The finishing ring is what I call the bit with the "Pentax-M 50mm" lettering on, it's just screwed into the filter thread. Once you've removed that then the front element group can be unscrewed (conventional thread, unscrews anticlockwise). You will need something to fit into the little notches and twist it - I did use a couple of small screwdrivers last time but there's a risk of them slipping if it was tightened by the Asahi works Sumo squad!

I've just bought a proper lens spanner for £15 on ebay, it should turn up in the next day or so and I'll be able to sort the 200mm f4 out. It's basically a couple of steel bars with a pair of angled prongs attached, you move these to the right width on the bars, tighten them with a couple of small bolts, then put the prongs into the notches and use it to unscrew the component. Due to the design it cannot easily slip and damage the glass.

Problem is, this lens doesn't have a finishing ring around the outside of the glass, just a metal collar about half a millimetre wide that screws into the filter thread to hold the front element in. It does have a couple of notches in but I'm scared to try using a couple of screwdrivers to turn it in case they slip. As it's a Pentax-A 200mm f4 I don't want to risk damaging the glass - they're not exactly easy to come by!

With something like a £5 50mm f2 I'm less worried, and in any case they have the finishing ring so you can get a grip with a bit of rubber. I use the strap from one of those "boa" jar openers.

Gareth, that 50mm will be an absolute doddle to fix. You just unscrew the finishing ring and then unscrew the front element group. You won't need to adjust anything, just refit them a little more than finger tight.

I've used vinegar on a cotton bud to clean fungus before now, all you need is a mild antiseptic that won't harm coatings. Rescued three 50mm lenses that way!

I do have another lens with some worrying specks (hope they're just dust) but I can't persuade the front to unscrew and it's a bit too valuable to risk pressing harder. Instead, I stored it on the radiator for a couple of weeks to make sure it was thoroughly dry and it now lives in a lens case with a sachet of silica gel.

Pentax don't seem to make any cheap primes, the only way to get a prime for that sort of money is to go 2nd hand.

The reason for the M and A series 50mm being fairly plentiful is that they were kit lenses with millions of SLRs for two decades or so. When Pentax adopted auto focus the zoom had taken over, leading to swathes of cheap and cheerful zoom lenses while primes became a high-quality and high priced niche market with no economies of scale to bring costs down.

From what I understand the 55mm was offered as a cheaper alternative to the 50mm in kits. Certainly this would fit - if you look at the K series 50mm they offered f1.4 and f1.2 IIRC, so a 55mm f1.8 would have been a mid-range alternative with the f2 as the base spec.

I'm also given to understand that the only difference between the f1.8 and f2 versions was a different aperture ring to allow you to select the wider aperture.

They can't have been sold with the K series bodies for very long. My Dad has a K2 that he bought in 1978/79 and it came with an M 50mm f1.7 from new.

If you want pure image quality then I think it always comes down to primes, certainly in terms of value for money. You'd have to spend a fortune on a zoom to get one able to beat the old K 55mm f1.8 or the various 50mm f1.7 variants.

That's a 50mm macro lens - designed for extreme close-up work. You can use them as a normal lens too but they're a lot more expensive than a conventional 50mm (even a new auto focus one) and their widest aperture isn't as good as a conventional 50mm. Nice to have, but a bit specialised for me. If I had that sort of money to spend on a single lens I'd be looking at the DA 40mm or the FA 50mm f1.4, both of which are superb performers.

Don't just look at Argos by the way - they sometimes have reasonable prices but often others can beat them. There's a thread on here about reliable suppliers.

Aha, yes, that would make sense. I have another push-on rear cap without that flange, so that may be more correct.

Sourcing original front caps isn't just OCD. The Pentax caps look better and are far better at staying on the lens than the cheap unbranded caps. After that, you might as well source one of the right style for the lens!

I'm not sure how original mine is. I found a black plastic cap with "Asahi Pentax" lettering on a crinkled background (as opposed to the smooth background on the M series and A series). A bit of research suggested that this was the right vintage, so I paid up.

The rear cap is a flexible plastic push-on job with "Asahi" on, which may or may not be period correct but was sitting in my spares box when the lens turned up without one. It looks identical to the rear caps supplied with current lenses, but with lettering.

It also takes 52mm filters, just like my kit lens!

I'm not sure how that lens could be better than the 18-55mm DA lens. Mine is one of the best lenses I own, only bettered by my collection of old primes (fixed focal length lenses, eg 50mm, 55mm, 135mm). I have a slightly broken example of the Pentax-A 28-80 like yours (it doesn't extend when you turn the zoom ring, I fished the loose screws out but have yet to dismantle it enough to reinstall them) and to be honest it isn't the best lens of that era.

What you will also find is that the camera wants to know the focal length of the lens for shake reduction, which gets annoying with a zoom as obviously this changes when you move the lens! Obviously this isn't a problem with primes as the length doesn't change. That said, some manual zooms are quite desirable - I have a Pentax-M 75-150mm f4 on the way which seems very well thought of.

If you want an AF 28-80mm lens take a look at this. I have one of these lenses in silver, not the finest but a perfectly acceptable piece of kit.

When buying MF lenses have a good look around ebay. I saw several 75-150mm zooms for £50-£60, but waiting saw me net one for £25. I've even managed to find things like a Pentax-M 50mm f2 for £5 (needed a bit of mould removing but is now excellent) and a 55mm f1.8 for £8.50 (pretty much perfect aside from a little dust).

Anything from before the A series lenses will require you to use some special techniques in order to make it work, as the camera body won't be able to read the aperture from the lens. There are plenty of threads on here about how to do this and it's really simple when you have the camera in your hands.