It is not always true that by moving the lens further away from the recording medium (film/digital sensor) will attenuate the image quality. To state in another way, it is not always a disadvantage by moving the lens further away from the recording medium (film/digital sensor). The first point which is the type of the recording medium itself and it's property.
In the film case, the closer the lens to the film plane, the less challenge the design of the lens resulting in compact lens with faster f/stop and less distortion while still maintain high resolution and colors and low vignetting.
In the digital case, the lens we are discussing here are film not digital lens, the closer the lens to certain digital sensors (Sony NEX7 and PhaseOne IQ180), the image quality is compromised by color freezing and softness to the corner and in some cases even astigmatism may occurred. Therefore, symmetric (compact) design lens will not do as well as retro focus lens (bigger). However, even the advantage of retro focus film lens on these challenging digital sensors, the image quality is still compromised. The Focal length of the lens become another determinator of proper image quality. Therefore, rangefinder lens from 45-50mm (shortest focal length) are safe with the challenging digital sensors.
So the question one should ask is why focal length from 45-50mm is the shortest focal length limit to work well with these digital sensors and how it ties together slr versus rangefinder lens comparison?
Best Regards,
Son