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05-08-2014, 07:40 PM   #1
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SMC Pentax A 100mm f2.8 won't focus at infinity?

I bought an SMC Pentax A 100mm f2.8 some time ago here on the forum and used it for some great shots. I love it size, reasonably close focus distance, colours and contrast. Last week I tried to shoot some night sky images and they were completely useless and I wasn't expecting that at all. I thought it would be a great and sharp night sky lens. Then I wondered if maybe it suffered from significant coma and that was the reason. Today I used it for some distant landscape shots and realised the lens doesn't focus to infinity. I stopped it down and down and down and nothing helped. Although by f16 diffraction is most likely making the image soft anyway.

After more testing it seems this lens will only focus to about 30m (say 100'). I was wondering if anyone else has experienced a similar problem or if anyone knows of a way to rectify the infinity focus on this lens.

The lens is in perfect condition, no signs of abuse, no dings, no obvious signs of abuse at all.

Any advice most welcome.


05-08-2014, 09:24 PM   #2
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I had the same problem with an M lens. Here is the link to the thread Lens got fixed and worked fine. I later sold it with no complaints from the new owner.

Last edited by pixie; 05-08-2014 at 09:31 PM.
05-09-2014, 10:31 AM   #3
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I don't have that lens. Based on the sliding hood lenses I do have (M135/3.5, M120/2.8 and A135/2.8), you have to remove the rubber focus grip first. Slide something flat like a plastic knife under it, run it around to break the glue bond, then slide the grip off. It will expose a set of three holes in the metal focus ring. At some point (probably infinity) those holes will line up with three tiny JIS screws. Remove those and you can pull the hood off. The screws are pretty small and often very tight, so I highly recommend a JIS #00 screwdriver. A Phillips #00 will just strip the heads instantly.

Then there is a washer around the front element which should just fall off. It's hiding three screws holding the focus ring to the focus helix. You can loosen the three screws, turn the focus ring toward the 30m mark, tighten one, then see how that works. The moon makes a nice target for setting infinity exactly.
05-09-2014, 12:12 PM   #4
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Thank you both for your helpful advice. Now to find a JIS #00 screwdriver...


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