M accessory shoe light meters
In Jan 2002, John Collier provided the following summary on the kinds of accessory shoe lightmeters available:
The M meters mount in the accessory shoe of the M camera and couple to the shutter speed dial. You set the desired shutter speed, take a meter reading and set the lens to the aperture displayed.
The M and MC meters use selenium cells which have the benefit of not needing a battery and a colour response that closely matches our own. They also have limited low light performance, clumsy booster cells to try and work around this and the metering area is very wide (65 degrees which is not that useful at times). The cells also oxidise over the years and require replacement. There are only two places that I know of still doing selenium cell replacement, fortunately, it is not that expensive.
The MR meters use powered cds cells and a have good low light sensitivity. The metering area corresponds to the area of the 90mm lens so semi-spot readings are easy to do. They use a 1.35v mercury battery which is now difficult to find but there are several easy fixes for this. The colour sensitivity tends to darken reds and yellows if I remember correctly and the meter is subject to memory problems if exposed to bright light (it can sometimes take 2 or 3 minutes for the meter to be accurate again). The MR has a "T" shaped meter on switch out the side that interferes with the rewinding on M4 and up cameras. The MR-4 has a meter on switch on top of the meter which solves this problem.
The ISO/ASA dial is easily bumped on all the M meters so keep an eye on it.
I use a MR-4 on my M2 and M4-2 cameras and, with careful metering, accurate exposures to with a 1/3 stop are possible. It is easier and much quicker to use the built in meter of my M6TTL and so I tend to use that instead. So I meter with my M6TTL and transfer the settings to the other cameras.
Although rather than drag around a 2nd M6 body, a compact hand-held incident light meter like a Gossen Sixtomat Flash would probably make better sense.
Also, if you *must* use an accessory shoe lightmeter, then don't forget the new clip-on Voigtländer meters with their modern compact design, silicon cells, LED exposure indicators and use of sensible batteries.
An interesting compromise which became available in August 2002 is the Gossen Digisix hand-held meter, which can also be mounted on the camera accessory shoe via a special-order camera mount part.