Front focusing means, when the picture is taken, the area of sharp focus is at a point in front of area where I focused it using the AF point.
back focusing means the area of sharp focus is behind the AF point location.
It happens with some 3rd party lenses. Because they pretty manufacture the same lens and put on different mounts after. there are 2 ways to calibrate the AF, one is to do it in the camera using AF micro adjustments (called "AF tuning" in Nikon) which are usually available in higher end camera bodies, or just get the lens adjusted physically (by a technician or service center). Some brands do not really come with this problem, probably they have a process that will calibrate the lens according to the lens mount it is being manufactured in. This will definitely increase costs, and that cost will translate to higher price. For Sigma, no problems, just bring your lens and your camera into sigma SC for a free calibration and they will adjust it nicely for you.
Bought on 30-Mar-10. I could say very very good price cos I'm trying my luck with my offer and she say okie. Almost the same price if I get in HK and with this filter she gave me, its better than HK price now. wahahaa. ;p;p
