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07-02-2010, 07:10 PM   #1
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focus test issues, FA 50mm 1.7 & DAL 18-55mm

Hey all. I took some focus tests with the 3 AF lenses that I have:
FA 50 1.7
FA 100 2.8
DAL 18-55

I purchased the 50/1.7 over the 1.4 because it was supposed to be sharper wide open. Stopped down, it's fantastic, but wide open it's pretty ugly. Not just soft, but some sort of aberration going on. Is this a bad lens, or is this typical for the 1.7?

Second on the list was the kit lens. As expected it's pretty soft wide open, esp at 55. What's interesting though, is that the focus offset changes through the zoom range. In fact, with the camera and target fixed, you can intermittently press the focus button while zooming out. Focus doesn't change from 55 down to 28, but from 28-18 it (audibly) refocuses. If anything, with the DOF I'd expect that sort of thing to happen at the other end of the zoom. The target was centered, these are cropped 100% and to the side so the markings show. Is this atypical? Can anyone else replicate?

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07-02-2010, 07:11 PM   #2
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Third is the FA 100 2.8 It's a little soft wide open, but I don't really see any problems - just putting it up here because I have it. Of course if anyone wants to tell me it's a bum lens while I'm still in the return window I'd appreciate it
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07-02-2010, 09:49 PM   #3
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Very interesting. Did you took the test indoor? Need more lights.
07-03-2010, 06:47 AM   #4
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I recently made my second purchase of a FA50/1.7 and the behavior of both is similar to yours; above f2.0 the focus is spot on but at 2.0 and 1.7 it is all over the map. It was much worse with the first lens which I returned but even the second try exhibits the issue. I learned to check the focus by eye and just refocus if it seems off. My FA35/2 does not have the hit-or-miss problem wide open but it may still be a short-coming of the camera combined with the FA50. It looks like your shot with the FA50 at f1.7 is just very front focused and if you refocused on the same spot it might be right on.

07-03-2010, 09:37 AM   #5
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Testing indoors under tungsten light on any Pentax DSLR but the K-7 also pretty much guarantees front focus. And the psoted crops don't appear to show the test target very flat or straight - they seem just curved enough to also guarantee a certain amount of apparent FF.

As for the 50 at f/1.7 in terms of overall look, the general softness and "glow" you see isn't out of line with what I've experienced with M and A variants of the lens and samples I've seen. Apparently sharper than the f/1.4 version at that aperture, but the out of focus areas do look odd. Although I suspect the point of sharpest focus is *between* two of the marking on your chart. I find stopping down even to to f/2.0 makes a big difference, and from f/2.4 on it's probably as sharp as anything else I have. At f/1.7, I'd say it's impressive *for f/1.7*.

Regarding the kit lens refocusing as you zoom, I'd assume the lens simply doesn't hold its focus point perfectly as you zoom. People sometimes make a big deal about parfocal versus varifocal lenses, but I don't care what a lens is officially - I'd still expect some amount of refocus necessary for pixel-peeping sharpness.

BTW, random thought I just had - when one is finding FF or BF and trying to resolve how much of this is camera versus lens related, do people ever try testing "regular" AF versus liveview?
07-03-2010, 12:29 PM   #6
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Thanks for the feedback. Was mostly concerned with the 50 as it's used and I had til Mon. to return it. It was a KEH BGN lens for $144, so since the wide open is apparently not far out of line from other lenses, it looks like a keeper.

I will mount the target and reshoot under better light to see what happens.

Will also try the Live View to compare, but would expect that Live View would always result in perfect focus. Wouldn't expect it to show any problems with camera (unless defective), but it might show issues with chart implementation (e.g. curved plane)
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