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"I received my Nikon 8-15mm lens about a week ago and have had it in the water once since then.  Because I shoot with a Nikon D800, my interest in this lens is as a close-focusing full frame fisheye lens.  I have very little interest in circular fisheye images. \n  \nI initially did a few topside shots comparing this lens to my Sigma 15mm f2.8.  My initial impression is that in dim light, the Sigma may pinpoint the focus in the center of the image a little better than does the Nikon lens. Probably due to the fact that the Sigma focuses at f2.8 vs f4.5 on the Nikon lens when set to the 15mm full frame.  But on those same test shots, the sharpness near the edges of the images were consistently better with the Nikon lens. \n  \nUnderwater I have used both the Sigma 15mm and now the Nikon 8-15 behind the Seacam Wideport which has a radius of curvature of about 75mm, meaning that if you had that port as a full hemisphere, the diameter would be about 150mm or about 6 inches. I also own the Seacam 4\" fisheye macro port, but pretty much stopped using it because you needed to cut off much of the lens shade on the Sigma 15mm when using this combination and therefore do not have it with me on my current trip.  I feel that cutting back on the Sigma lens shade resulted in too much image degrading flare. \n  \nWith the wideport, I could use the Sigma 15mm with the manufacturer's full shade in place and although I did get slight vignetting at the corners of the image, I feel the image quality was better enough to use the lens this way.  With the Sigma 15mm, I use the wideport directly attached to my Subal housing body. \n  \nI switched to the Subal housing body for my D800's because I felt Subal gave me a much more complete set of controls to the D800 body functions than did the Seacam housing body and with the help of a good machine shop I was able to modify the port mount on my Subal housings to take my Seacam ports rather than buy a new set of ports. \n  \nBecause the Nikon 8-15mm lens @ 15mm is physically longer than the Sigma 15mm, I am able to shoot the Nikon 8-15mm lens, with its Nikon lens shade mounted, without any vignetting when using the Seacam wideport in combination with a Seacam 20mm extension tube. With a 25mm extension tube, there may be a very slight amount at vignetting, but still far less than with the Sigma 15mm. \n  \nBelow is one of the shots from my only dive so far with the Nikon 8-15mm lens. It shows a pair of small octopuses mating underneath the Edithburgh jetty in South Australia.  The image was shot late in the afternoon at 1/250th at f16 using ISO 3200 on my Nikon D800. \n  \n \n  \nWhen anyone does tests to determine the nodal point of the 8-15mm lens, or any other lens, they should be aware that the nodal point of the lens lens can change with both the zoom setting and the focus point of the lens.  I found quite large differences when I did a series of tests on Nikon's 24-70mm f2.8 several years ago. Using the manufacturer's given nodal point is also likely to be at least slightly inaccurate for underwater use because the manufacturer's nodal points are given for lenses focused at infinity rather than at the close distances the lens is likely to focus at when used behind a dome port.  \n"
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