AF is Anisotropic Filtering. It's a system to sharpen textures. Best result is when you view a texure on a surface from a very low angle, ie as if you are very near it looking along the polygon. This means the texture will appear very differently from how it was drawn by the artist so chances are it will loss detail and look fuzzy. AF is a system to counter this. There are various different settings like with AA, with each higher setting looking better but running slower. I think it's powers of 2, so you'll have 2xAF, 4xAF, 8xAF and 16xFA.
With a Radeon it's enabled under the Direct3D and OpenGL tabs of the advanced display control panel. Don't know how to enable with an nVidia card, sorry.
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