Something a bit off the wall...

Pentax-M 28mm f3.5

It's all manual, but a very sharp, compact lens. I tend to leave it on my K-m now as the default choice. It makes the camera not much bigger than a large bridge camera, so it's very unobtrusive for walkaround stuff.

That was the one - and I'm unlikely to get bored any time soon! It's usefully faster than the M 135mm and handles really sweetly too. In fact, I've tracked down an original "crinkle" finish lens cap for it as it came with a later Pentax one.

The Pentax 28mm lenses are odd. The f3.5 is a better lens than the f2.8, it's just much sharper and seems to have less fiercely-curved elements, which presumably helps it show less distortion too. I just wish I knew why Pentax dropped the optical formula as by most accounts the later A, F and FA 28mm f2.8 just can't match the K or M 28mm f3.5. I haven't tried the f2 as they seem to be unbelievably rare.

I am thinking of saving for a DA 40mm f2.8 Ltd. Mostly as it's the cheapest of the Limiteds, I like the design/field of view, and I quite fancy an autofocus prime. Won't be for a while, say two or three months depending on how much they're going for. My target price is £320, Amazon UK currently have them for that. Plus it's only 3x the average price of a used M 40mm and the DA is apparently a much better lens!

I'm still being told that old glass is full of imperfections and (even funnier) that new 3rd party zooms are better than my bundle of old Pentax primes. Thing is, there are only so many degrees of precision to which you can grind a lens. We might have techniques enabling us to push the glass further now (hence why there was never a Pentax-M 28-200mm but there was an FA version) but aside from coatings there are only so many ways you can manipulate glass. A good lens design will be timeless as the laws of physics do not change.

I think I need to crack on with finding some decent subjects to let my old lenses show what they can do on digital. It must be something to do with how Pentax program their sensors (as opposed to Sony...), as I don't seem to have any of the problems that I apparently should be having (aside from the odd out of focus shot!)

I would - apparently pretty much all pros, if asked, will freely admit that AF does the job better than they can!

Plus the AF lenses are not nice to focus manually - very little tactile feedback.

I take many photos of unboxing it, then load a film and start using it. James May has the right idea on "collectible" items - use them carefully and enjoy them rather than stuff them in a case!

Edit, lens prices:

I paid £8.50 for my K 55mm f1.8, nothing wrong with it other than the usual paint scuffs. My A 50mm f1.7 was £42 and my M 28mm f3.5 was £50, all on ebay. The only lenses I have spent over £50 on were my K 120mm f2.8 (£65) and my A 200mm f4 (£99) from a camera shop website.

I think you need to get in touch with Pentax Service and see what they advise. I know I had an MZ-50 which made similar noises, in that case it was a cracked cog on one of the drive motors stopping the camera from winding on the next frame (it just ran the motor continually as it wasn't managing to reach the next stage in the procedure). The K100D will be a completely different design, so I have no idea what's causing the problem with yours.

The little pin that pops out is the drive for the autofocus - it engages a socket on the back of the lens. If you have the lens off the camera and turn the focussing ring manually you should be able to see a small circular thing turning on the lens mount, this is the socket that the pin engages with to focus the lens.

Interesting that you mention the K 55mm f1.8 - I have that and the A 50mm f1.7. The A 50mm is obviously much easier to use but there's just something about the 55mm that other lenses struggle to match.

I just don't see why people are willing to pay massive sums to gain .5 of an f stop over the common variant (or .2 of an f stop over the slightly-saner-priced f1.4 versions). If I saw one cheaply (say the urban legend charity shop scenario) I'd pounce on it, but I won't spend over £100 on a manual non-telephoto lens.

Or (and this is a very common one) is the aperture ring set to "A"? I've seen more than a few people who think their expensive lens is wrecked when all they need to do is press the button and turn the aperture ring a bit further.

The F series (and later) lenses should be fully auto on all Pentax DSLRs - they have the same auto focus drive and electronic aperture coupling as modern DA lenses.

It's the A version, judging by the shape (and the fact that I can just make out the A button!)

Silly money for a manual focus 50mm - it can't be that much better than the f1.7!