I had a play with one at SRS back in march, based on very limited fiddling it was nearly as good at 250 as my FA80-320. That doesn't make it stellar, but it makes it usable and highly impressive for the range.

I believe the only thing that makes it clearly not of Pentax heritage is its lack of quick shift manual focus override. It is the only DA which lacks this.

the only other thing I can suggest is that the aperture stop-down lever on the lens can sometimes become ever so slightly bent. If that is the case then it doesn't contact properly with the stop down coupler on the camera.

TBH though, that should only effect one lens, and I think it should stop the lens opening up rather than stopping down. I'm afraid it sounds like theres a fault with your K100D which is preventing it from going into M mode with an old lens attached. It is behaving as you would expect if it were in any mode other than M, but M is not engaging properly.

As a last ditch, have you tried resetting your camera to factory defaults then just changing the Aperture ring one to "Permitted"?

I agree with John, the OOF eyelashes in particular are distracting, I would like to see more depth of field. Great idea and nicely focused.

So called K series lenses are the original K mount lenses which arrived with the K1000 and friends in the mid 70's. Mechanical only, generally optically good for their time.

M series lenses get an M suffix on their name, eg Pentax SMC-M 28mm F2.8. These came along a little later to coincide with the M series of cameras, ie MX, ME, MV, MG, ME-Super etc. They are still fully mechanical with no electrical contacts at all. They are generally smaller and lighter than K series lenses (although one or two are just re-badges of the K series versions). Optical quality again is generally very good for their age. A few are not quite as optically good as the K series, having suffered from the efforts to make them small and light. For example, the K series 200mm F4 is said to be slightly better than the M 200mm F4.

A series lenses match up to the A series and P series cameras. These are the last non AF Pentax lenses. They gain electrical contacts which allow matrix metering, shutter priority and program modes. Crucially for us, the contacts allow aperture control from the camera body, making the A series fully usable on Pentax DSLRs.

Edit: oops, John types faster than I do!

At that price it would need to be truely stunning, since the Pentax DA12-24 is cheaper!!

As mentioned above, the 12-24 is not a * lens, but don't let that fool you. From an optical standpoint, it should be. My Dad has a K10D with 12-24 and 50-135 for architectural work and the 12-24 is the sharper of the two.

I have no real idea of the price for an A* 300, but my Tamron F2.8 300mm cost me £350 without a mount, I would expect a genuine Pentax and a * lens at that to cost a LOT more.

Since going digital I've been operating with the 18-55 and an 80-320. This is a rather larger gap than you're looking at, and has rarely been noticeable let alone a problem. I am now using a 16-45 and 80-320, I hope to get a 55-300 at some point soonish, but the gap doesn't often worry me.

now THAT was a bargain, you could legitimately charge that much for either the mount or the lens!

I'd still pay the extra £5 and get it from SRS, but either way its a great lens for the money.

Still since the OP was talking about scraping together the £50 for a 50-200, it seems unlikely he'll be able to find another £150 down the back of the sofa for a 55-300.

I'm going to guess the DA* 50-135?

very very nice whatever you were using.