Leica monochrom cameras are now on their 3rd iteration.
Want much sharper images with the Leica M10 Monochrome? Here are tips proven in the field.
Always focus with magnified Live View
Rangefinder focusing has inherent accuracy limits, being woefully inadequate for a 40MP sensor (or 24MP) even if in perfect adjustment (impossible to be accurate over the focusing range, let alone to compensate for optical focus shift with stopping down).
Focus errors with the rangefinder increase dramatically when some color filters are attached (orange, red, dark red), to the point of making the image obviously blurred. With orange and red filters, the focus shifts rearward substantially*, even with Leica M APO lenses.
* With ordinary glass types, red light focuses notably farther away than green light, and blue light focuses closer than green light. An APO lens (with special glass types) is supposed to focus the entire visible spectrum identically, but companies take huge liberties in designating a lens “APO”. Indeed, the very best Leica M APO lenses are really faux APO, far less well corrected than Zeiss Otus lenses. A fact which is easily shown in magnified Live View with a red filter vs without a filter. The ONLY lens that is truly apochromatic that I have used is the Coastal Optics 60mm f/4 UV-VIS-IR APO-Macro, and it’s mostly made of fluorite elements.
Always use a color filter
All-arounder (general, people, etc): orange filter (dark yellow filter is OK, but less helpful for most subjects).
Landscape: red filter or deep red filter
A green filter can be useful at times, but in general I find it much less useful than an orange filter.
A color filter blocks other wavelengths. This makes the image sharper because the sensor receives a much tighter-focused dot stemming from a much narrower wavelength band, instead of a smeared dot accruing from the full spectrum of light which the lens does not focus the same across the visible spectrum.
Without a filter, peak micro contrast isn’t going to happen without stopping down 2 to 3 stops; a halo occurs around every imaging point. This is damaging to micro contrast at wider apertures, which Leica’s own MTF charts attest to.
See the as-measured Zeiss MTF charts for visible light vs green light for how a narrower spectrum can greatly decrease astigmatism, even in a lens extremely well corrected for color. See as well the discussion of color correction in Zeiss 15/2.8 Distagon Q&A — Optical Performance.
For landscape photography, a red filter (or orange filter or dark red 091 filter) have several benefits: the blue sky is darkened for more dramatic effect, and blue atmospheric haze is stripped away, increasing contrast for distant scenes.