I used to own both the Fisheye MC Zenitar-K (Pentax) 16mm f/2.8 and the MC Mir-47 Pentax K 2.5/20. I have since replaced them with a Sigma XQ 16/2.8K fisheye and a Carl Zeiss Jena 20/2.8 Flektogon, an M42 screwmount that has been permanently converted to a stopdown K mount.
The Zenitar fisheye's colors look artificially pastel. Also, the lens is not very sharp in the corners. For not much more money, the Sigma is a much better (though much bigger) lens. It also focuses twice as close: About 6 inches, vs. 12 inches. The XQ was the middle of a succession of three fisheye 16/2/.8 lenses that Sigma introduced in the 1970s and early 1980s. It is very well built. After buying the Sigma, I sold my Pentax 17/4K; the Sigma was sharper in the corners and nearly as sharp in the center. Its only weakness is flare, and it's a big weakness. The Zenitar 20/2.5K is a more successful design. It is a true 20mm lens, while the Zeiss is more of a 21 or 21.5mm. (I compared them together.) Saturation is weak, but the colors are reasonably natural; if your main goal is a good JPEG, you can juice up the saturation on your computer. The Zeiss focuses closer (0.19m, vs. 0.3), is sharper in the corners, and has less distortion than the Zenitar. It also focuses and click-stops with a much more satisfying precision. The Zeiss uses the same optical formula as the Pentax 20/2.8 PKA; it even uses 67mm filters. The Zenitar cannot accept a front filter or an accessory hood. So the Zeiss beats the Zenitar, at least for me. Of course, the Zenitar was designed for the K mount and can meter at open aperture. And the f/2.5 aperture would give you bragging rights. If you value absence of distortion or close focus above flare resistance, the Zeiss Jena 20/4 (single coated, 77mm filter, 0.15m close focus) is an even better choice. Another top choice is the Russian-made Zenit MC Mir-20 M (20mm, f/3.5), available only in M42 screwmount. A 9-element, 8-group design, it can focus to 0.18m. It is somewhat prone to flare but in other respects is a better performer than the Mir 20/2.5K. And you can find it new for less than $100. One eBay seller sold his Pentax 20/2.8 PKA after buying this Mir; another Mir-20 owner found it as sharp as his Zeiss 20/4. My final advice: For a fisheye, get any of the old Sigmas. For a 20mm, get a used Carl Zeiss Jena (the f/2.8, f/4, or both) and a new Zenit MC Mir-20 M (f/3.5). Test each with a K mount adapter. Keep the lens you prefer and sell the other(s); you won't lose much money, and you can share your findings with us! Paul Franklin Stregevsky
