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The fast and the infuriating?
Soviet Helios-40-2 85mm f/1.5 lens for M42 Cameras

What is it? This Helios (sometimes called Gelios by the Ukrainians and Russians) lens is in M42 mount. It fits Pentax screw-mount cameras, Yashica screw-mount cameras, Praktica SLRs, Soviet Zenit SLRs and anything else with the native M42 mount.

Using adapters, you can get this lens to mate with Pentax K, Konica AR, and Canon manual-focus bodies. With these, you should be able to get infinity focus. For Nikons, you can either get an M42 adapter with glass (which will focus to infinity) or the Soviet one without (will not focus past about 12 feet, but it does not degrade the image). The lens is easier to use with aperture-priority bodies. There is no autodiaphragm.

What versions? The lens comes in four versions.  All three versions have identical optical and mechanical designs.  They also have more than 10 aperture blades, forming a perfect circle iris.

1.  The Helios-40 is raw aluminum ("chrome" or "white" color). It has single-coated optics (reddish-purple coatings) and takes odd M66 filters. It has its own fixed tripod socket intended for use with Soviet M39 SLRs. The fatter front ring is for a preset aperture - you set the click-stopped aperture ring to the desired aperture and turn the wide ring (which has no detents) until it stops. The rear scalloped ring is for focusing (1m to infinity). 

2.  The Helios-40 "B" is the same as the earlier version except that it has a rotating tripod collar that secures with a thumbscrew.  This is a significant step forward.  If you tried to mount the original version on an M39-M42 ring, you would find that the tripod socket would end up in an odd position.

3.  The Helios-40-2 is the same as the Helios-40 "B", except that it is black, it focuses to 0.8m, it takes standard 67mm filters, and it is multicoated (allegedly).  The Helios-40-2 goes for about 100% more than either silver version, and I think two things make it worth it. First, the bare aluminum tends to age poorly; the black finish does not deteriorate as quickly. Second, the standard filter size is a lot easier to deal with.

4.  The Helios-40-2 export is the same as version #3, except that the inscription is in Romanic characters.  The 1979 version I recently drew is very, very sharp wide open.  I'm not sure whether this was just a good sample or whether the export models received more thorough inspections.

Strengths. The strength of the 85/1.5 lies in its incredibly shallow depth of field. The 56.67mm clear aperture makes the depth of field almost as thin as on an 80/1.4 Summilux-R (costing 15 times as much...). This allows you to take portraits like the one below. The lens is outstanding for facial portraits at about a meter. A lot of people go for 85/1.4 Nikkors and 105/1.8 Nikkors to shoot that kind of picture, but in my experience, neither of them falls off as gently from the point of focus to the remainder of the frame. Outdoors, at f/4 and faster, the lens has plenty of contrast. The lens is solidly-built and inexpensive.

Wide open, about 1m

Weaknesses. The weakness of this lens was that it was designed as an oscilliscope lens. Performance wide-open (not that contrasty to begin with) drops off radically as the subject distance increases. I can't tell whether this is due to flare or uncontrolled spherical aberration, but the solution is to close down the aperture by about a stop for every extra meter of distance from 1m. At f/5.6, the lens is sharp along the entire distance range. In terms of performance on-film, with black-white, you can get very nice results, since you can vary the contrast in development or output. On typical, snappy color film, this lens tones things back a bit and makes for very pleasing results. The lens does have a high propensity for flare under backlit conditions, which may be something you want:

Wide-open, about 2m

Upshots. If you a sharpness fanatic, you should probably choose an 85/1.8 lens from the manufacturer of your camera body. If, on the other hand, you are looking for a little something extra in terms of portraiture, this may be just the thing for you.