Hey Dennis, the DMK 21AF04 was $490US when I was looking to buy. The DMK 21F04 was $390US and both appear to have dropped $100US.
Have a look at the specifications via the links, but I was told by the Imaging Source people that the "Af" version had significantly improved noise properties - and that that was the distinguishing feature - which I regarded as important for our sort of thing!
. Improved noice was meant to be obatined in the way the thing was put together.
cheers,