Would a +4 diopter help increase corner sharpness?

Hello, I have an 18-55mm Nikon lens with a +2 diopter and a 5503.50 port (as recommended by the Ikelite port chart). At 18mm, I found the corner sharpness to be too much of a problem.

Would a different configuration increase the corner sharpness?

Would a +4 diopter, or not using a diopter, or using a different port help increase corner sharpness?

Attached is a wide angle picture of a seastar (the seastar was in the corner of the wide angle shot). You can see how blurry the left side of the seastar is compared to the right.



Or is the solution, not to frame the subject in the extreme corners?

Thanks for all the help.

When a land lens is used behind a dome underwater, an apparent image is created on the radius of the dome. Since most shooters tend to use a center focus point in the camera, the center of the image will almost always be "in-focus" while there will be "fall off" as you look to the edges of the frame, resulting in image softness. The BEST way to control this is to improve your depth-of-field. This is accomplished by closing the aperture down (setting to a higher f-stop number). The background in the frame may tend to darken as you close the aperture down since the foreground and background lighting will be unbalanced, however your depth-of-field will improve resulting in a sharper overall image.

A diopter brings the minimum focusing distance closer to the camera (as well as infinity distance ). I would try it with and without the +2 diopter. Some shooting situations may show slight edge improvement with the +2. Going to a +4 diopter will not likely help.

If corner sharpness is still not acceptable, consider purchasing an 8" dome as it is less convex than the 6", requiring less "depth-of-field".

One other thing to note: You will find a spot somewhere in your zoom range where you will get the sharpest edge-to-edge shot. I like to call it the sweet spot. This is the optimum distance between the lens Nodal point and the dome apex. Zoom lenses have floating Nodal points which is why the sweet spot exists only at one zoom setting.

Hope my explanation hasn't been too confusing. If I can be of further assistance my e-mail address is "brett@ikelite.com".

Regards,

Thanks Brett. I will try your suggestions.