I've looked into back focus problems here and on the larger Web, and it's at the edge of what my brain can understand, but all the discussions seem to involve Canon/Nikon AF systems, and not much of that seems applicable. Can an A-series Pentax lens have back focus problems?
I have a Pentax K100D Super and a Pentax A series 50mm f/2.0 lens.
I keep taking shots that look like this:
(enlarged to show texture):
In the viewfinder, it looked as though the people were in focus. I wasn't trying to photograph the flowers. I get shots like this fairly often. The plane of focus seems to be about two feet (or 20 percent?) behind the thing I think I'm focusing on. Classic back focus problem, right? But can back focus exist
in the lens? Doesn't back focus come from a discrepancy between the lens-to-CCD distance and the lens-to-focus-screen distance?
Settings for this shot were f/2.4, 1/60, Aperture priority -.5EV, ISO 1600, Anti-shake ON, hand-held and looking through the viewfinder. It was an evening barbeque; pretty dark.
I haven't noticed similar problems with my other lenses on this camera, but my other lenses start at f/3.5 and I use them mainly on auto-focus. My other lenses are 18-55 kit and 10-17 zoom fisheye.
I downloaded some focusing chart PDFs, but got frustrated and after a while I no longer trusted my own squinting eyeballs, so I got a little more home-brew, literally.
My roommate has an *ist Ds, so I borrowed that to see if the problem is me, my lens, or my camera. I set up beer bottles, some lamps, and a tripod in the kitchen. The 50mm lens has distance markings for
15ft ...
3m ...
8ft. I took pictures of a beer bottle 13ft from my camera lens, using the focus scale, NOT my own eyes, but here goes:
All shots at f/2.0 Wide open, tripod, Anti-shake OFF, 2-second timer release so I wasn't touching the tripod when it went off, ISO 800. (*ist Ds doesn't have anti-shake)
My camera, distance scale at 13 feet
*ist, distance scale at 13 feet
My camera, distance scale at 11.5 feet (halfway between
15 and
8)
*ist, distance scale at 10 feet (*ist at 11.5 image accidentally erased)
It seems to me that the lens's "real" focus is about two feet behind what the distance scale says. That jives with my real-world photos of the outdoor party.
I'd love for this all to be the fault of my $60 eBay 50mm, but I worry that it's a problem with my DSLR. What should my next step be? Many thanks.