1. Minimal shutter lag.

This is essential for capturing the decisive moment.

The classic (film) rangefinders have extremely brief shutter lag times (msec):

Leica M3         16

Leica M7         12

Compare this to the following digital rangefinders, which have much longer shutter lags (msec):

Leica M8         80

Leica M9         80

(source: Wikipedia)

 

2. Optical vs. Electronic Viewfinder (OVF vs. EVF).

My preference would be to have the OVF retained.

If the decision is made to move to an EVF (to improve focusing accuracy, avoid the rangefinder drift that plagues current rangefinders, etc.) the following criteria should be met:

• The view should consist of a simple outlay free of visual clutter, distracting blinking lights, etc. Ideally, only the framelines should appear (or, at least, the option should exist to turn off all displays so that only the framelines appear).
• No perceptible EVF lag through a wide range of light (bright to dim).  I don’t believe current technology is able to address this satisfactorily yet, hence one of the several reasons the OVF is still favoured by many.

 

3. Robust build.

Reliability is a priority.  This should be the minimum expectation for a luxury/professional camera.

Moisture sealing.  The expectation is not that it should be as impervious to the elements as a professional DSLR, because that would add too much bulk/weight, but that it should be able to withstand water spills, light rain, etc.).

Excellent battery life.  With current technology, this may necessitate a slight increase in the size of the camera thickness (for example thicker than the Leica M10) to accommodate a larger battery.  However, this is an acceptable trade-off given the benefit of longer battery life.  Also it is unrealistic to expect film-era camera body thickness in a digital M when modern lenses themselves have also grown in size and weight as compared to their film era progenitors.  The camera body-lens pairing should balance nicely to avoid grip fatigue, etc.

 

4. Quick operation.

Current frame rates are acceptable for a rangefinder.

However, shorter card-writing times and larger buffers are always welcomed.  This too may require a slightly thicker M to enable adequate heat dissipation.

The ability to review photos quickly, at 100% magnification (with the touch of one button) to be able to quickly verify focus, and to maintain 100% view while scrolling through a sequence of images, etc would be desirable.

 

5. (Bonus)… this is unrelated to the M line of cameras but will be arbitrarily included in this list:

A large (medium format) digital sensor rangefinder would be desired by many current M photographers.

This is best envisioned as a modern day Mamiya 7 but with a digital sensor.

The rangefinder format would allow for minimal camera size (width).

Together with manual focusing lenses built to the same quality/performance as M lenses this would offer an extremely desirable level of image quality.

However, it must be conceded that pricing for such a system would potentially place it out of the financial reach of a significant proportion of photographers/consumers.

In that case, a fixed-lens version (along the lines of the Leica Q but with the aforementioned medium format sensor) may be a more viable (attractively-priced) option.  A design of this type (fixed lens matched to the sensor) would also potentially allow for a smaller lens size, since custom software corrections for lens design compromises would be possible (again, akin to the Leica Q).

 

—Peter.