Sluggish Diaphragm - odd story

[George Collier]

I recently bought 2 Nikkormats on Craigslist. They came with a 50 f/2, a 24 (a fairly old one) and a Vivitar 135 w/teleconverter.
I tested the cameras through all the f/stops and shutter speeds with the same roll of film, going from top to bottom on the speeds, using the 24mm lens. Both cameras were virtually identical at all speeds, but I noticed something odd. The higher shutter speed exposures were all just a bit weak (film was HP5 box speed), but as the speed slowed down, the exposures were much heavier. Also, I noticed that in some later shots, just walking around using up the rest of the film, the depth of field was rather shallow.
Some of you know where this is going, but in trying to figure out what happened, I pressed the DOF preview button and the stop down was rather slow. I put in my own newer 24, and it was instantaneous.
I had never seen this happen before, but it explained everything. The slower speed shots used smaller apertures, and the diaphragm was still open too far when the shutter fired, also the issue with the DOF.
Anyway, does anyone know what this might cost to fix, and is it worth it? It's an old version of the lens, and I already have a newer one.

 

[Mike Wilde]

If you are not afraid of the thought of taking one apart, it is usually that case of some tiny amount of oil migrating from a linkage onto one or more of the aperture blades.

A wipe with a q-tip wet with naptha, and then left to dry will usually fix this situation.
The next step is to wet with one q-tip, and wipe dry with the other. A very light touch is required.

You may want to check that the external stop down pin, etc has not been bent. Or rubbing or mating wrong. If so fix the outside bits before delving deeper into the lens.

 

[BobD]

Remove the lens, set it to smallest aperture and look through rear of lens while
flicking the diaphragm coupling lever at the rear of the lens. The diaphragm should
move with a crisp, snappy motion as it opens and closes down. If it is sluggish, it
needs cleaning.

 

[fschifano]

A good cleaning will usually fix that problem easily. I'm sure that any competent camera technician can do it for a reasonable fee if you're not comfortable about doing it yourself. If the 24 mm lens is a Nikkor, it would probably be worth the price. If it's one of the third party aftermarket lenses, you might be able to find one for less than the cost of the repair.

 

[dehk]

If you wanna do a home remedy if you are bored or just wanna try something yourself: Rubbing Alcohol does the trick after disassembling the whole thing. Its not rocket science really. Also, if you wanna make a homemade lens opener (spanner wrench), go to home depot and buy a Caliper, file or grind down the 2 points. Just remember how the whole things goes together. I know it doesn't sound professional, that's why I call it a home remedy, fixed a Pentax 50mm f2 that way.

P.S. make sure the diaphragm is completely DRY before re assembling.