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04-04-2010, 06:11 AM   #1
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Poor performance from A-50/2 on K-x

I've just picked up a K-x kit which I'm enjoying very much. When I use my A-50/2 on it the performance of this lens wide open is atrocious. At f2 the image has what looks like a double image as if the shutter speed is too low, but it occurs under all situations regardless of whether the SR is on or off, at slow shutter from 1/60 to above 1/1000, even with the flash on. I have owned this lens for a long time and while it was never great wide open on film, the image was still acceptable. On the K-x when stopped down to f4 the image is "okay" and by f8 the image is excellent.

A-50/2 at f2
crop full

For comparison:
K-55/1.8 at f1.8
crop full I only just got this lens along with a KM. It appears very sharp although the colour rendition is a bit odd.

18-55 II at 55mm f5.6
crop full

As you can see the old K-55 and the kit lens perform great under virtually identical conditions. I also have a old manual focus Tokina 70-210/4-5.6 which surprisingly is pretty good on the K-x too (once the Ricoh pin was removed).

So what gives, is my A-50/2 is just a lousy lens that can't cut the mustard on a modern crop sensor compared to film?

04-04-2010, 06:18 AM   #2
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Do you have a filter or something on the lens? I've seen ghosting like this caused in low light shots by a filter mounted on the front of my lens.
04-04-2010, 10:15 AM   #3
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That shot is not normal at all. either the world's worst filter is mounted, or something has slipped inside the lens.
04-04-2010, 04:11 PM   #4
Just1MoreDave's Avatar

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QuoteOriginally posted by Marc Sabatella Quote
That shot is not normal at all. either the world's worst filter is mounted, or something has slipped inside the lens.
I agree. I would see if you can move either the front or rear lens groups.

The front group is held in by a brass washer with three screws holding it down. If those screws have worked loose, the three lens elements in front of the aperture can move out of alignment with the two elements in the rear. If you can unscrew the beveled trim ring just inside the filter ring, you can get to these screws.

04-09-2010, 09:49 AM   #5
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Looks like you might have a defective lens. I have the Pentax-A 50mm f/2 and I don't see such a ghosting problem. In general I find the A 50/2 to be sharper than the kits lens at 50mm and at the same aperture.
04-09-2010, 09:53 AM   #6
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Judging by the photos there is something definitely wrong with that lens. If you were getting shutter speeds of 1/1000 then it couldn't be from camera shake. If you shake the lens can you hear anything moving inside?
04-09-2010, 11:27 AM   #7
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Because it happens only for the range beyond the focus point, and not on the between the lens and focal point it really seems like the lens elements a creating this stange bokeh. Some misalignment?

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