I was planning on writing an article about my three wide angle lenses (14mm f2.8, 10-17mm fisheye, and Zenitar 16mm fisheye). I started taking a few sample photos with each and noticed something strange. The images at wide aperture with the 14mm f2.8 looked really poor. Initially I was thinking that user error was involved, so I went through various processes about… 3 or 4 times with the same result. Eventually I tried the 14mm f2.8 on my other camera body and received an extremely sharp image at the same aperture setting.
It’s just a fact of life that there are tolerances and levels of variation when electronic and mechanical devices are aligned and configured. What I know now is that my K10D has an issue with something called front/back focusing, so far I only know it happens on the 14mm f2.8. The mechanical device inside is slightly miss-aligned so when I take a large aperture shot, the intended area of discernible focus isn’t where I expected it to be producing a seemingly un-sharp image.
As you can see from the image that when I use manual focus (I took 10 shots while slightly rotating the focus ring, selecting the sharpest image), the K10D image is very nice. So that leads me to believe that it is an auto-focus issue. From my initial research about the K10D, I have a few options to correct the focus issue.
1. I could downgrade the firmware to a version that allows software adjustment of the autofocus. I’m not 100% certain it’s necessary to downgrade the firmware, so I’ll be checking to see if version 1.31 (eg. current) allows for some type of AF adjustment. The negative of this is that I would probably lose the ability to tether as that feature wasn’t in the K10D until firmware 1.20 I think.
2. I could open the bottom of the camera and tweak the front focus/back focus screw a bit. This actually sounds like the most fun. I’ll have to do some more research first.
3. I could send the camera to Pentax to have them align the camera. I’ll assume this will cost me some money in shipping fees at least. The camera is “new old stock” with an extended warranty, so I have a good two years of warranty available. I’ve never once sent a product in for warranty work in my life, as I’ve always been the type of person to do something myself when it’s a reasonable fix.
Before I do anything I need to figure out if this is just limited to this single lens or every lens. If it is just the single lens I might have to just leave it as is. More tests are in order with say my larger aperture lenses like the 50mm f1.4 and 40mm f2.8. With small depth of field it’s necessary to have spot-on focus, otherwise an image won’t turn out as intended. Overall it’s a minor issue, and actually kind of interesting to see what I can do to fix it.