[Petrock]

I was given this lens a couple of months ago. It's a good lens and worked fine, but now all of sudden this problem cropped up. I can only get good exposure wide open at f4. I can just see the slightest darkening on the very edges of the aperture blades. It seems this is lubricant contamination. Is it practical to do a home aperturectomy for a cleaning, or do I have to bring it to a lens doctor?

[chunkiness]

Unless you have special tools, or a wiz at making them yourself, its best to send it to an expert. There are a lot more parts there than it seems.

[marcusBMG]

Test the iris for stickiness by putting the aperture ring on a high f number and testing the movement of the iris holding the iris open with the aperture lever and then letting go. If the return spring isn't consistently and easily closing the iris there is a problem.
Repair: I am not familair with this lens. My thread on the viv 70-210mm gives an idea of what can be involved:

Vivitar (komine) 70-210mm, PK-A mount, sticky iris repair. - PentaxForums.com

Some diligent internet searching may pull up some disassembly info on this lens, or a similar tokina zoom. You only need access to the iris to then try a bit of solvent (coleman fuel/similar) to remove any oil/grease residues.

[Petrock]

Test the iris for stickiness by putting the aperture ring on a high f number and testing the movement of the iris holding the iris open with the aperture lever and then letting go. If the return spring isn't consistently and easily closing the iris there is a problem.
Repair: I am not familair with this lens. My thread on the viv 70-210mm gives an idea of what can be involved:

Vivitar (komine) 70-210mm, PK-A mount, sticky iris repair. - PentaxForums.com

Some diligent internet searching may pull up some disassembly info on this lens, or a similar tokina zoom. You only need access to the iris to then try a bit of solvent (coleman fuel/similar) to remove any oil/grease residues.

Thanks, marcusBMG. Those are detailed instructions. I think I'll just bring it to a professional! Luckily, we have a large camera shop down the road just outside of Boston, Hunt Photo. I'll call them for a quote.

[marcusBMG]

Well it's not worth it TBH, will cost more than the lens is worth! I see these crop up on ebay every so often, they are not one of the sought after vintage MF lenses even though when s/o comments on them its normally very positive so they rarely bid up above (equiv of ) $40-50.
Just bide you time to pick up another one, or look for something similar. The adaptall 70-250mm 104A for example is a good lens. And the SP 60-300mm 23A is even better - that does have the cachet to bid up but it's well worth the best part of a hundred bucks (tho I wouldn't pay more there's plenty of these around)

[Petrock]

In a practical sense, my problem was solved to begin with. It's only when I've tried the lens hand-held that I had the problem. But with 2 sec. delay on tripod---the way I virtually always shoot---the problem does not appear, since the delay gives time for the slow diaphragm to close. It took me awhile to figure this out......duh....Thanks for all the good advice, anyway !