Can you design a lens out of spite?  The difference between Konica and Leica is that for the last 20 years it made cameras, it made them for self-amusement and technical gratification.  In the mid-1980s, Konica pressed into the compact motorized SLR market with the FS-1 and FT-1 Motor - when it already was in the second or third tier of SLR sales.  Want a half-frame flat 35mm camera?  Try the AA-35.  An ultraquiet press camera?  Here's a Hexar.  A motorized, autoexposure Leica M6?  You could consider the Hexar RF.  When, in the late 1990s, Konica entered the Leica-mount lens market, it released lens after lens that simply rubbed it in Leica's face.  The 50/2.4L Hexanon outperformed the 50/2.8 Elmar, the 35/2 UC-Hexanon evenly matched the 35/2 Summicron ASPH wide-open, and the 60/1.2L Hexanon brought back the aberrated days of the 1950s.  The M-Hexanons were no different.  Similar performance to a Leica lens, equal build, half the price.  This reached its peak with the 21-35mm Hexanon Dual, which when released was the widest varifocal lens for the Leica M.

More bang for the manufacturing buck.  The M-Hexanon Dual, according to Konica's white papers, was designed to be a killer lens at 2m and at apertures all the way down to f/11.  On top of that, it was designed to have easy collimation at both focal lengths.  From an engineering standpoint, it was a much more elegant design than the Tri-Elmar 28-35-50mm (at that point, the only other lens that could do more than one focal length).  It did not have the clunky automatic frameline selection (nor did it need it, since no M camera at the time had a 21mm frameline).  By allowing the aperture to be variable (half a stop slower at 35mm), Konica was able to eliminate the multiple cams required in the Tri-Elmar to keep the aperture at a constant f/4.  And by making the zoom and focus take place inside the lens, Konica was able to lighten the focusing effort to a feather touch.

In the box.  Typical of completist Konica packaging, the Hexanon Dual was packaged with a 21-35mm frame finder (like a larger version of a Cosina or Leica brightline finder), a case for the finder, a case for the lens, a bayonet hood, and a case for the hood.

Physical aspects.  In terms of size, the lens extends approximately 60mm from the front of the camera.  Most of the barrel is 55mm in diameter.  The barrel flanges out to 67mm where it connects to the hood and necks down sharply to a 62mm filter thread.  There are two hood-mounting pins (a la 10.5cm P.C. Nikkor) that sit just behind the filter threads.  The aperture ring has ball-bearing detents and half-click stops.  The focusing ring is not knurled, but it does have a Leica-style tab.  Focus is marked in feet and meters.  Nominal close focus is 0.8m, which is about as close as you would want to get given the relative poor performance of Leica M rangefinders at closer than a meter.  Everything on this lens feels first-rate and solid.  It's not a surprise that when released, this was twice as expensive as any other M-Hexanon.

Performance with the M8.  From this point out, this site will evaluate lenses primarily using digital bodies.  Digital cameras are more demanding and have flat imagers that approximate the "ideal" film plane.  In general, if a lens works well on digital, it works well on film.  These are the basics:

• Resolving power: excellent at f/3.4, superlative at f/5.6.  Performance is roughly the same at both 21mm and 35mm settings, and the lens produces enough resolution and contrast at any aperture to generate moire on the Leica M8's sensor.  In practical terms, this means that it's hitting 73 lp/mm or greater.  This should be no surprise, since the lens is designed to return 90% MTF at 2m at all apertures down to f/11.  At the 35mm setting, it outperformed the 28-35-50mm Tri-Elmar at the same focal length and all equivalent apertures.  Not bad.
• Chromatic aberration: very little to note here.  What passes for "chromatic aberration" in many so-called tests is better classed as "overcontrast bleed."
• Distortion: very low pincushion distortion; about the same as a Cosina Voigtlander 21mm lens - so less than most 20 or 21mm prime lenses for SLRs but more than a Biogon or a Kobalux.  The picture at the top is shot at 21mm.  There is no noticeable distortion at 35mm.
• Vignetting: not much, but it's not covering a 24x36mm frame.
• Bokeh: if you believe in it, this lens has it.
• Focus accuracy: excellent out of the box; tended very slightly toward back focus (that is not detectable on a film camera).  Easily corrected via master collimation, which is right under the lens mount.  Critical use of any lens with an M8 will require recollimation, even when the lens has "Leica" on the front.
• Coding: sadly, making this lens show as a 21mm lens with 6-bit coding is not possible without permanently modifying the lens mount.  It's something that is only relevant to in-camera JPGs, and CornerFix software takes care of it (and radial color shift) quite handily.
• UV/IR filters: B+W makes 62mm 486 filters that fit this lens.  Leica does not.  It's not a big deal, because you can shoot an auto-correction profile for CornerFix and be done with it - even on RAW files.
• Framing accuracy: Leica M8 stock framelines vastly underestimate what will be in the frame at distances past 2m.

All in all, this is a great lens for the M8.  It's not pocketable, but then neither is the camera.  It also has the advantage of bridging the focal length ranges of both Tri-Elmars.

Performance on film. I'll add a couple of notes here to fill out some unique issues with film.  The following are based on observations of negatives at 15x, shot on a Leica-spec camera body.

• Vignetting: there is some slight vignetting at the wider apertures, particularly at 21mm.  This is mostly gone by f/5.6 and gone by f/8.
• Sharpness falloff: at wider apertures, the sharpest area is the middle 24mm diameter circle, with gradual falloff.  At around f/5.6, the entire field is sharp.
• Focusing accuracy: nothing abnormal noted.
• Viewfinder: you will need both an external viewfinder and a bubble level to use the 21mm setting effectively.  Although the lens comes with a nice finder, the Leica Universal Wideangle Finder M makes a better choice, with zoom framelines, variable parallax correction and an integral bubble level.

If you really like wideangle pictures, this makes a great lens for film, too.

Upshot.there are only three varifocal lenses for the Leica world.  The 21-35mm Hexanon dual is a better performer than the 28-35-50mm Tri-Elmar and a far more useful lens than the exhorbitant 16-21mm Tri-Elmar.