F5.6 i think which is pretty good for a long lens that dosnt cost thousands.. sounds like a very nice buy..

the *ist DS has a reasonable ISO performance which is basically what makes these lenses very usable.. the shake reduction on my k100d makes them even more usable.. sr and a usable nearly noise free ISO 800 makes all the difference..

definitely a tripod job with tele converter on thow.. i doubt the feasability to be honest.. a 2 x tele will lose 2 f-stops and i think the auto focus as well..

trog

ps.. i dont have the lens mentioned just two or three third party long zooms.. a tamron 18-200.. tokina 35-300 and a sigma 28-300.. the first one is digital only and a nice walk around lens.. the second two are older film lenses and new but cheap experimental bargains.. he he