diopter for canon 17-40

what diopter for canon 17-40 behind an 8 inch dia dome?

For which housing and camera? More details would help you.

20D in an aquatica housing

don't really understand why the camera and housing should make a difference in theory. if the "nodal point" is located pretty nearly to the center of the dome, then don't see why it would make a difference whether or not the camera is ff or croping -- but then i've never heard a convincing explanation for choosing a diopter for dome other than trial and error either.

Hi Stever,

I did some testing last summer with my 40D and the 17-40L. Results and comments are here;

http://wetpixel.com/forums/index.php?showt...;hl=Canon+17+40

Steve

thanks for the link, should have done a search

i have the +2 and a bit more than 50mm extension which Aquatica recommends and i calculate to be pretty close so i can add the +2 and see what happens

i last used the Tokina 11-16 without diopter (because i forgot it) with pretty good results (appears to have about the same nodal point as the 17-40 -- typically internally zooming lenses don't shift the nodal point like external zooms)

saw a mention in the other thread about lift from the dome - i've added about a pound of lead to hold it down

i've been testing the 17-40 on land and although it's a very sharp lens, corners are much better at f8 to f11 (even though the center falls off a bit on a crop frame camera from diffraction) which is pretty typical - particularly of zooms.

i'd still like to hear some theory on what diopter - if any - to use when