Originally posted by ars Gear: Pentax K-5II, Sigma 100-300 f4.
Hi everyone!
I've just got Sigma 100-300 f4 in mint condition a week ago. Indoor testing showed perfect results. The problem I'm experiencing now is a presence of severe back focus at long distances. When focusing at 2-5m it is almost correct, but at infinity or distant objects, everything is out of focus. On the other hand, when using Live View, all the shots are perfectly sharp. AF adjustment +10 (maximum a can make on K-5ii) improves focusing accuracy, but doesn't solve the problem completely. I see several solutions to that, and would be grateful for your suggestions!
- Using service menu and adjust the AF to higher "+" values. But, all other lenses will have focusing problems afterwards (by now, DA 18-270, FA35, FA100 macro and F-50 1.7 I have focus perfectly accurate). Will I lose the guarantee for the body?
- Use MF or Live View. But the lens was thought for definitely the other kind of shooting.
- Is it possible to calibrate the lens some how? Or this is only possible for HSM Sigma lenses?
- Can Pentax do something, or all non-Pentax lens problems are not recognized by Pentax?
Thanx
I have a Sigma 100-300 and had a K5iis. I now own a K3 and here are my findings/experience:
With the K5iis, no matter what I tried, it did not play well with the Sigma, I, too experienced severe back focus on many many shots even after calibration. On occasion I would get a keeper (I'd guess about 50% of the time) but it required a technique of focus, stop, refocus if the subject lacked contrast such as animal fur etc. High contrast/texture objects it generally did very well with, but if that high contrast/texture was behind my subject, the lens would lock onto that object like dog on a scent. I'd have to recompose, focus intentionally on an near object, then refocus and hope that the lens would choose my subject. The problem was that often I could not tell in the viewfinder, only once I could see the image at 50-100% on my computer. I have posted several rants about this very issue over the years, but I won't link them here...lol
On my K3, after all my frustrations with the K5iis, I have been much more thorough in my investigations and slightly happier with the results. My first impression is that the smaller center AF point of the K3 was a BIG help, right off the bat. The 100-300 was more consistent and reliable. So much of your frustration is due to the "poor" AF system of the K5ii(s) bodies.
The next step took about 3 hours and a test target/calibration ruler. Using combinations of LV, MF and AF I discovered some quirks of my 100-300. At 300mm and f4 at less than 10m I needed a +8 adjustment. At 300mm and about f8 at the same distance my adjustment was 0! I tried it at 50m with a 300mm focal length and found that my needed adjustment was also based on aperture, ranging from +7 at f4 to -2 at f11. Essentially, subject distance AND aperture affected front/back focus response. I have no clue why or what causes this so I'm chalking up to the fact it's a legacy lens and there's probably been some movement of elements or what not. I've just resigned myself to "missing" some shots and if I can, trying to tweak focus manually.
Here's the other thing I noticed, at least in my copy, the physical focusing mechanism has some play. If I had to guess it would be the equivalent of about 2 feet (.6m) of depth of field as we get to the mid to long focus distances. Close to minimum focusing distance, it's a little more accurate . This means that the lens will tell my K3 body it is focused at say 25m when it can actually be 24.3 to 25.6m. At 300mm and f4, that's the difference between a nose in focus and the tail being in focus instead. It was even greater if I have the quick shift ring in MF mode and the body in AF. There's probably 3-4ft (1-1.3m) of play, especially close to infinity.
My conclusion is that the Sigma 100-300 is a great lens optically, but it doesn't really play well with the K5 series because of the HUGE auto focus points. It could be sample variation, because others have raved about how well this lens performs on their bodies, but you sound like you have a quirky copy like mine.
The "only" thing you can do is be aware of the issues and develop techniques to try and compensate, but for quick action scenes you may not have that luxury. You may also consider going to a K3. Lately I've been using 9 point AF-C and shooting in 3-4 shot bursts and that seems to have upped my keeper rate. Single shot and AF-S is still inconsistent.
In terms of service, CRIS Camera told me last year that there were NO parts even available for service or maintenance work, so there was nothing they could do to help. They are a Sigma Authorized Repair Center, so they would probably know. I haven't talked to them recently after my marathon testing session in January, but I'm not sure if anything can be done for the focus mechanism play anyway.