beavo451 said:
According to the titles, the first lens has Internal Focus while the second does not. IF implies that the lens length will not change when focusing.
Yes, but since this is a very very new lens and they only make one type of this lens, I'm guessing Amazon is actually referring to the same lens, although it may not seem like it.
The "DC" means that this lens can only be used on the smaller sized APS sensors. The lens was designed for smaller sensors so that they could make the lens physically smaller, which is fantastic.
Of course there's a negative side to this. Firstly, if Nikon were to ever come out with a full frame (ie: 35 mm film sized) sensor, and you bought that camera, the Sigma 17-70 mm lens you're looking at would be useless, because it wouldn't work. Actually, it WOULD work, but you'd only get a photo in a part of the frame, and a pitch-black corners because the lens is too small to spread the light over the entire sensor. Call it "vignetting" if you want.
I will only buy lenses designed for 35 mm film AND digital sensors so that when Nikon goes with a full-frame sensor, my lenses can still be used. I made an exception when I bought my Tokina 12-24 mm f/4, but that will be very easy to sell.
Secondly, corner sharpness is incredible on digital DSLRs with the smaller APS/APS-C sized sensors when coupled with a larger lens designed to be used on digital and 35 mm film cameras. Why? Because the centre of the lens is the sharpest. Since the lens is, in a way, too big for the sensor, you're using mostly the centre of the lens, and getting sharp sharp photos as a result, and with less vignetting.