Originally posted by offertonhatter I tried both AF-C and AF-S but no difference,
However I thought with moving objects, AF-C would be better, mybe I am wrong.....
I had a look at one of the otter shots you'd posted on FLICKR. From what I gather, you were using the Sigma lens as the exif showed 70mm, f/2.8, 1/1000 second and ISO 500.
A couple of things to try:
Stop the lens down a bit. At f/2.8, you don't have much room to keep things sharp, whereas at f/5.6 you'd have some forgiveness with the added DOF and your shutter speed would still be fast enough to freeze some action.
Were you set to continuous drive, or single shot? (when you were using AF-C AF mode) Sometimes rattling off a sequence of 5 shots will be enough to ensure one of the shots is in sharp focus. It's generally how I get shots of my dog running at me head-on.
Lastly, I'd suggest something just a little bit longer. Sometimes focus is a bit ahead or behind the subject even though it looks good in the viewfinder. Being able to go to 100mm might might be enough for you to verify a more critical focus than 70mm would allow you without compromising your framing too much.
Originally posted by flyer If you can invest in some SDM lenses, the autofocus will be as fast as any other make. With moving subject, I have good results with the center sensor (or any other you wish to select, as long as it is not on auto) and AF-S.
I'd wager that largely depends on the lens being used and how the AF is programmed in the camera (for lack of better terminology) as opposed to SDM (generically) being faster. I've seen conflicting reports by pentax users about the speed of the 50-135mm and from personal experience, the 16-50mm is close enough to the same speed as the DA 14mm that I don't worry about it.