The 35 2.0 FA is aparently out of production. However Park Cameras still had some stock a couple of weeks ago or so when I got mine. Personally I am very pleased with it. Optically it has a fantastic reputation, and it has two advantages over the 31 (aside from being much cheaper) - the removable hood means I can use my Cokin fiter system with it, and it is significantly smaller and lighter. I suspect it will be on my camera as much as my 16-45.

I also discovered you can leave the hood in place even when using the flip-up flash - it is small enough not to cast a shadow (though deeper than the hood on the 31).

Just seen this...

http://www.dpreview.com/news/0706/07060101pentax50thdslr.asp
Looks quite fun (like the 'takumar' version of the DA 40mm limited)

But it isn't going into production

The Cosina 19-35 is in fact a suprisingly good lens for the money. When I bought my DS I bought the body and used this as the standard zoom (until I go the 16-45) and the results were good. I had had good results on film with it before, and it has had several reviews which said it was much better than expected for the money (I think it was about £109 new - astonishingly cheap for such a wideangle lens) and pretty much held its own against considerably more expensive competition. It is a pit plasticy but you can't have everything. I still have it to get ultrawide on film.

The FA 28-105 3.2-4.5 is hardly "incredibly heavy" - it is about the same size and weight as the 50-200 and lighter and smaller then the 16-45. It is useful as a sort of midrange zoom for parties and snapshots - having a pasable 'wide angle' of 42mm equiv with a reach up to about 160 equiv - about right for candid portraits. (24-90 would be better but it is bigger, more expensive, and I don't have one!!)

It is also a pretty good lens!

Pentax did do an FA 24 f2 but they hardly ever turn up second hand (too good to part with I believe)

What about the 21mm f3.2 limited??

Sigma do fixed, fast, good 20, 24 and 28mm lenses

Well, there are plenty of pentax 28mm lenses out there - A pentax SMC M 28mm 2.8 in good condition. shouldn't set you back more than about £40. Third party lenses can be had for much less and some of them are quite good. The joy of Pentax is if you buy the old manual lenses they will work on the newer digital bodies (though because the digital sensor is smaller they don't cover as wide an angle on the DSLRs)

If you want to go wider and keep the budget down your choices are less clear. The Pentax SMC 24mm 2.8 and SMC M 20mm 4.0 lenses both fetch about £150 used and are relatively rare. The Fisheye 17mm is rarer still (though I have one!)

On the other hand there are a number of their party 24mm lenses which can be had for reasonable money. Both Vivitar and Kiron produced a 24mm F 2.0 lens. You should be able to pick one up for less than £50. Both have a good reputation (in fact they may be the same lens). There is also a flat field (not fisheye) 17mm manual lens - I think by Tokina or Tamron, and a Russian 16mm fisheye.

If you go digital and want ultra wide it is going to set you back a bit - the cheapest ultrawide lens is the Sigma 10-20 (which is equivalent to 15-30 on a 35mm camera) or the Pentax fisheye 10-17 - both of which are near the £400 mark and that is before you buy the camera. The standard zoom with the Pentax DSLRs is a not very distinguished 18-55 (equivalent to 27-80). The (much better) 16-45 (24-70) is just short of £300.

If you do plan to switch to digital in the years to come it is worth looking for the 'A' series manual lenses as they are easier to use with the digital bodies - but as a consequence they tend to be more expensive.

I have the SMC A 2.8 which is also 1:2. I've had no problems with chromatic aberation, ghosting or flare with that lens.

Magnification - at 1:2 you fill the frame on a Pentax DSLR with an image which is about 47x31mm in size. On a 35mm camera (where full frame is 36x24) it would take a lens capable of a magnification of 1:47/36 or approximately 1:1.3 to acheive the same result. So it is like using a lens of that magnification on a 35mm camera (only the depth of field is somewhat improved - by about a stop)

As to larger magnification - tubes or even a bellows is the way to go I would think - macro lenses are designed for this. I think Pentax even used to do a dedicated tube for the 50mm macro to make it 1:1 (the 50mm auto extension tube acheives that I believe)

I have the Sigma 170-500 (pre DG) which I am quite pleased with (though I have not used it that much as yet) Used they can be had for about £250. They are significantly lighter than the 'Bigma' (1345g vs 1842g)

http://www.sigma-imaging-uk.com/lenses/telezoom/telezoom.htm

Reviews:

http://www.ephotozine.com/equipment/tests/testdetail.cfm?test_id=389

http://www.ephotozine.com/equipment/tests/testdetail.cfm?test_id=431 (this is the 135-400 but I suspect the perfomance is very similar)

Also worth considering are the Tokina 100-400 (which lacks a tripod mount and may be the same as the Cosina) and the Tamron 200-500 (but which according to some reviews suffers from Chomatic Aberration.)

Review:

http://www.ephotozine.com/equipment/tests/testdetail.cfm?test_id=386

Actually I got a SMC F 35-135 for -£30, bundled with another SF7 and an AF400 FTZ flash - though I guess I did have to buy a battery for the camera and give it a field test with a roll of transparency film so I could sell it as fully functional, so I ought to factor in another £15. Bundle cost £70. I sold the flash for that and the body for £30.

I've stil got the 35-135 but having now acquired the 50-200 it is somewhat redundant so it may appear on ebay in due course. Not a great lens but perfectly servicable.

Just to annoy people...

I got my SMCF 50 1.4 a few years ago as part of a 'job lot' with an SF7 and Sigma 400 5.6 (AF but sadly not the apo one - however I got a few good pics out of it), a Sigma AF 70-200 of no merit and the SMC F 28-80 for £200. I was only intersted in the 50 and the 400 so I sold the 70-200 on ebay for about 30 quid, the camera and the 28-80 to a friend for a very reasonable £70. Later I sold the Sigma 400 on ebay (after acquiring a 170-500) and got £90 so my 50 1.4 cost me a tenner...