like most (all?) zooms that extend, my Canon 17-85 nodal point changes with focal length -- about 1 1/2 inches between 17mm and 85mm
is it best to optimize dome positiion toward the short focal length or long for general use - that is as a zoom lens not knowing in advance what focal lengths will be used.
Dr. Mustard has almost convinced me to take the lens to Raja Ampat. I was never very happy with it's results in the past, however, i've since discovered that Aquatica's extension recommendations for domes are not necessarily correct (at least in accordance with "dome theory").
I do not intend to spend dive time on the trip doing lens testing
I use the 17-85 lens behind a Sea&Sea fisheye dome port and SX extension with acceptable (to me) results; the edges are relatively sharp. But I do have some problems focusing when zoomed out and up close but it it's probably because I am not using a diopter. If I continue to use this lens I will be buying one.
Ben
okay, did the numbers and the dome location with Aquatica's recommended spacer is "correct" at about 22mm (the aquatica spacer is 1.4 in, and 1.7 is theoretically correct at 17mm). as the lens is zoomed, the nodal point recedes toward the lens mount as the front elements move out. in any case, lens clearance zoomed required the spacer and the nodal point is too far from the front of the dome by 1 1/4 inch or so and the front element is quite close to the dome and not much of the dome is being used. probably acceptable with a +2
