Originally posted by elho_cid One year ago I compared the AF speed with slot driven lenses with a friend using Nikon D7000. He also has a D300.
He was surprised to see that K-5 drives comparable lenses faster than Nikon, it probably has stronger in body AF motor. Adding the SR to the equation, it makes sense to try getting these or similar lenses in K-mount...
At that time we compared FA*85 against some 85mm Nikkor. I'm not sure to what extent can this finding be generalized, but I'm curious what are the experience of others.
The in-body motor of the K-5 is very fast/powerful, and could very easily be on par with the D7000 and D300 in terms of pure driving speed. However, I still contend that the D300 is superior in terms of overall AF performance, which of course will depend somewhat on how you're using the camera. I also agree that the FA*85 focuses extremely fast on the K-5. I've used the FA*85 on the K-5 as well as the 85/1.4D and 85/1.4G on the D700, and there's no question the FA*85 is a very fast focusing lens. Keep in mind, it is probably the second fastest focusing K-mount lens I've used (the DA40 being the fastest). At the same time, the 85/1.4D and 85/1.4G are certainly on the slow side as Nikon lenses go.
I also agree that adding SR into the equation does make a somewhat compelling case for switching to Pentax. To be honest, the in-body SR system is what attracted me to Pentax 6 years ago when the K10D came out. However, as the OP points out in the post above, if switching from a D300 to a K-5 leaves him without a budget to buy the lenses he wants (which will no doubt be more expensive in K-mount as compared to F-mount), then it's not really an option.