Is this a preset lens or does it have an auto/manual switch (with the little silver pin visible on the bottom rim of the lens mount closing the aperture down when pressed)?

T2 mounts are an interchangeable lens mount. The lens is standard but the shop will bundle a different mount according to what camera you buy it for, said mount simply screws onto the back of the lens. I have a 500mm mirror lens with this fitting which came with a K mount, but I could easily swap that to an M42 mount or another brand entirely.

The presence of the silver pin suggests to me that this is a fixed M42 mount, as in my limited experience T2 mounts are only found on mirror lenses (with a fixed aperture) or preset lenses (where you close the aperture manually with a second control ring after preselecting an aperture with the normal click-stop ring).

If you can't swap the mount then you need an adapter like this one:

http://www.ebay.co.uk/itm/Pentax-M42-42mm-PK-K-Mount-K-Bayonet-Adapter-Ring-/110...

Note that it doesn't have the flange present in the other link, and will therefore allow infinity focus. I would still advise hunting for a genuine Pentax one though as they are far better made than the copies.

Incredibly lucky to get it that cheap! They didn't have a K 50mm f1.4 for about the same money did they?

I'd say the best value current lens is the 35mm f2.4, the image quality isn't far off the Ltds in the right hands.

Best value classic lens? Probably one of the M 50mm variants. Even the f2 is still a pretty impressive performer despite being the cheap kit lens of its day.

Have a look at the mount on your lens before you buy anything else

Many 3rd party older lenses use a T2 mount which can be swapped, so you can replace the M42 with a K mount rather than adding an adapter. A new T2 mount isn't expensive and it'll be much easier to use. Judging by photos I've found of the Optomax 400mm f6.3 I suspect that it is a T2 mount. Just loosen the three screws in the knurled bit next to the mounting thread and unscrew the mount from the lens, the K mount fits on in the same way (but can be rotated to put infinity focus on top).

Funnily enough, no!

I note that it's from a well-known or possibly infamous seller...

Came across this a while ago after buying a couple of Auto Takumar 55mm lenses (an f2 and f2.2), his collection is rather lovely. I really must fabricate a proper new release pin for the f2 and find an S2/H2 to put it on.

The f2.2 is rather harder. While in better condition it has a black aperture ring, which apparently means I'd need a black S1 for a correct set. Not many of them about...

I wouldn't bother. Marks on the mirror have no effect on images and can't usually be seen in the viewfinder. Plus as others have said you run a serious risk of damaging it.

Thing is, you pay £20 for it and use it until it expires. At which point you have it fully serviced for about £100 and you now have an overhauled LX for less than the price of those with completely unknown history you see for sale...

I seriously considered buying a jammed one with a databack from Rocky Cameras (£50!) a while ago with a view to doing this. Only his dubious reputation put me off.

This is part of the reason I'm not leaping to pay £99 for that 50/1.4. I already have a small pile of M 50mm f1.7s and f2s! The 100mm on the other hand is slightly more tempting as I can see it being very handy as a pocket telephoto to go with my ME Super/40mm f2.8 combination. Only thing is I already have the 120mm which is a similar focal length and has the same max aperture.

Av mode and preset lenses aren't more accurate on my K-m by the way, I have an old T2 mount mirror lens which needs a lot of exposure compensation to work properly in Av. Meanwhile my K and M lenses meter perfectly from wide open to around f11 when used with the green button (well, ok, it's the exposure compensation button but the function is the same).

That'd be why I won't pay the going rate for a lens without seeing a front view then!

I'll only buy unseen (by which I mean lens has caps on making it impossible to check optical condition) if the price is very low. Otherwise I want to see shots of both ends and a shot demonstrating that the lens is genuinely clean internally.

I notice Ffordes have an M 100/2.8 up for grabs for £69 at the moment. It's listed as having "slight signs of oil on iris blades" which they say doesn't affect performance, although I'd be wary.

The classic lens triplet of the era would have been a 28mm, 50mm and 135mm.

Look for anything sensibly priced with Takumar in the name, ideally Super Takumar or SMC/Super-Multi-Coated Takumar as the aperture on these will stop down and open automatically (as opposed to the earlier Auto Takumar where you had to re-cock it, and the even earlier preset lenses where you have to stop down manually before pressing the shutter and open again after).

I'd suggest looking for a Super Takumar 55mm - the f1.8 and f2 are essentially identical apart from the maximum aperture, but if you can have an f1.8 for sensible money then go for it. There's also a 50mm f1.4 but that goes for much more money.

The Super Takumar 135mm f3.5 is plentiful and a fine performer.

For a wide angle look for the Super Takumar 28mm f3.5 or 35mm f3.5.

That will give you a top quality lens line-up, but there are loads of M42 lenses about to try if you can find them cheaply.