Originally Posted by Darius4522 
I have a little money to spend on a lens, but am uncertain which one to buy.
- The Pentax 50-135mm (possibly a little short at the long end). $1,600AUS ($1,100AUS+postage from Hongkong)
- The Sigma 70-200mm (longer reach) $1,398AUS ($998AUS+postage from Hongkong)
- The Pentax 60-250mm (perhaps longer than I really need). About $1,200AUS+postage in Hongkong, but have no idea of the price in Australia....
Any other suggestions would be gratefully accepted.
But I want, for the money, the best IQ I can get...
I mainly shoot tourism, landscapes and street photography.
My price limit is around $1,500AUS (about $1,200US).
Darius, from your list I read, that you are explicitly looking for the tele photo range. So I won't suggest a 16-50/2.8 or the like. I personally have a Sigma 70-200/2.8 (pre-HSM, pre-Macro version), which is simply excellent. It is one of my two mainly used lenses (the other probably being now the Pentax 16-50/2.8) It is ideal for street scenes, if you prefer a bit of shooting distance (as I do). You can always add a tc to get even longer, and arrive at basically everything the Pentax 60-250 would provide, except on the shorter side.
With the original Sigma 1.4x tc you would be very hard pressed to find any loss of IQ with the 70-200 - I never saw any difference in my pics. But currently the HSM function is not supported by the Sigma tcs, as they rely still on the old scredriver AF.
I never even seriously contemplated the Pentax 50-135, as I find it too short a focal length. If I need a small, unobtrusive lens, I use primes and not a zoom at all. So if you go for the Sigma, an FA 35/2 would be a cheap and very sensible addition to it (as you already have the 50/1.4)
Ben