There is one persistent trend: the demand for more pixels. When Leica announced the M8 with its 10 Mp, there was an immediate response that this amount was too timid. The M9 had 18 Mp, but spread over a larger area. The pixels size did not change (6.8 micron in both cases). The M240 family and the new M10 has a pixel count of 24 Mp with a slightly reduced pixel size. The Leica Q uses selective cropping to emulate several focal lengths, combining a 24 Mp sensor with a basic 28 mm lens. The idea is that the definition of the lens can be used to enlarge sections of the sensor area. It is the same as using several focal lenght’s from one fixed standpoint to enlarge a small part of the scene.
The same idea can also be used with the MM2 and the Apo-Summicron-M 50 mm lens. When you take a picture of a model the result may be like this:


If you want only a part of the face, you can use a 90mm lens or enlarge a section of the picture, see below.



This picture is a non-post-processed sectional enlargement of the original file. The definition is excellent and all detail is clearly and crisply rendered.
Selective enlargements are as old as photography itself. It is one of the main arguments for using high definition, monodispersion emulsions.