This is the mirror lens design, called “catadioptric”, which bounces light twice inside the lens. The goal of Nikon lens designers was to make a telephoto lens that was as small and light as possible. This is essentially the same optical design as the Hubble telescope.
All designs involve trade-offs, and this design has huge trade-offs. On the plus side, the lens has acceptable sharpness, weighs next to nothing for its focal length, has essentially zero chromatic aberration, and it’s cheap. On the negative side, its fixed aperture is slow, there’s lower contrast, and it has a minor hot spot in the center.
With the miracle of digital processing, most optical drawbacks can be minimized or eliminated. One optical issue, however, is this lens’ biggest weakness: doughnut-shaped bokeh.
Reflex-Nikkor C 500mm f/8
My lens copy is from 1981. This is from the Nikon era of supreme build quality, so the lens works as well as when it was brand-new. It comes with 39 mm rear filters, including yellow, red, orange, neutral-density, and the L37c UV. A filter is required for the optical design, so I leave the L37c mounted. It comes in a nice hard-leather case, and the filters fit inside the case lid.
The lens focuses down to 13 feet, and weighs 2.2 pounds. The real transmission of this f/8 lens is closer to f/11. It comes with a built-in tripod mount, with a button to switch between horizontal and vertical. It’s 3.6 inches in diameter and 5.4 inches long (6 inches with its included screw-in lens hood).
The manual focus rotates about 180 degrees, and mine is silky smooth. You need this kind of focus throw, due to its long focal length and narrow depth of focus.
The “C” in the lens name indicates that the optics are fully multi-coated. Back in the day, this was a big deal.
These lenses presently sell for about $300.00 US in pristine condition. Less than the sales tax on most big telephotos. Like I heard once, it’s dollars to doughnuts.
It should go without saying that you need to stick with shutter speeds above 1/500 second. The lens has little inertia, so you need to watch out for vibrations. With modern cameras, ISO is no longer the huge problem that it was in film days, thank goodness.
Catadioptric optics with two mirrors to bounce light
The optical design shows how it’s great for minimizing physical length, but it also shows how the front mirror blocks a huge amount of light from entering the lens. Mirror lenses are inherently dim.
This is one of those “Non-CPU Lens Data” camera setups, which lets you use aperture-priority automatic exposure. I tested this lens on the Nikon D610.
I mentioned above that the lens has acceptable sharpness. Maybe I should have said ‘barely acceptable’. The peak MTF50 values on my D610 were 31.8 lp/mm, where 30 lp/mm is the lower end of acceptable resolution. Corners and edges were typically around 22-27 lp/mm, which is sub-par by today’s standards.
You really notice how the center of shots are brighter than the edges; it has a different character than normal vignetting. This is fairly simple to fix in post-processing.
I used “live view” at 100% to try to get the best focus on the resolution chart. I leave the shots as un-sharpened raw for the MTFMapper resolution software analysis.
Nikon D610 MTF50 results
The resolution results are interesting, in that the meridional and sagittal directions look almost like mirror images of each other. Sort of a yin-yang.
The peak MTF50 of 31.8 lp/mm equates to 1526 lines per picture height for this camera sensor.
Honestly, nearly all modern lens designs will smoke this lens when it comes to resolution.
MTF 10-30 Contrast plots
You can really see the low-contrast characteristics of the lens in this plot. Very unusual to see the edges seeming to improve compared to the center.
Overall, results are underwhelming when compared to modern lenses, but not by a huge amount.
Humming bird looking at doughnuts
The out-of-focus background has that strange doughnut characteristic. Providing full disclosure here, this is the sharpest out of about a dozen shots I took. Depth of focus is razor thin, and auto-focus was sorely missed. You’ll probably want to leave your camera on “continuous-high” and shoot away as you tweak focus.
Auto-focus spoils you rotten. It removes the biggest challenge associated with long lenses. Technology is a wonderful thing. On the other hand, the humbling experience of focusing a big lens yourself can be an interesting challenge.
The out-of-focus Protea reminds me of sea anemone. Again, a pretty strange effect. Not a terrible effect, just different. Notice the center is brighter than the edges; many times this is just fine and you can leave it as-is.
This lens is one of those things that is an acquired taste, which most people will never acquire. If you really, really need to pack a long lens into a small space, then this is about as small as you can get. I have used this lens backpacking; I would never be able to haul my big telephoto on those trips.
Long lenses are almost unusable without auto-focus, as far as I’m concerned. When you combine the manual focus with the strange doughnut bokeh, this lens just has too many strikes against it for most uses.
It’s easy to see why Nikon and most other lens makers have abandoned this kind of design, auto-focus or not.