Calculating the Distance of Wildlife in a Telephoto Shot

While enjoying a calm and misty morning at the pier in Malibu, California, I spotted dolphins. After observing them for a while, I noticed that their swimming was synchronized. There was a natural explanation for their circling: a feast had begun, quickly attracting sea lions, pelicans, and seagulls.

After shooting a few photos from a distance of almost 1km (~0.62mi), I switched to 4K video. Having a Nikon D850 with a Nikkor 300mm f/4 PF, and 1.4TC III, the 4K DX mode helped to give me the extra reach. This short movie was taken with a 420mm focal length and 1.5x DX crop, thus equivalent to 630mm on a 35mm camera.

I don’t usually take a tripod on trips like this, so I just rested the camera on my Think Tank Photo camera back and fixed it in place with a denim shirt. Nikkor’s Vibration Reduction took care of the rest of the stabilization. The 4K video was further cropped to FullHD for maximum magnification, and here is the result:

Although it’s not the best quality, the gear was pushed to its limits – without a tripod, with a lens that weighs only 755g (~26.6oz). I consider the Nikkor 300mm f/4 PF to be the best small telephoto lens ever made, and it accompanies me on almost all my travels due to its small size and superior image quality.

So how do you measure distance on the open sea? That’s a tricky one. First, with programs that can access the full metadata (e.g. ExifTool) of your photographs, you can check the distance, only to find out that the number isn’t correct even in the order range. I wouldn’t expect much precision for these high distances — it’s still just a camera and not a range meter.

For this picture, the metadata shows a focus distance of 31.62m (~103.7ft). The real distance is more than 20 times further. How did I calculate it then? The key to this are the dolphins themselves. There are three common species of dolphins in Santa Monica Bay. I think this photo shows the short-beaked common dolphin (Delphinus delphis). Their body length ranges from 1.5 to 2.7m, an average of 2.1m, which could correspond to this area of the image.

The whole horizontal field of view of the D850 would then be roughly 76m. The 420mm lens has a horizontal field of view of about 4.9 degrees and from that, we can go back to our school math lesson with triangles, calculating the distance to roughly 887m. While this result cannot be precise, I think it’s pretty safe to say that the dolphins were filmed from a distance of 800-1000m.

And that’s how photography can be combined with biology and math.

About the author: Alexandr Pospech is a photographer and filmmaker based in Prague. The opinions expressed in this article are solely those of the author. You can find more of Pospech’s work on his website, Facebook, and Instagram. This article was also published here.

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