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Another blurry edge question......

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#1 EddieR


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Posted 22 July 2012 - 11:10 AM

Here is my dilemma. I usually photograph people underwater, so my subjects aren't small. My shots are tack sharp in the center of the image, but blurred around the edges. I shoot with a Nikon D200, in an Ikelite housing. The lens is a 10.5 mm and the port is the Ikelite 6" dome. I understand that the port isn't ideal and is probably the cause of the problem and that a switch to an 8" dome along with the proper extension ring is suggested (by Ikelite)

My issue with this suggestion is that at this time, I am unable to afford the dome or the extension ring that it requires. Is there another (less expensive) option?? I've tried looking for info on diopters, but apparently the 10.5 mm will not accept a diopter. Has anyone tried to adapt a diopter successfully to a 10.5? Will using extension rings with the 6" dome work? Is there another wide angle or fisheye lens ideally suited to the 6" dome? I'd appreciate any help. I'm tired of shooting my subjects at a distance and cropping out the blurry parts.

Sincerely Ed

#2 Lwang



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Posted 27 September 2012 - 07:57 PM

A dome port creates a spherical virtual image in front of your lens, but your lens is designed to focus on a flat plane, thus when your center is focused, the edges would usually be less focused.

You can shoot with as small an aperture as possible so you can get as much depth of field, plus, instead of using AF, which would keep the center sharp, calculate the DOF for the given f-stop and focus your lens so that the center would be right at the extreme end of the DOF so as much of the non-centered part of image will be within the DOF.

Or, you can shoot with a flat port, where your focus distance would be in feet instead of inches.

#3 jcclink


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Posted 27 September 2012 - 08:02 PM

Does the 6" dome also require an extension ring to properly locate the dome center of curvature at the nodal point of the lens? Check the geometry of the combination.

Google turned up the following for 10.5mm lens nodal point
46mm from base of lens
80.5mm for center of tripod screw hole at base of camera body

Edited by jcclink, 28 September 2012 - 11:54 AM.

Nexus D300, 10-17mm, 12-24mm, 17-55mm, 60mm, 105mm VR
S&S YS110's & YS27's

#4 tdpriest


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Posted 29 September 2012 - 09:11 AM

Using a 10.5mm lens on a cropped sensor at a reasonable f-stop offers the greatest depth of field of any optical combination and is less affected by the choice of dome and extension than a mid-range wide-angle lens would be.

The arrangement of subject around the lens will affect what is in focus: an few examples might suggest a different way of framing the images, arranging the subject on the curved focal plane...

Edited by tdpriest, 29 September 2012 - 09:13 AM.