Off topic question, but why does the M8 not have the "correct" filter on the sensor? Was it a deliberate design decision? I get why some cameras omit the AA filter, but a weak UV/IR filter seems unusual.
Have to dig back through the cobwebs of memory...
🙂
This was early days for digital sensors in photography and not all was known. And the M8 was a difficult project largely because of the short flange-to-sensor distance resulting in fairly high angles of incidence of light coming in from the lens. And the wider the angle of view of the lens, and the closer the lens projected in toward the sensor, the more problematical it became. An SLR avoids this issue by having much larger distance flange-to-sensor to clear the mirror.
Digital sensors have their individual sensor sites in recesses and groups of four: red, blue, green, green. If the light from the lens comes in at a steep angle, some sensor sites are shaded by the depth of the pit, resulting in loss of some color information. Sensors "like" to have the light come in perpendicular to the surface.
The M8 attempted to solve this issue by firstly making the sensor smaller than full-frame, 18x27mm rather than 24x36mm. Secondly, and directly relevant to the question, the sensor cover glass was made very thin, only 0.5mm thick, for optical reasons. The thin cover glass resulted in reduced filtering of IR light frequencies.
This was originally thought not to be a problem in photography, and indeed it seemed to benefit B&W tonality. Like others early-on, I didn't think it would be a problem... until I came home from a Mediterranean trip and saw the brownish-greens of foliage and the magenta tint of artificial black fabrics due to reflected IR. Leica had become aware of this too and began a program of supplying two UV/IR Cut filters free to each M8 customer.
The M9 was a big technical achievement, as it was then widely thought a full-frame M camera was just not feasible. IIRC, addition of micro-lenses near the cover glass aided the light paths, while a thicker cover glass at 0.9mm took care of most of the IR problem. Most, but not all IR was filtered out, so I have continued to use UV/IR Cut filters on all digital Ms. Most other digital cameras have thicker cover glass than this, even.