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"Hi Adam et al,      Some more comments on the fisheye lenses for Nikon and the camera you can use them on. \n  \nAdam, it seems you have misread my comment comparing the D500 to the D800/810.  I specifically wrote that I did not see a whole lot to be gained in image quality by using the D500 vs the D800/810 in the DX crop mode. I totally agree that the D800 in the FX mode will provide a significantly higher image quality than the current crop of Nikon DX cameras including the D500. And that is one of the reason's I shoot an FX camera.  \n  \nHowever, you have made the comment, \"Strictly technically, the Sigma does actually focus closer than this lens (by 1cm). I note that Nikon list the FOV of the lens at 175° at 15mm on FX.\"  If you look at the lens specifications, as provided by the lens manufacturers, the Sigma lens may focus that 1 cm closer. However this lens specification distance is usually measured from the subject to the film plane or sensor plane in the camera body.  But underwater, and behind a dome port, how close to the front of the lens you can focus (which might be called the working distance) is more important due to the fact that with the dome port you are focusing on virtual images that are much closer than the actual subject. And with a small diameter dome port, those virtual images are quite close to the dome itself. The fact that the Nikon fisheye zoom is physically quite a bit longer than the Sigma fisheye means that at minimum focus, the Nikon lens focuses closer and provides a larger image (more magnification) than does the Sigma.   \n  \nI shot a couple quick comparison shots with the two lenses at the minimum distance at which each lens could focus and the camera still fire when shooting with my D800 set to single focus auto-focus with being in focus the priority. To do this, I started with the lens almost touching the little spice bottle and then backed it away in tiny increments until the camera was able to shoot the picture.  Here are the two shots. \n  \n \nThis is the image shot with the Nikon fisheye zoom at minimum focus and at 15mm \n  \n \nThis is the image shot with the Sigma 15mm fisheye at minimum focus \n  \nAs you can see, the vertical height of the little bottle is not completely shown in the image taken with the Nikon fisheye zoom.  But there is space above the top and bottom of the bottle in the Sigma lens image.  I made paper \"feeler gauges\" to measure the actual space between the front element of each lens and the bottle.  The distance between the Nikon zoom's front element and the bottle was 24mm.  The distance between the Sigma's front element and the bottle was 51mm. I have not had the opportunity to shoot this comparison underwater, but will when I get home and can try both lenses behind both the Seacam wideport and the Seacam fisheye macro port. \n  \nSomething that also shows in these two shots that I was not expecting is that if I look at the background of the Nikon lens vs the Sigma, the overall coverage with the Nikon lens is a bit wider than that of the Sigma lens. I would have expected there to be less difference as the glass bottle position remained constant and thus the Nikon lens was a tiny bit closer to the background as well as being 27mm closer to the bottle.  \n  \nSo I would conclude from these images that the Nikon zoom is capable of providing a larger image (more magnification) of the subject at its closest focus than the Sigma fisheye can. And that at very close focusing distances, it includes a bit more of the background (is a little bit wider). \n  \nFred \n"
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