Sorry to be a bore, but what is CFD, google didnt give me the answer. Close focus something I guess ???
Short question, long answer.
CFD=Close Focusing Distance, the closest distance which the lens is able to focus on. It is important when using a dome port, because the port 'creates' a curved, virtual image some distance in front of the dome, which the lens focuses on. The distance to the virtual image is aproximately double the dome diameter. So ideally you want a lens that can focus closer than the distance to the projected virtual image with no add-ons. The things to play with is getting a larger dome, extensions to move the dome and image further away and then dioptres that alters the cfd-capabilities of the lens. I picked up the Sigma 10-20 because it simply didn't need any add-ons. I could use it without extra stuff and get good results (at least to my standards). This off course differs depending on what equipment you have and - what your standards are. Also worth mentioning, because the image is curved - it is always a bit tricky to get acceptable sharpness in the edges and corners with rectilinear lenses behind dome ports. Shooting a higher aperture is normally needed. Dioptres certainly can help, even if your lens is already within 'range'.
/christian
Another example of 10-20, in blue water:
