Not much old, good glass out there at 400mm+ I think, but good luck. Some long Takumars were made, they're probably your best bet but they won't be very cheap and will be very heavy I imagine.

I don't think there should be any harm to the contacts. Just use an M42 body cap when it's not in use. You could always modify this with some plastic or felt to cover the contacts when it's on.

Bump.

Lithiums are incredible. If they weren't so expensive I'd use them all the time.

Normal Alkalines are rated for about 80 shots in my K200D. Alkaline batteries are not ideal for DSLRs as the voltage quickly drops below what is necessary to run the camera.

Eneloops or other low-discharge rechargeable batteries are the cheapest option. If you want to use non-rechargeable batteries then they should be lithiums, which will last for a very, very long time (rated for around 1100 shots on my K200D, I think).

I haven't used all that many but my favourites are all SMC Taks. The 35/3.5, 55/2 and 50/1.4. I was so impressed with the 55/2 I ordered the f/1.8 version too, but I've yet to get my hands on it. Of the three mentioned here, the 50/1.4 is the weakest, despite it's reputation, though I have been using it lately and it's better than I originally thought.

However, the 55/2 & 35/3.5 are truly great in sharpness, colour and contrast.

I had to shim my Katzeye. It's far more sensitive than the normal screen so if you want it to work well you'll probably need to shim it.

That's a very nice collection. Have fun with them.

All K-mount lenses will work with the K5. HOWEVER, there are two things you need to be wary of:

1. Extra long aperture lever protection flanges. I have a Sigma lens with one of these. In order to mount it on a DSLR the flange needs to be cut back to avoid damage to the inside of the camera.

2. The infamous Ricoh pin. This is a pin used on some lenses for I forget what, but it's in exactly the same place as the screw-drive AF on SLRS and can lead to the lens being locked onto the camera and requiring a technician to remove it.

Neither of these is very common and never appear of Pentax-made lenses.

Be aware that there are cheaper options to the Katzeye. Some people like them, some complain. I decided to skip the risk and go for what I understand is the best. Katzeyes need to be ordered direct, unless you can find one second hand, which is rare.

Lens series:

K - first fully manual series. Metal.
M - later fully manual series, functionally identical to K. Metal.
A - Auto aperture. Aperture can be controlled from the camera body, allowing various auto exposure modes, Made of plastic, but quite strong.
F - first auto-focus lenses. Made of plastic but far lower build quality than older lenses.
FA - newer AF, functionally identical to F
DA - As FA but for digital therefore may not cover the image circle of film.
DFA - Updated FA. Covers image circle of film.

Other abbreviations:

SMC - Super-Multi-Coating, increases contrast and light transmission and reduces flare.

WR - weather resistant

I use medium format (Pentax 645) but I live quite far from East Anglia. There are other medium format users on the forum, but I don't know where they live.