Attempts to dry out a damp tent on the wet grass outside have been called off. It's now in the study. As it's a pop-up tent it doesn't need to be pegged down to keep it upright, just about a two cubic metre space.

It comes down to a simple calculation - how much did the lens cost, how much will it be worth when cleaned, and how much does professional cleaning cost?

In the case of a 50mm f2 with fungus costing £5 from ebay I principally bought it to learn how to fix lenses. It was really simple to fix and would probably go for £20+ on ebay now if I sold it on, although I have no intention of doing so as I bought it to go with an uncommon chrome finish MV1 which I'd restored.

In the case of my Pentax-A 200mm f4 which turned out to have some fungus I sent it to a professional, as the lens had cost £100 and I didn't fancy fiddling around with it.

You can never have too many 50s!

Between my Dad and I we have the full set of silver K bodies, I have the KM, K1000 and KX while he has a K2 that he bought in the late 1970s with M 50mm f1.7. The original manual and (I think) the receipt are still with it. I like the KX, although I'd be hard pressed to choose between it and the MX. The MX's LEDs are easier to read in low light and it's a bit more flexible in that you can add a motor wind and change focusing screens.

I have seen a black KX for sale (was going to bid but forgot), apparently there are black KMs out there. There were also motorised versions of the KX and KM which must be even rarer than the K2 DMD.

One thing I've never been able to figure out: The original Pentax K (like that one) has an M42 mount. Where does the "K" come from?

It's always a risk, and I wouldn't open anything particularly rare or expensive. My A 200mm f4 was sent to a professional to have fungus removed but I'd happily open up a lens I'd paid under £50 for now. Just don't force anything and work logically. Lighter fuel can be useful for freeing stuck threads, it evaporates to almost nothing so you won't have to clean it up afterwards.

One remaining snag with the 28-50mm, I can't work out how to get that funny circular plastic bit which goes around the rear element out (and thereby unscrew the rear element to get the last bits of fungus out). I doubt it makes much difference to the pictures, but I'd like to remove it as it just feels wrong to have a fungus-infected lens around.

I more than remember the ME Super, I've got two of them (one silver, and just bought a black one). Lovely shutter sound and a pleasing weight, although the MX is probably tougher.

Jessops have a very poor reputation hereabouts, with good reason from what I've seen and heard of them. They've gone from being a photography shop to an electronics retailer who stock cameras. IMO the rot set in when they stopped selling used gear, as there was no longer any need for the staff to be knowledgeable about the product beyond what they could learn from the box!

I wish to cast no aspersions on either side, however...

It occurs to me that proclaiming the negatives not to be genuine would be a smart move on the part of someone whose livelihood depends on their control of such a lucrative source of income as the proven genuine negatives they own. I would have been more surprised had he said "Yes, those are real, my grandfather must have left them in storage somewhere" as this would further authenticate negatives with the potential to make their owner tidy sums of money.

The "about 19,600" is a reference to the original Asahi Pentax, which is indeed rare and collectible in good condition. This however is a K1000 which was made in vast numbers, judging by the photo I'd say it's the same age as mine, which I think is probably one of the early Hong Kong versions.

The KM and KX I have must be far, far rarer as they only made them for two years. I paid less than half that for both of them together!

As the links should show, generally you unscrew the finishing ring/nameplate after which you can unscrew the front element group. The dust is usually between the front and rear element groups as this space isn't airtight - the aperture operating lever goes into it.

Do not disturb the focusing helix or the infinity stop as otherwise you'll have to re-set this, which is annoying. Assuming the 24mm is basically the same design as the 50mm then you shouldn't need to. I've dealt with a couple of lenses of late (K 28-50mm with fungus and a Tokina 28mm with a dislocated focusing ring) where there was no way to avoid disturbing these parts and I really wouldn't care to open either up again!

You might want a lens spanner like this, it makes it much easier to unscrew the notched rings used to hold some lens parts in without risking slipping and damaging the glass.

I wish I could find a 24mm for £45, with or without dust! I saw one go for £60 a while ago with a declared fungus problem, I was very close to buying it but felt the price a bit daft for a lens with a fault.

I'm not sure about the K 24mm but I've opened a few 50mm and a 55mm lens before now to blast dust out, it wasn't too hard. You will never get it completely dust-free unless you have a clean room, mask, hairnet, white coat, etc. I have never had a 24mm in my hands so I've no idea how it comes apart - hopefully someone else will turn up who has done so! I find a can of compressed air (air duster) very effective for this. I even managed to blast most of the dust out of a zoom without opening it up, just by putting the air duster nozzle into suitable gaps and giving it a few good squirts.

The K lenses usually produce cooler tones than the more recent glass, I always put it down to the SMC coating recipe having changed over the years.