Originally posted by nzcjs Also, how awkward is it to use the older K-mount lenses?
I have a K 55 f1.8 which I use occasionally. A full manual lens like that (or an M series lens) will only work in manual mode, and the aperture will remain wide open all the time, unless you press the metering button (AE-L on a K100/200, Green button on a K10/20), at which point the camera will stop down, and meter.
If you want an 'almost' auto metering experience, you can use the lens wide open (and wide open only) in Aperture priority mode, and that works as expected. This is particularly hand for faster primes.
Please note that there is an entry in the "Custom" settings which must be changed to allow the use of lenses with aperture rings. If you have any Pentax A series lenses, they will operate automatically in aperture priority, and shutter priority modes, do not require stop down metering.
If you want to try it out, the guys at Auckland Camera Centre in New North Road (who I have no afiliation with by the way) are pretty helpful, and last time I was there had a couple of K20s, and some older second hand lenses as well.
With regards to your flash, as Class A mentioned, older flashes tend to have high trigger voltages which can damage modern SLRs, and the K series manuals contain warnings about this. However, the K20 has a PC-Sync terminal, and there is speculation that may be able to cope with higher voltages.
If you'd like to play it safe, you can buy various voltage regulating buffers, such as this one:
Camera Flash & Acc - Flash Controls, Cables & Accessories - SMDV SM-512 Safe Camera Hot Shoe Adaptor for fairly modest sums.
(I am not recommending that, I have no experience with it, it's simply what I found with a quick search)
Also, on the subject of the 16-45, it is worth noting Pentax has since released the DA 17-70 f4, which is a bit longer. However, it is also more expensive, and seems like a bit of an unknown quantity.
I see that we're both in Auckland though, so if you'd like to have a quiet play with some Pentax gear outside of a shop (I don't have that much though), drop me a PM and we might be able to sort something out.