Originally posted by TenZ.NL IMHO a comparison between the K3 and the 7D would be apples vs oranges, not only because they don`t share the same lenses but also because of the ageing 7D. Even when it is a draw, that`s quite impressive for an allmost 5 year old body
The 7D still beats the 70D for continuous auto-focus and burst mode. If the Canons you were referencing as being miles ahead in AF are Full Frames, then I won't argue. For APS-C, I don't believe it.
Originally posted by TenZ.NL Well, it slows down because you`re loosing light. A 1.4 T/C will cost you 1 stop of light, a 1.7 TC costs 1½ stop and a 2x TC will cost you 2 stops so a DA*300 with 1.4x TC will behave like a 420mm F5.6. From what I understand is that most AFsensors work upto F5.6 (except a few high-end bodies that will go upto F8) and when going over that limit you will loose AFcapability.
You are talking about Canon. Pentax AF is not limited to f5.6 or f8 and will work beyond that. With a DA*300, you lose a stop, from f4 to f5.6. That may affect whether the camera can find focus in some extreme or marginal condition, but not the speed at which the AF moves. The main complaint with SDM is that it moves the lens elements too slowly, especially with the portrait lenses; DA*55 and DA*50-135mm. I'm pretty certain the DA*300 is as fast as a screw drive or competitors' standard micro-motor lenses. It is slower than expensive ring drive lenses.
Note that when stopping down a DA*300 to f8 with a TC mounted, the effective f-stop is f/11, but the focussing is done with the aperture wide open, at f5.6.