Stumbling Toward 'Awesomeness'

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I have been researching the best options available for the D300 when it comes to quickly generating some lightprobes/panoramas. This of course means fisheye lenses. Currently, Sigma is the only company that makes a 180 degree circular fisheye. They come in two flavors, 8mm, and 4.5mm. The 8mm projects a full circle onto a full 35mm sensor (full frame), but on an APS-C sensor it is cropped. The 4.5mm however, throws a perfect circular image onto an APS-C sized sensor; I believe it is the only lens that does this.

The Pixels

You would think that the 4.5mm would be the way to go, I did until I took a look at both. It really comes down to the pixels. The width in pixels of the image thrown by the 4.5mm lens is roughly 2285px in diameter. So while you can shoot less, an entire panorama taking about 3 shots, it will come out as a <4k equirectangular. However, using the 8mm, you need 4 shots, plus one zenith (5 shots total) and it generates an 8k image.  While the 4.5mm does generate a 180 degree image across, as you can see it is very wasteful.

So why doesn’t the lens have full coverage in at least the short dimension? I think it’s because it’s a lens designed to fit Canon and Sigma cameras, not just Nikon. Canon sensors have a 1.6 crop factor and Sigma’s Foveon X3 has a 1.7 crop factor (13.8mm)! The coverage is so small because Nikon DX format has a 1.5 crop factor, the APS-C sensor is much larger than Canon or Sigma. The actual circle measures 12.3mm, even small for the Sigma, which makes me believe they future-proofed it for Four Thirds.

For an APS-C sensor like the D300, I would recommend the 8mm, unless you really need a full uncropped image. The 4.5mm, while being more expensive, also has an aperture of 2.8, compared to the 8mm (f/3.5)

I am not super constrained on time, if you are on set and shooting bracketed probes between takes or something, the 4.5mm will save you two shots (18 pictures) and this might be preferable. That said, it will only generate a 4k image in the end (which might be enough)