Canon 100-400L II & UV Filter

sliverstorm wrote:

Thought I'd throw my hat into the ring on this matter.

I was trying to diagnose poor sharpness in bird photos- the focal plane would look decently aligned, but even on subjects that filled a lot of the frame, there was no sharpness to the bird's eye and feathers.

I decided to work on AFMA with the FoCal software. Especially with my 1.4x teleconverter, I was getting poor results- poor focus consistency, astigmatism, and my AFMA results moved each time. I thought it was problems with my teleconverter, as results were less terrible without it.

Then, off the cuff I removed the Hoya HMC UV filter (It seems they are being discontinued so they are inexpensive, but tests apparently show it's really good http://www.lenstip.com/113.15-article-UV_filters_test_Hoya_72_mm_HMC_UV-0.html )

Results immediately improved a lot. Then today, I started looking at the image samples in the report:

f/8 1/320s ISO400, 7D MKII, 100-400L II +1.4x, AFMA 10

f/8 1/320s ISO400, 7D MKII, 100-400L II +1.4x, AFMA 10

Care to guess which is which?

These results were even consistent across all the AFMA adjustments. Here's a comparison at AFMA -20 (the best value for my lens is around +9):

f/8 1/320s ISO400, 7D MKII, 100-400L II +1.4x, AFMA -20

f/8 1/320s ISO400, 7D MKII, 100-400L II +1.4x, AFMA -20

These tests were twenty minutes apart with identical in-camera parameters.

One thing changed, I taped the target to a board instead of the wall it was on, and used a little more tape (I was pulling out my hair over the poor results, and thought PERHAPS a breeze could be moving the target a few mm) but now that I'm discovering a number of older threads with folks experiencing sharpness problems with a 100-400L + UV filter combo, I think the filter may be the root cause after all.

Especially considering my equivalent focal length here is 900mm, it potentially makes a lot of sense. We know meniscus front elements were used on the supertelephoto primes, and that plus my results suggest to me that long focal length increases susceptibility to sharpness problems with filters.

Ideally I would redo the tests doing my utmost to isolate the filter as the single variable (e.g. not touching the target between tests), but I won't have a chance to do that for another two weeks.

P.S. Bought the filter from BHPhoto, so the seller is reputable. Though, I bought it from their used dept. If you wanted, you could argue maybe it's a knockoff and BHPhoto didn't notice when they acquired it.

I have used my 100-400 II with a B+W UV filter with no sharpness problems, including with a 1.4x converter.  I think you have a bum filter.