Leica M Biotope 

The first Leica had no rangefinder mechanism, but from 1932 (Model II) a rangefinder was built-in. This was the camera that Cartier-Bresson started to use. His comments on the effectiveness of the rangefinder are worth reading: he was impressed by the clarity of the finder, the accuracy of the framing (it showed exactly the scene as it was recorded on film) and the speed of use of the camera that supported his style of photography. During the next twenty years Leitz engineers studied all properties of the rangefinder, technically and artistically with feedback from thousands of users. The result was the M3 combined view/rangefinder, the most elaborate and well-designed finder ever encountered in photography. The designers choose the 50mm frame as the baseline and not without reason: the 50mm lens and angle of view gave precise framing at close and distant settings and the 1:1 magnification allowed the photographer to be in touch with the subject. Last but not least the rangefinder spot and its split images could be aligned very accurately and with speed.
When the 35mm lens became the focal length of choice for many documentary photographers Leitz produced a finder with an expanded view (the M2), but one could not frame the scene as accurately as with the 50mm lens. The frame lines could not be all captured at one glance and the eye had to move around to the edges to see the frame lines. This movement distracted from the goal of instant composition, which required that the eye could focus on the whole at one glance.
Rangefinder dilemma
The dilemma is quite simple: if you wish to view a scene with a wider angle, you need to reduce the magnification and when you have less magnification, the accurate framing and the immediacy of the scene are lost. The designers of the M3 had to fit the rangefinder into the physical shape of the camera body and then you cannot use a large finder and you are limited in the fields of view you can  squeeze into the finder area.
If you want to use wide angle lenses (any focal length below 35mm), the reflex finder is the better option as this device combines framing and focusing. The Leica camera had to be used with additional finders and you had to switch between focusing and framing to take a photograph, which is not very easy to do and interrupts the natural flow of viewing-aiming-picturing. In addition the camera becomes larger and less elegant.
The geometry of the rangefinder mechanism is such that the precision of focusing increases when using wide angle lenses and decreases when using narrow angle lenses. Beyond 135mm the accuracy of focusing has dropped too much and the accuracy of framing is also questionable.
But the focusing of wide angle lenses is less critical than the focusing of 35 or 50mm lenses. The extra precision of the rangefinder is partly lost in the extended depth of field and partly lost in the reproduction scale of the scene: the objects are reproduced quite small in the auxiliary finder and selection of the plane of sharp focus is not easy.
High-precision compact camera
The Leica M is basically a high precision compact camera for unobtrusive documentary work that can deliver pictures of very high image quality, close to what one can expect from medium format devices. The celebration of 60 years of Magnum Agency is a fine trigger to look at the images made in that period, many with the Leica camera. There are only a few really outstanding wide angle pictures to be seen. (wide angle in the Leica biotope is a focal length below 35mm, the natural limit for Leica rangefinder photography).
In architectural and (urban) landscape photography a 21mm or a 15mm lens can be employed quite effectively, but in documentary and reportage style photography, such a lens feels a bit strained except in gifted hands.
The focus on the core characteristics
The Leica M camera, based on design, construction and photographic style is at its optimum when lenses are used from 35mm to 90mm: then all aspects fit: compactness, readiness, accuracy, immediacy etc. One could make a case that the domain might be extended to 24/25mm.
The Leica M is a quite versatile camera system and it can be used with lenses from 12mm to 640mm, but when you go outside the natural biotope of the camera, the advantages compared to competing camera systems (like the SLR) in these situations are becoming quite thin. One could argue that the excellent quality of the Leica lenses would provide a decisive edge when considering the purchase of a lens or camera system. But most photographers are a quite conservative lot and hardly anyone will buy to a system based on one or several excellent lenses.
Leitz did realize this too and the birth of the Leicaflex showed that he understood the market movements, what is not the same as being happy with these trends.
For a long period the Leica lens system was focused on lenses in the range form 28mm to 135mm. With an expanding user base this restraint was sensible: new users needed to buy new lenses. But from the moment that the user base is stable or even shrinking and the market is flooded with used equipment the sales of the core group of lenses will also show a negative growth.
The market will demand new lenses too, partly to expand photographic possibilities, partly to expand the collection of lenses owned.  The Leica M-system has always been strong in the normal focal lengths and the medium tele-lens range. The natural direction for expansion is the wide angle domain, where Leitz for a long time relied on outside companies to provide good lens designs (Schneider and Zeiss). The technology of aspherical surfaces gave Leica the opportunity to deliver very high quality designs combined with compact dimensions. But with wider and wider viewing angles, the advantages of the rangefinder concept start to diminish.
Digital challenge
With the introduction of the M8, the rules have to be rewritten. The digital capture technique demands a higher focus accuracy and the reduced angle of view by 1.33 requires a finder with less magnification, in fact two contradictorily demands. The classical range of focal lengths from 35 to 135 has be adjusted from 28 to 90, or even better from 24 to 90. The 24 mm lens has an angle of view corresponding to a 32mm lens, the 35mm becomes 47mm and the 90mm becomes 120mm. It would be nice if Leica would develop a series of lenses specifically tuned to the 18x27mm sensor size. Sch a move would of course imply that these designs would not work on a filmbased Leica M camera, but it would signify Leica's commitment to the digital format. The argument that the current designs cannot fully exploit the capabilities of the 18x27mm sensor in combination with excellent post-processing software, will be explained in a forthcoming article.
The optimal lens range for the Leica M (when you wish to take pictures for which the M has been designed in the first place) is between 24/35mm and 90mm, for digital and film-based image capture. For this range, the camera concept, the view/rangefinder design and the focal lengths fit beautifully. Results as far as image quality is concerned are beyond reproach and the photographer can fully concentrate on excellent quality pictures in the documentary and human interest domain. Technique and artistic/expressive goals mutually support each other. And the camera system is a viable alternative to the ubiquitous DSLR and other digital camera systems.
There is one area where the M system is unbeatable: the wider angle lenses have a very small volume, compared to the (D)SLR systems and here the M systeem cannot be equalled, let alone surpassed. Do you need a compact system with wide angle lenses? There is only one choice: the M system! (the companion RF systems: Bessa and Zeiss Ikon and Hexar RF and in the digital sphere the Epson RD might qualify too).
A broader lens range is very pleasant to have and to buy into, but it is not part of the core competence of the M system. For the ambitious photographer the challenge is simple: work within the natural domain of the M camera and produce the pictures that the best lenses in the world are capable of.