autofocusing issue with my Canon 300mm f/4 lens
Feb 17, 2015
Hello,
I have an autofocusing issue with my Canon 300mm f/4 lens that I just can't seem to get resolved. In live view (Canon 5DIII), the resulting sharpness depends on how the lens was defocused prior to AF. This is not (that is, no longer) a problem using quick mode or the view finder. In live view, back (toward infinity) defocusing produces a poor result, while all other modes are OK. The following images were captured raw, then cropped around the target and converted to jpg in DPP4 (sharpening turned off, all other default settings). I took 5 shots in each mode (each of the 5 were about the same) and posted a representative one:
Live view, back defocus: http://1drv.ms/1CxKj5M
Live view, front defocus: http://1drv.ms/1CxKu0Z
Quick mode, back defocus: http://1drv.ms/1CxKw90
Quick mode, front defocus: http://1drv.ms/1CxKCxi
Is this sort of asymmetry expected? This is a repeatable result. Have you seen this with your own equipment?
Camera/Lens settings:
Central Sensor AF point (no surrounding points enabled)
Manual Mode, ISO 100, 1/125, f/4
Target at 33ft from target (33 focal lengths)
Live View set to stills
Focus beep on
Single frame advance
Image stabilization off
Remote shutter release
Mirror lockup for view finder
Raw mode image
tripod mounted lens
Background:
Making a long story short, I've never been very pleased with the results of this particular instance of this lens for my wildlife photography. Granted, there are many variables (me being one of them), but the autofocus was just never that great (I always shoot through the view finder, by the way, and only occasionally using live view). After some searching around, I decided to microadjust the lens. After much experimentation with different approaches, I found that microadjustment indeed was needed, but different values were needed depending on the direction of lens defocus. I noticed an asymmetry in live view as well. I borrowed a friend's 5DIII and mounted my 300mm and got the same result. I rented another 300mm f/4 -- no problem with that one mounted on my camera. So, I sent the lens to Canon (Irvine, CA) for repair. The autofocus system was deemed "broken" and replaced. Upon return, I again tested the lens using the same setup. The asymmetry was gone from phase detect AF (microadjustment was needed), but the contrast detect AF was still an issue. Again it went back to the same repair shop along with the body this time. Adjustments were made to the lens/camera so that no microadjustment was required. But the contrast detect AF issue was still present. Again, the camera/lens pair were sent in for repair (again, with very detailed repro instructions and raw images on CD along with references to previous repairs), this time to Canon in Newport News, VA. Again they found that the AF system was "broken" and replaced it. However, the contrast detect AF was still not fixed. BTW, this all started 9/15/2014 (warranty ended Dec 4, 2014). The last repair was received from Canon on 2/13/2015. I'm not sure what my next moved will be...
Thanks for reading and I would appreciate any suggestions you may have to help me resolve this problem!
Jeff