11-30-2017, 08:20 AM
I am strugging with a Tamron 17-50 has well (non-VC). It has a tendency to severely front-focus at times. We're talking static shots where I point the lens at infinity, I have only the center AF point activated, yet focus is somewhere near close-focusing distance. The next shot could be spot on.
Since it's an intermittent problem (50% of shots or more) this is not something for AFMA, which fixes lack of accuracy rather than lack of precision. It irks me that in each case I get focus confirmation.
Focus in live-view works flawlessly, as does focus on my old 400D (and 300D before that, now sold).
I just had an idea. What if I started using AI servo instead of one-shot, even for static shots. It should hunt when AF goes awry. I'd lose focus-recompose unless I use back-button focus. I'll have to give this a try later today.
I had the Tamron 17-50 F2.8 non VC in the Pentax mount. It took me a while to realize why the lens front focused.
After having calibrated it, when shooting fully open the lens was accurate, and from F2.8 it was accurate all FLs and all apertures up to F5 ....as soon as you stopped down to F5.6 or more it front focused........something in the AF algorithms within the lens created this problem, the focus ring would turn about 2mm more front focused, than when F5 was selected on the camera or higher.
I posted here about it asking for anybody to try this test.....no response.....maybe nobody here had the lens.
I also posted my results on DPreview Pentax forum, and Bingo, five or six people ran the test and found the same result!
Many suggested including Klaus that maybe it was there to counteract RSAs, which would seem reasonable..........but it wasn't that......it was a flaw in the programming maybe confined to the K mount, I never looked further at other mounts.
Once I found the problem I rarely stopped down to more than F5 and all shots were in focus.
It was the "sneakiest fault" though, so anyone having front focusing issues with the lens should try the test......check whether the front focus is confined to apertures smaller than F5.
This is of course PDAF though the viewfinder!