I think I'm just about spent up on lenses, so it's time to go out and start making some more images with the ones I've already got.

In a couple of weeks time I'll be putting various Pentax and Olympus classic lenses and bodies on eBay. These will be screw thread and OM vintage and at first glance seem to be very nice indeed. A professional friend of mine has retired and just handed me a huge Billingham bag full of these items.

Every time i open another pouch on the bag some more enticing goodies fall out, but there's been no time to ssess what is there yet.

My bid would be £5, a little short of expectation.

I think if you want straight lines buy a rectilinear lens, and they don't come much better than the Pentax 12-24mm.

The 10-17mm is fine, but trying to straighten it out on a routine basis would be a waste of time and the lens, in my opinion anyway.

Yes, I'd forgotten that one. You're probably right.

At some indefineable point in the past "creepy-crawlies" became known as "mini-beasts" - does anyone know when this strange transformation of language occured?

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It probably is wide open at 55mm and f5.6. If the lens is set on its widest aperture and zoomed, the aperture doesn't close but the f number changes.

A simple 50mm lens 25mm diameter = 50/25 or f2
A simple 100mm lens 25mm diamter = 100/25 or f4

This is why zoom lenses aften have variable apertures, because no correction is made during zooming. optical or mechanical correction is needed to maintain a constant aperture.

The perfect companion to your 18-55mm could be the pentax 50-200mm. Very compact, same filter size, very very good quality

A search of the forum will bring you the full details from previous threads, but in brief summary:

M42 = 42mm screw thread (Takumar, Super Takumar, Super-Multi-Coated Takumar and SMC Takumar lenses)

K mount = K bayonet with no electrical contacts (SMC Pentax, SMC Pentax-M lenses)

Ka mount = adds electrical contacts for communication with electronic camera bodies (SMC Pentax-A lenses. These work fully on digital bodies)

KAF mount = Autofocus lenses

KAF2 mount = Autofocus lenses with power zoom connections and now also the facility to have built in focusing motors.

The exceptions to total compatibility are few, but relate to whether or not the lens has electronic contacts, an aperture ring or in the case of the latest digital lenses whether it covers the APS-C sensor or a full frame on film.

If you are using the DSLRs the basically anything from the SMC Pentax-A lenses onwards is fully compatible. Obviously manual focus lenses are still manual focus!

It seems you have a simple K mount lens without electrical contacts.

The real answer is that if, for whatever reason, the lens does not seat smoothly and easily, then don't try to force it and don't use it.

It isn't worth causing damage to the camera for the sake of an old and cheap lens.

Fair enough, but beginners seeking advice are unlikely to need the complicating detail. Some of the tables of compatibility published are so complicated that a newcomer would be totally dazzled and bewildered.

In essence, DA lenses are for APS-C format DSLRs.

In reality some can be used on film bodies, even the 16-45mm being fairly acceptable. They work well with my MZ-3, but only in P and Tv modes of course.