Well it does have a couple of small chips...

Only snag is that due to the need for a delay between removal and replacement the insurance would definitely smell a rat and refuse to pay for it!

I've done this with a dire Jessops-branded 2x PKA-fit TC. Given I paid about £20 for it over ten years ago I wasn't too worried about value, it works out cheaper than a PKA extension tube. I did test it with my A 200/4 (the best long telephoto I own) and found that images taken with the lens alone and then cropped were better than images involving the same lens and the TC!

You need to look at how the elements are held in. On the Jessops TC they were part of a solid module rather like a loupe, screwed in with a locking nut to stop the module from shifting. I just unscrewed the locking ring and then unscrewed the element module, leaving me with all the auto contacts and aperture lever completely untouched.

I'm waiting for a quote for a new windscreen too.

The last owner evidently had it replaced, but the monkey responsible damaged the paint where the sealing goop sticks the screen to the body. This is now bubbling visibly and needs to be resprayed, but doing so will need the windscreen to be removed. Can't do it on insurance either as it isn't actually cracked, but probably won't come out intact. So the old screen needs to come out, then the paint damage needs to be repaired, then a new screen needs to go in.

The only other lenses I'd like (if they come up for sale when I have the spare cash to buy them) are the K 50/1.4 and the K 300/4

Given how infrequently either appears for sale I think my wallet is pretty safe!

K 28/3.5
DA 35/2.4
K 55/1.8
DA* 50-135
K 100/4 macro
K 120/2.8
K 200/4

I've worked out why it's seven. One fewer than I'd actually like! This kit would keep both a digital body and a KX or MX happy.

I managed to limit LBA pretty effectively by deciding only to buy K series lenses. This tends to mean higher per-lens spending but you don't buy as many.

As for bodies I'm only really interested in M series or earlier, although if a nice P30n came up cheaply I'd probably buy it to complete my P30 set.

Now have the K 100mm f4 macro. It's a bit good...

The 35/2.4 is excellent, definitely the modern version of the old 50mm f1.7 as a reasonably priced standard prime. It will not be embarrassed next to your 50-135mm - I have both and they're a good match.

I have the original K 100mm f4 macro and TBH the only compromise I've noticed is the comparatively slow maximum aperture (which is still faster than most current consumer zooms at that focal length). It's a relatively compact and incredibly sharp telephoto as well as being a macro lens.

I'm not quite sure what you mean - are you talking about picking a lens in good condition from a shop's used stock?

Take a small LED torch and your camera with you.

First inspect the lens. Is it clean? Are there dents in the filter ring or lens barrel? Minor paint rubbing and "shine" on plastic parts is normal but dents suggest the lens has been carelessly handled in the past and may be best avoided. Don't be put off buying a lens with paint wear if the price is right, so long as the optics and functions are perfect. It can be a useful bargaining tool though if you're haggling!

Next look at the glass - check for scratches and rubs on the front and rear element. Open the aperture as far as possible and shine the torch into the lens to look for fungus. You will always find some dust in older lenses, but it doesn't usually cause trouble.

Check the focus and aperture rings move correctly. The focus should be smooth and not overly stiff, the aperture should click precisely. Close the aperture right down and flick the lever on the back of the lens mount to check that it opens and closes freely. Check for oil on the blades which will make the mechanism sluggish and will cause exposure problems. On an "A" lens push the button and set the aperture to A, a small pin should pop up from the flange of the lens mount (like the contacts on your kit lens). This is how the lens communicates its aperture range to the camera. The pin should retract when you move the aperture ring from A.

Mount the lens on your camera and check that it will focus properly, if it has been poorly serviced in the past then it may not be able to focus on infinity. It's easy enough to fix but again, either a deal-breaker or a bargaining tool depending on price and whether you can sort it out. The lenses you're after are common enough that you can wait for a perfect example. Try a couple of test shots to make sure it works on the camera.

Yep, for use on a DSLR A is preferable to M.

The only less-desirable thing on the A lens is the plastic aperture ring, but on a DSLR you set this to A and leave it alone thereafter so it won't bother you at all.

The focusing on both should be ultra-smooth, so have no worries there.

To give an idea of prices, I've paid anywhere between £10 and £30 for an M 50mm f1.7 (the latter included a tidy chrome ME Super, the former needed internal cleaning to remove mould). The only A version I have of the f1.7 cost £42 alone, at the time this was relatively cheap as it had been poorly listed.

The f2 variants are cheaper, but not quite as good as the f1.7. There isn't much in it though, especially when you stop down a bit. I have shot with a fungus-ridden 50mm f2 I bought for £5 and cleaned the mould out of, the resulting images were perfectly acceptable.