3

I have a lens that gives perfectly decent results at f/22 and f/32. Here's its capture of an ISO 12233 test chart:

SMC Pentax-A 135mm/2.8 at f/32

It's not the sharpest lens I own, but whaddaya want, it's a quarter century old design. (SMC Pentax-A 135mm/2.8)

This lens starts getting soft at f/16 and below. Between about f/11 and f/5.6, it's reasonably sharp in the center, with an increasing amount of that "vaseline on the skylight filter" look at the edges. By f/4, the soft focus effect reaches the center, but it's not so bad that you couldn't purposely use it as a soft-focus lens. At f/2.8, it's an, um, "creative" dreamy lens. Yeah, that's it, dreamy:

SMC Pentax-A 135mm/2.8 at f/2.8

Note the doubled image of the triangles at the edge of the test chart.

What does this test result mean?

I've asked two lens repair companies to quote me on a repair for this. One said "talk to Pentax," and the other didn't even bother responding.

I ended up buying a different copy, which performs properly through the full aperture range.

That leaves me with a set of choices:

  1. Sell it, including these test chart images of course, hoping that someone wants a "creative" lens.

  2. Keep it and use it as such myself. But, I have Photoshop, so what do I need with a soft-focus lens?

  3. Take it apart and try to fix it. If it doesn't work out, hey, I get a pile of bare lens elements to play with. That could be fun.

There is no visible problem when looking through the bare lens, and it rattles only a tiny bit when you shake it hard. I'm guessing the tiny rattle is a loose lens element. Does the test image support that guess?

  • Just out of curiosity. What is the price of such lens? – Umberto Dec 11 '14 at 11:53
  • @fasttouch: They're selling on eBay for US $125-150, at the moment. – Warren Young Dec 11 '14 at 12:09
  • Then I would say have fun with it :) – Umberto Dec 11 '14 at 12:10
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From an optical testing standpoint the main aberration that shows up on axis is spherical - and it varies only due to (F/#)^4, which explains why you are getting a soft focus at a larger aperture. If you look at the top and bottom the the image you see triangles aren't imaged the same, which would probably be astigmatism. I could tell you more if I saw more pictures, but that doesn't really matter; I think one of your lens elements is de-spaced and/or de-centered. I don't know how easy it is to fix it though; I have never aligned a lens myself, though I have seen instructions on the internet on how to do it. If you have another copy I see no reason not to mess around with it.

  • I agree with @Alex. Hopefully it's the front element (easiest to fix), which will be held in place by either a threaded ring with writing on it, or a black threaded metal ring. The latter will have two notches in it for a lens wrench to grip; the former will have no notches but may be able to be tightened with your fingertips (if loose). If you are confident in your abilities and have the right tools (size 0 and 00 phillips screwdrivers, lens wrench, rubber grip cloth, fine toothed tweezers, microfibre cloth, pressurised air), I would go with option 3). – HamishKL Jul 26 '15 at 12:13

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