Nikonos RS-13 Fisheye Lens (Adapted for Sony Mirrorless)

Introduction

For a long time, I held off getting a fisheye lens since I mainly photograph wrecks and really appreciate the “straight lines” that a rectilinear lens provides. But, there are advantages to a fisheye and I finally took the plunge but with a very vintage lens adapted to a modern Sony mirrorless camera.

The Nikonos R-UW AF Fisheye 13mm f/2.8 lens is legendary (usually abbreviated to simply “RS-13”).

It was originally designed for use with the Nikonos series of underwater cameras that were produced between 1963 and 2001. Below is a photo of the last mainstream model produced, the iconic Nikonos-V.

The RS-13 lens would fit on the front of the camera and provide a very wide 13mm fisheye view. The lens would be manually focused and the aperture adjusted via the two knobs shown in the picture above. The RS-13 was built with the RS mount which allowed better sealing.

The optics of the lens and integrated dome port are very good. In fact, many underwater cinematographers have been using this lens adapted to modern film camera for years. This is a very good article about the benefits of this lens. It was used extensively in filming Avatar 2.

Over time, in order to use this amazing lens with modern DSLR cameras, there have been a number of adapters and ports built that allow the lens to be mounted to newer underwater camera housings. I use Nauticam housings and this is an example of such an adapter. There are really two things to consider when adapting this lens. The first is the mechanical aspect to ensure that it seals with the housing to prevent any water leakage. The second is electronic conversion of autofocus.

Given the heritage of the lens, the first adaptations (AFAIK) were done to Nikon body cameras. There were also some adapters built for other cameras that required manual focus but I wasn’t really a fan of that. Then I read about Isaac Szabo who had a very ingenious approach to converting to Sony cameras, including autofocus….

This thread on Waterpixels has a really good discussion about the converted lens:

https://waterpixels.net/forums/topic/721-its-time-to-talk-about-the-nikonos-rs-13mm-again

The Conversion Process

The process to get and convert a lens isn’t hard, but it also isn’t easy. 🙂

Part 1 – Acquire the Lens

The first issue is the availability of good condition RS-13 lenses. They exist but are obviously all used and the primary marketplace for buying one is Ebay. As this lens has gained in popularity and the quantity of good copies of the lens have been depleted, the law of “supply and demand” is alive and well. You will have to pay quite a sum to get a good lens and you will have to be quick. The prices have increased in the last six months since I bought my lens.

Quite a few of the lenses have some fungus on the inside and slight scratches on the dome portion. Also, quite a few of them are located in Japan and I’m not sure what the current tariffs or import fees would be. Mine happened to be in the US when I purchased it.

Isaac helped with this part of the process and I lost out on a couple of bids before I finally got a lens. The bidding starts out slow but gets heated in the last few minutes of any EBay auction. After finally “winning” and getting a lens, I had it shipped directly to Isaac. The seller knew Isaac and so he was willing to do that.

When the lens arrived, it had a few more flaws that were not visible in the on-line pictures. The seller was very good about it and offered to help pay Isaac to buff those out. Overall, it was a good experience.

Part 2 – Have the lens converted

This is the real magic. There are two things that Isaac does that are very cool:

• Take the “dome” part of the original lens and fit it into a custom, 3D printed port for Nauticam
• Take the “guts” out of the RS-13 lens and “port” those into a donor Sony 50mm lens

The result is a modified 50mm lens with the internal optics of the RS-13 and a port. This also allows the Sony camera to use autofocus through the 50mm lens controls. What an ingenious idea.

I had asked Isaac to take some photos of the process of converting it which I’ve included below:

As I mentioned, my copy of the lens had some scratches but Isaac has built some tools / methods to polish the glass and remove them:

The next step is to take apart the “donor” Sony 50mm lens so that the optical elements of the Nikonos lens can be inserted:

Isaac then 3D prints the port for the dome and the other parts:

He then “builds” the system

The last step is testing the final system to make sure everything is working correctly. The optics must be very finely tuned in order to get the best results:

One option that Isaac offers is the “Close Range Correction” system. This allows the focal plane of the lens to be changed depending upon how far you are from the object you are photographing. It effectively alters the focal plane and I’m still working to understand it and test it. It cost an extra $100 at the time of the conversion so it was worth it to have the option although I’m not sure how much I will use it since I don’t do a ton of “close focus wide angle” (CFW) photography.

Below are some pictures of the port on the Nauticam housing. I know it might not seem like it, but it is TINY compared to other wet lens or dome port options.

Alternatives

There are really two mainstream alternatives to the converted RS-13 lens.

The first is to buy a normal fisheye lens (the most often used for full frame is the Canon 8-15mm) along with a converter (if you aren’t using a Canon camera body) and the appropriate dome port. There are a LOT of people doing this and it was my plan if I couldn’t get a RS-13 setup. I believe that the specific lens has recently been discontinued but it is still relatively easy to find. This is a very good article about the setup with the Sony and two dome port choices.

The second alternative is to use a “wet lens” from Nauticam (or others) which converts a “regular” zoom lens into a wide angle view. This is somewhat in between a rectilinear lens and a true fisheye. For example, the WWL-1B is a lens I use a lot and at a 28mm focal length, the result is a 130 degree field of view. The photos don’t look quite as “fisheye like” with the pincushion look but they are also not rectilinear either.

Size Matters

The downside to this setup is size and weight. Below is a comparison of what the WWL-1B looks like compared to the RS-13. Nauticam also recently released a “Fisheye Conversion Port” (FCP) which converts a standard lens to a true fisheye view but it is even heavier and even more expensive. (US$7,500 at the time of writing).

Admittedly, they don’t look that much different, but they feel very different. One thing that isn’t clear from the photos is the weight difference (which can be made neutral underwater). I know this isn’t a real “apples to apples” comparison, but here is the weight of each of them without covers, etc:

This is a good thread that shows the size & weight differences of the Nauticam wet lenses. Realize that the WWL-1B is pretty much one of the smallest options and it is bigger and heavier than the RS-13 port.

Results

I took the lens out last weekend for a dive on the Yukon . The surface conditions were amazing but the visibility on the wreck was only 5-10 feet. Interestingly, the pictures make it look a LOT better and I guess that is one of the advantages of a fisheye lens vs a rectilinear in a large dome – you can get closer to the subject with less water between you and the subject.

The photos turned out well given the conditions. I’m still learning to use the lens, but I’m very happy with it so far — especially the form factor.

Pincushion Effect

Below is a good example of the guns and how the pincushion effect becomes more obvious even by getting a little closer to the object (I have only adjusted highlights and exposure and not cropped any of these photos).

Below is a photo near the bridge with a diver in the background and shows the pincushion effect:

This photo shows the area near the bridge with the radar dome. There is an advantage of the fisheye since it really highlights whatever is in the center of the frame:

Corner Sharpness

One thing that the lens is known for is corner sharpness even at relatively open apertures.

I took some photos inside the engine room where the corner performance could better be analyzed (vs open water backgrounds). I’m still learning how to position my lights when taking the photos, but you can get a sense of the performance and it looks pretty good.

You can clearly see the “pincushion” effect in the photo below. I have adjusted the exposure a bit but did NOT use any AI tools or adjusted the Clarity, Dehaze, etc.

If I look at 100% magnification in the lower right corner, this is what it looks like. That is pretty darn good given the resolution of this camera (61 MP) and that it is a “full frame” camera.

Below are some additional photos from inside the wreck. Again, I’ve done very little post-processing other than adjust some exposure and boosting shadows to better show corners.

Next Steps

I have only one dive with the lens so I need to spend more time shooting with it. Besides just getting more time in the water with the setup, there are two things I want to work on:

• Lightroom Lens Profile – Lightroom has the ability to automatically make some corrections based on a specific Lens Profile. There isn’t a lens profile for the RS-13 lens that I have been able to find. Also note that Lightroom sees this as a “Sony FE 50mm F1.8” which is appropriate since the that is the “donor” lens. I have played around a little bit with changing the profile to the Canon 8-15mm Fisheye which is probably the closest equivalent in the LR profiles. So far, the results are not promising. It essentially “flattens” the image and crops it and has a really “reverse distorting effect” to my eye (before and after below):

• Photogrammetry – I’m guessing that photogrammetry will not work well with this lens. Similar to Lightroom, Metashape looks at EXIF data to determine the type of lens and make corrections for that before doing alignment. I’m not sure if Metashape has a profile for the RS-13 lens (I doubt it). It might still work since I tend to shoot a LOT of photos for a model so the alignment would work but I’m guessing that the dimensions would be slightly skewed.

Thanks To…

Obviously, a huge thank to Isaac Szabo.

He really did an amazing job removing any imperfections in the source lens and creating a beautiful product. Being an engineer, I’m really in awe of the ingenuity in what he has created. He has quite a backlog of work to do so be patient and realize that the end product will be worth the wait…