John Gulliver
Contributor
What counts for the length of the extension is where the nodal
point of the lens is, not the dimensions of the lens.
This is beginning to get too technical for me! Apart from the (important) fact that the dimensions are almost exactly the same, so that the lens will fit in the port, I was assuming, based on simple multiplication, that a 10-20mm digital lens was optically the equivalent of a 17-35mm analog lens. Is it more complicated than that?