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02-16-2010, 02:37 PM   #1
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DA40 Ltd focus advice sought

Just got a DA 40mm ltd, and noticed that the focus seems to be off. It looks like it focuses "in front" when auto focusing. I also have a 21mm ltd and the kit 18-55mm, and if there is front/rear focus issues with these lenses, I can't make it out. I have a K-x body.

Attached are three images. The focus chart is at a 45 deg angle, and the camera is pretty close to being normal to the focus chart, and centered. I set the focusing to be spot, and verified that the focus "hunts" when just above the center in the white area, or just below center in the dark area. So, I know the camera is focusing right at the center of the chart.

The three shots are at F2.8, F3.5 and F5.6, maybe 2 ft from the chart. At F2.8, the focus is way off, and it's still noticeable at F5.6. The results are similar for both sides of the focus chart, so the camera is pretty square to the chart.

Should I return the lens?

I didn't think this was really possible, considering how the phase focus works on an SLR.

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02-17-2010, 12:21 PM   #2
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It's possible. Search this forum (or the net in general) for the term "residual spherical aberration" in conjunction with the term "focus" for one plausbile explanation.

On the other hand, neither the 21 nor the 18-55 will get as shallow a DOF as you get with a 40 at f/2.8, so I wouldn't rule out the possibility that this is a camera issue, not a lens issue. The fact that you see the issue at even f/5.6 is of course disconcerting, but since focus is done at f/2.8, it still seems possible that a misaligned focus sensor could cause the DA40 to appear off while other lenses didn't. Maybe.
02-17-2010, 04:28 PM   #3
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Thanks for your response. I'm thinking it's a combination of the camera and the lenses.

Worst: 40mm ltd, even at f5.6, good light, real photos, still see front focusing.
Medium: 18-55mm kit lens. At 20mm, just a bit of front focusing. More at 45mm.
Best: 21mm ltd, at f3.2, a hint of front focusing, not noticeable beyond f4.0

I returned the 40mm ltd - it was within the 14 day supplier return policy, and it just seemed really far out of wack with the camera body.

I will try adjusting the K-x camera front/back focusing according to ricehigh's blog, carefully and with fingers crossed.
02-18-2010, 09:15 AM   #4
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QuoteOriginally posted by leadfoot Quote
I will try adjusting the K-x camera front/back focusing according to ricehigh's blog,
Good plan! We know what an expert he is on Pentax.

02-18-2010, 08:25 PM   #5
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Actually, I don't know if ricehigh is an expert or not, and don't care. He/she obviously knows alot about the insides of Pentax cameras - guess if you are into disassembling Pentax firmware, go for it. I appreciate the tutorial on how to get the debug stuff to work.

The debug menu worked on the K-x, and I was able to fix the front focus. Dialed in -70 um's, and the front/back focus is pretty darn close on my lenses. Why don't they do this at the factory? (rhetorical question) Unfortunately, I sent the 40mm ltd back already - that's ok, I can always get one in the future.

Some pix from the kit 18-55, and the 21 ltd, in extremely challenging light - shooting almost directly south, into the sun. Even with a polarizer, still some flare. Mt Hood in the background, the Columbia river gorge and the Bonneville Dam complex, from the top of Table Mountain.

K-x is an awesome camera, and now it auto focuses properly Of course, today was very bright, and not a good test of the focus, but still a gorgeous day to get out and shoot some pictures.
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