Today I received a supposedly fully-operational SMC Pentax-M 50mm f2 lens but it has sticky aperture blades and stops down too slowly to work correctly. In all other respects it's fine and a quick test shows how sharp it is compared to my kit lens. I reckon I got it cheaply (£11 + postage), so I'm undecided about whether to return it or risk trying to repair it. I'm generally pretty good with mechanical stuff but haven't tried anything like this before and I'm worried about it releasing loads of tiny springs or ball-bearings! Is it relatively simple to repair a lens such as this or should I just return it, bearing in mind that I assume a replacement would cost a fair bit more? Cost is a big factor at the moment and I don't particularly mind that it's an M rather than an A lens, nor that it's the f2 rather than the f1.7. Thanks.