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01-12-2009, 03:15 PM   #1
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SMC A 28mm 2.8 problem(?) question

Hi,
I'm new to this board and to Pentax DLSRS, just having bought the K2000 (my first Pentax was the Auto 110 years ago...). I bought a used A 28mm f2.8 lens for $50. It works but (1) it appears less focused than shots with my other lenses (kit 18-55 or DA 40mm pancake) and there appears to be a tiny spot in the middle of the bottom of the lens which I can't clean off. Could this be some sort of dried liquid on the interior? If so should I try to open the lens up just to see if I can clean the inside bottom-most lens?

Also, (2) The lens from time to make makes a rattle but there is nothing I can see that is moving inside or out.

Thanks for any advice!

Brad

01-12-2009, 03:44 PM   #2
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Hi, I have this lens.

QuoteOriginally posted by bsimon Quote
It works but (1) it appears less focused than shots with my other lenses (kit 18-55 or DA 40mm pancake)
Hmmm, mine focuses as well as my DA18-55 @ 28mm, assuming that I focus it, of course (could your problem be just inexperience with manual focus lenses?). You could try the focus trap method to get the camera to take the photo when it thinks that the image is in focus.

QuoteQuote:
and there appears to be a tiny spot in the middle of the bottom of the lens which I can't clean off. Could this be some sort of dried liquid on the interior? If so should I try to open the lens up just to see if I can clean the inside bottom-most lens?
I'm not sure what "middle" or "bottom" refer to, can you post a photo?

Remember that the digital camera crops the image, so marks at the edge of the lens will not effect the image. (But scratches will, because they bend the light differently to the lens element and so a scratch will add flare). If the mark is not in the centre of any of the lense, then it is likely that it will not effect the image.

QuoteQuote:
Also, (2) The lens from time to make makes a rattle but there is nothing I can see that is moving inside or out.
For me, that would be worrying. It may be a screw that has come loose, and there is always a danger that it might get on the aperture blades.

I've taken apart a few lenses, but not the A28/2.8.
This site suggests that the M28/2.8 is easy to disassemble and the A28 is the same lens formula (but it does have auto-aperture).

The A50/1.7 is known to have a problem with the aperture ring, which sometimes need fixing. I thought it was a problem with the A50/1.7, but this post suggests that the A28/2.8 can suffer from the same problem. Can you move the aperture ring to A and if so, can you change the aperture with the K2000? Or alternatively, can you move the aperture ring off A and use it to stop down? (In both cases, use M mode on the body and the green button to see if the lens stops down.)

Incidentally, if you choose to take apart the lens, bear in mind that the front elements bend light a lot, and because of this the edges of the lens elements are painted black. Be careful not to scratch the black paint, otherwise the lack of that paint may cause flare (although you can do a reasonable repair of such a scratch using a Sharpie).

Hopefully, that will get you started on trying to determine what's wrong with your lens.

Richard
01-12-2009, 03:52 PM   #3
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I had one of these that was pretty disappointing until I discovered the front lens group was loose. I was able to screw it back in with finger pressure on the front element, clean my fingerprints off and notice a big improvement. I now use a Pentax-F version that's the same optical formula, and I know it's sharper than the kit lens, so your lens definitely needs help.

I know the Pentax-M versions are pretty easy to disassemble from the rear, but the A might be harder. It's amazing how big a spot has to be to make a difference, so it may not be worth doing.

Rattles are not uncommon even in a perfectly working lens. The aperture mechanism has to be a bit loose to work quickly. This rattle is often quieter if you have a finger on the aperture lever.
01-12-2009, 04:06 PM   #4
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Thanks for the comments. I've attached 3 jpegs I just took. 2 landscapes of the same view--one at f4 (middle image) and the other at f22 (bottom). Focus was set to infinite on both (which hopefully accounts for my vision/manual focus ability challenges). The f4 is less focused it seems throughout the image than the f22. Maybe this is normal but I can't find a part of the f4 image in good focus. The other image of the flower was shot at f3.5 and seem ok. For all shots I set the aperture to A and entered 28mm in the k2000 when it asks for focal length. But honestly none of these images look focused to me.

I see the "spot" if I look at the side of the lends that mounts to the camera--it appears to be right in the middle, very small---too small for me to take a photo of it. Hopefully if it's a problem it would show up in these images? Or only affect part of them?

The rattle is a mystery..it seems to come and go; sounds like something is moving but I'm not sure it's affecting anything.

Thanks for your comments... I'm not quite ready to take apart the lens.

Brad

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01-12-2009, 07:21 PM   #5
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One of the closed blossoms (the one just above and to the left of the open one) looks reasonably well in focus, although it's hard to tell. Definitely some shallow DOF - and folks say wide angle lenses are inherently deep! They are when shooting distance, but not closeup!
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