The Nikon Fisheye Nikkor F5.6 6.2-mm SAP is said to be one of the world's rarest camera lenses. The bulbous piece of glass, which is actually able to see behind itself, was never made available to the public, and only three examples are believed to have been produced. This month, one of those will go up for auction, where it's thought it could sell for up to US$50,000.

Developed in the 1960s, the Nikkor F5.6 6.2-mm SAP is one of the lenses which paved the way for the likes of the mammoth 6-mm F2.8 Nikkor. With a crazy 230-degree view, which is a little wider than normal human vision, the SAP-230 lens produces images which look slightly behind itself. It has 10 elements in seven groups, uses an aspherical front lens to match the projection formula exactly, and has an aperture range of F5.4 to F22.

Extreme fish-eyes like this were billed as being for scientific and industrial applications, such as meteorology and astronomy, or photographing in extremely restricted spaces. However, they typically sold in low numbers, with other subsequent extreme fish-eyes only being produced to order. As such, it could be argued that they were made more to demonstrate to customers (and rival camera manufacturers) that they could be made, than as viable commercial products.

The particular Nikon Fisheye Nikkor F5.6 6.2-mm SAP which is due to go on sale – one of only three said to have been produced – has the lens number 1014. It's described as being in "almost new" condition, and comes with caps and leather case. When lens number 1015 was sold at a camera show in Tokyo a number of years ago, it fetched around $40,000.

So, should you have a spare $50,000 lying around, you might be interested to know the lens also comes with a copy of the technical description for this lens by its designer Masaki Isshiki.

The lens is listed for sale at the 28th WestLicht Camera Auction in Vienna, Austria, on November 21st, and is expected to fetch €35000-45000 ($38,000 to $50,000).

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