Diagnose a damaged lens from focus test results

Question

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:

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:

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?

Answer 1

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.