Adapting circular fisheyes to Nikon D1
by Bjørn Rørslett
What do you do when you wish to use a full-format fisheye lens on the Nikon D1, and there isn't a mirror lock-up to allow mounting the lens, and besides, the image circle doesn't fit the smaller frame of the D1?
This problem became evident when I planned to press my old OP-Fisheye-Nikkor 10 mm lens into service on my D1. I had visual applications in mind which only this odd lens could produce. This speciality lens is a non-retrofocus design so it extends deeply into the camera throat, ending just 10 mm in front of the film plane. Employing a relay lens to reduce the image circle of the fisheye lens would be the easier solution, and additionally would help push the image plane of the 10 mm lens further back.
The rear flange-to-film distance of a Nikon SLR is 46.5 mm. Thus, the relay lens needs to have its infinity focus at the equivalent position in space where the image from the master, prime lens is formed. Because I planned on using a combination of Nikkor lenses to give a relay system, this setup would have an infinity focus at the desired position. The front lens would need to be spaced 2 times this distance from the relay system, so an extension of 93 mm was called for. Thus, the image projected by the master lens serves as the intermediate image for the relay lens which generates the final image at the film plane. This end image would be upside down, and scaled by the magnification ratio of the relay system. For the D1, the 21 mm image circle of the 10 mm lens needs to be reduced to 14 mm, so a magnification of 0.7X is appropriate. I tried different lens combinations culled from my arsenal of Nikkors, and ended up putting a 58 mm Noct-Nikkor reversed onto a 35 mm f/2 Nikkor. This combination exhibited more reduction (magnification 0.6X) than I initially sought, but tests showed this particular relay system to yield very good sharpness across the entire D1 frame. Using any of my 50 mm Nikkors would also be a feasible approach, but end results were slightly less sharp.
Being reluctant over the years to throw away anything that eventually might prove useful, I already had plenty of extension parts on hand to complete my relay design. In particular, the old-fashioned K-ring sets came - once more - to the rescue. The following extension rings were combined to give the needed 93 mm extension (number of units in parenthesis),
BR-3 (1), K-3(1), K-4(1), K-5(3)
Presumably, other combinations would be possible but these parts were those readily available to me.
|Shown here is the finalised relay system for my Nikon D1, mounted with the OP-Fisheye-Nikkor 10 mm f/5.6 on the necessary 93 mm extension made up by several K-ring items and the BR-3 ring. The two lenses making up the relay system must have their apertures at the wide-open setting, or severe vignetting will result. Working apertures are to be set on the front fisheye lens only.|
An additional benefit of the choosen relay system is that it can be reversed, so instead of a magnification of 0.6X, it can give 1.7X in the alternate position. This entails nothing more complicated than just turning the relay system and attaching the 58 mm lens to the D1 instead of the 35 mm f/2. This allows D1 to capture either circular fisheye images, or format-filling pictures with incredibly steep perspectives (because of the short focal length involved). In either case, focusing the setup is conveniently done with the 35 mm lens only, leaving the other components at their infinity settings.
OK, so how about the pictorial possibilities? I haven't done much shooting with the system - yet , but the images below should demonstrate the inherent potential of the relay approach.
0.6 X image
1.7 X image
|© Bjørn Rørslett/NN 2000|
Because the OP-Fisheye-Nikkor was designed to be mounted on a camera with the mirror in a raised position, I had previously never been able to actually view and compose a picture with it. An auxiliary finder was needed and its small image did nothing to make framing the fisheye picture any easier. Additionally, the OP uses a different image projection formula than the finder and thus the resulting image is very different from that seen in the finder.
So, being able to use the 10 mm lens with full imaging on my D1 was a revelation. The relay system may seem an awkward contraption, but the OP Nikkor is a very light lens and the K-rings are true featherweights, so the setup proved quick and easily workable under field conditions. The OP-Fisheye is a fixed-focus lens by design, but having it mounted on the relay system enables it to be focused very close, so care must be taken not to bump the front element.
I have tested the relay setup with other circular fisheyes, and it works equally well with the 8 mm f/8 Fisheye-Nikkor (and presumably, with the 6 mm and 7.5 mm Fisheyes too). However, the 8 mm f/2.8 Fisheye-Nikkor really isn't suitable, because of its retrofocus design and in particular due to its bulk and weight.
I recently made another setup for my 10 mm lens, using the elusive Repro-Nikkor 85 mm f/1.0 as the relay lens. I had to incorporate a focusing collar into the relay unit because neither lens had a focusing means on its own. All this made the new setup slightly less "sophisticated" in external appearance, but imagery - as expected - was much improved.
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