Sticky aperture?

Hi All
So, I just purchased a long wanted lens, the F 100 f2.8 macro. The price was good and the seller assured me it was ok.
But I suspect iit is not.
It seems to be shooting wide open only. When choosing a smaller aperture either manually on the aperture ring or in the AV mode it is not stopping down and the images are overexposed.
When I take it off the camera and look through the lens I can change the aperture until it reaches the f2.8 setting where it seems to get stuck. Stepping it down on the aperture ring doesn't make the aperture blades follow. Only When I activate the aperture lever it "snaps down" again.
I have tried moving the aperture lever some. It may seem a little tight but not too much. It is not bent and doesn't seem to be dislocated. The aperture blades seems clean.
Is this a sticky aperture problem or something else?
Is this fixable?
If it is serious I will send it back to the seller but I would love to keep it if possible.
Thoughts, advise, anything will be much appreciated. Thanks.

If you are using the lens with the aperture ring on the 'A' setting, it should stop down OK.

If you are using the aperture ring to set an aperture and you are using a Pentax DSLR you'll need to change a menu setting to "Permit use of aperture ring". You'll find the setting in the custom menu somewhere. If this is not set, the lens will not stop down.

When detached from the camera, turning the aperture ring to any setting will not stop down the lens. It will only stop down when the aperture lever is moved to close the diaphragm to the chosen aperture.

All that said, if you can see that the blades are sticking, send it back for a refund,

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Peter E Smith

My flickr Photostream

It's possible it may have been dis-assembled and put back together not quite correctly. This is pretty common, especially with lenses bought on ebay.

Unless you are prepared and able to take it to bits, I don't know whether you'll ever find out what's wrong with it. Either this, or pay someone to take it to bits for you?

I think I would be tempted to send it back, or negotiate a pretty big refund.

Regards
David

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Flickr

Nicola's Apartments, Kassiopi, Corfu

Some cameras, some lenses, some bits 'n' bobs

Thank you for your comments/advice.
I had a good talk with the seller. He would take it back, No questions asked, but we agreed upon a very reduced price. I believe I can find good use for it shooting wide open. IQ at f2.8 is quite stellar imho.
Maybe at some point I will try to clean the aperture blades and see what happens
How much would it cost for someone skilled to disassemble and put it back together assuming nothing serious is wrong (just a rough estimate)?

If you want to use the lens for macro work it will be almost useless at a permanent f2.8 as the depth of field will be wafer-thin. If it's for more general use it may be OK. As for cost of repair, I have no idean but I don't think it would be very cheap.

Regards
David

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Flickr

Nicola's Apartments, Kassiopi, Corfu

Some cameras, some lenses, some bits 'n' bobs

I got a quote for £48 to clean an 85mm lens with a sticky aperture from Asahi Photo.

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You can see some of my photos here if you are so inclined