your image is in backfocus. The closer to the center the less bending effects you will get.
Who to blame now: Lens or Body...i would say the BODY. Some bodies have on-board fast lense database to offset the bending effects, but most don't and we do have a long list of canon fast lenses from 14 to 50mm.
I thought traditional focus shift was an optical phenomenon whereby you focus once (you can use center point), do not refocus for any subsequent shots, but just stop down and continue to shoot, you will see depth-of-field increase, but also shift. In some instances, I think dependent on distance to subject, the shift could cause your intended target to be outside the plane of sharp focus.
What you said ...works wells for an old film system where focus system and lens aperture can be controlled separately by the user. Our DSLR does not work that way. DLSR AF system performs it function while lens at wide open and our image sensor recording image right after the aperture stopping down. Below is a sample to illustrate the camera and lens sequence. Please, note the black square as our focus point.
Our AF focal plane stay the same where image focal plane has been flattered out by lens stepping down to f2.8. This DSLR focus shift phenomenon is a bit different than older film system
Edited on Nov 30, 2012 at 06:01 AM · View previous versions