Leica (the uber optics maker) in their infinite wisdom to resolve the UV/IR issue in the M8 gave every owner a set Filters to resolve this issue. is that not contrary to the mantra of the bare naked lens?
It is, but it also is the only way to solve the issue in a cost effective manner.
The underlying issue is that the incident rays of a non-retro focus lens design, especially on wider angle designs, will cause an ineffectiveness in a hot-mirror filter especially towards the corners compared to the center. Also, an added layer of plan-parallel glass just in front of the sensor with cause optical deterioration, which is exaclty what they wanted to avoid (and which bit them in the tail).
With the chosen compromise, depending on the filter used, now they only have to deal with the IR-reflection mis-match between center and corners (by reading the lens ID and 'correcting' in software). The optical degradation is probably less than from something close to the sensor.
Bart