Re: 24-105mm FOCUS SHIFT survey - submit serial numbers
Some information after optically testing the one I have that misfocuses stopped down.
1) Optically, it is identical to the dozen I tested that do not have the problem. Actually the shifter is among the better MTFs wide open, the field curvature is no different, etc. etc. This goes against a true focus shift; you would expect some optical difference if it was a true optical effect.
2) We tested for focus shift in the following manner: Lens is mounted, set at infinity focus by the camera at f/4, and focusing distance measured to within a micron. Then we stop down to f/5.6, f/8, f5.6, and f/4, remeasuring focusing distance each time (to the plane of image focus). Repeat for 24mm, 70mm, and 105mm focal lengths.
Other 24-105s tested this way change focus plane about 0.02mm when stopped down or opened up a stop.
The shifting lens at 24mm, changed 0.02 from f/4 to f5.6, and 0.03 from f5.6 to f/8, so about like the rest.
At 70mm it changed 0.312 from f/4 to f/5.6 and another .135mm from f/5.6 to f/8.
At 105mm it changed .73mm from f/4 to f/5.6. We couldn't measure f/8. It had shifted so far that it was beyond the range of our bench (the bench is working at close distances with mirrorless systems.
My factual conclusion is there's no doubt the shifting problem occurs in some copies and not others. This is an enormous difference.
My opinion is this is not optical focus shift at all but something different. I haven't been inside one, but it has a compensating element and focusing elements which are near the aperture, perhaps even part of the aperture assembly. I suspect stopping down, either from electronic or mechanical reasons, is interfering with the focusing / compensating element spacing in some way.
Right now, I only have a single misfocusing copy and this was a test with methodology not confirmed on multiple samples. So please don't say I found the problem or know what it is. I confirmed what you guys already knew, and said some things I'm pretty sure it isn't.
Roger
Feb 06, 2018 at 01:38 PM
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