Diopter w/EF 17-40?

Hello there

I'm completing the build of my UW setup, and have a brief question. I have a Canon 5DII with a 17-40mm lens in your housing behind the 8" dome and 5510.24 port. I notice in your "ports" page you don't say anything about the use of a diopter with this particular setup, yet I've read some posts about people using the diopter in this situation.

Could you let me know what your thoughts are regarding the use of a diopter here? Thanks so much,

John

John,

A diopter moves a lens's minimum focus distance closer to the camera, so infinity also becomes closer. If a lens does not focus down to 12", an apparent image cannot be created on the dome underwater. Since the 17-40mm lens focuses down to .9', a diopter is not needed. Some people may choose to use a +2 diopter which can improve edge sharpness at certain focal distances. We do not test for this since zoom lenses have floating nodal points. This means that the sweet spot or optimum focal point (nodal point to lens apex) must be found while using the lens. If you feel the lens is softer at a preferred focal point, then the +2 diopter may help. If you are shooting subjects beyond 6' or over/under shots, then the diopter would not likely be a benefit.

To summarize, don't worry about the diopter unless you feel your images are slightly soft on the edges. Remember also that closing the aperture down one or two stops can help alleviate this problem.

Regards,

Wonderful info, thanks so much :-D