I am looking for some advice on dipoters for wide angle lenses.
I have probably done something simple fairly obviously wrong and hope someone can put me straight.
I am using a Nikon D70 in a Sea & Sea housing and have recently bought a Nikon 10-24mm zoom lens to replace the Sigma 10-20mm I had been using.
I have two dome ports which I can use with the Nikon lens, the Sea & Sea Compact Dome Port with the SX Extension Ring which Sea & Sea list as being the correct port setup for this lens, however the front ring of lens makes contact with the inside of the dome at about 20mm focal length. I also have the larger Sea & Sea dome - the acrylic one not the optical glass one. The lens fits behind this without making contact with it so I use this one instead.
My problem is that I am getting very soft images which I wasn't getting with the Sigma. The Sigma just had a little softening in the corners but careful composition dealt with that. Perhaps significantly, the Sigma had the same port setup listed as for the Nikon (Compact Dome with SX Extension Ring) but it actually used to fit behind this so I used the smaller port with the extension ring rather than the large port.
I also have the Nikon 10.5mm fisheye lens and don't have any problems with this lens when used with either of the domes (obviously without the extension ring).
My understanding was that softening in the corners of an image was caused by the failure of the lens to focus on a part of the virtual image that was closer to the lens than the centre of the virtual image and that the addition of a diopter improved the close focussing ability of the lens which addressed the problem.
I bought a +4 dipoter for the Nikon lens but it seems to be making matters worse as it introduces distortion into the image without improving the softening around the edges. With the thickness of the diopter, the front of the lens makes contact with the inside of the small dome at about 16mm to 18mm focal length so I have continued to use the larger dome.
I have attached a couple of examples of the problem.
The shot of the two divers was taken before I bought the diopter and the softening is clearly visible around the diver on the left and in the bottom left corner.
The shot with one diver in it was taken with the dipoter in place and the distortion is very noticable.
The 10-24mm lens is nice and sharp if I use it topside, but the diopter introduces distortion.
I calculated the dioptre strength from a spreadsheet I downloaded from the Cameras Underwater website, but it is quite possible that I based this on the smaller dome rather than the large dome that I am actually using and so have a stronger diopter than is actually required.
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