Le filtre rouge crée des problèmes de mise au point (longueur d'onde).
RAS avec le filtre orange, jaune ou neutre.

J'utilise effectivement très souvent un filtre ND 3 stops 8x B+W sur le Summilux 50 f1.4 ASP.
Je n'ai jamais rencontré aucun souci de mise au point, vraiment jamais...
Et j'ai encore l'oeil de Lynx sinon

Yes, you will need ND-filters for all of your lenses

ND filters are Neutral Density filters. Or sunglasses for lenses.

The ND filter reduces the light going through the lens without degrading the quality or changing colors or anything. Just less light.

If you live in a Sunshine state, or visit a synny place with your Leica M9, you will need ND filters if you want to keep the lens fully open at f/0.95 or f/1.4 or whatever. You should always keep the aperture as open as the lens goes (as covered elsewhere in these pages).

Aperture is not for light control but for control of DOF (Depth Of Field), so only if you actually want more sharpness in the the front or back (for a group of three people portraits for example) should you use aperture to get more sharpness.

The rest should be done wide open. That is my opinion and how I always do it.

I keep stumbling into people who live in exotic countries with too much traffic and too much sun ... and yet they don't own a single ND filter. They use the aperture to step down the light.

And what a waste of good lenses!

8X ND-filters from B+W
You should get a 3 STOP ND filter for all of your lenses. Get B+W Made in Germany ND-filters, and get 3 stop (also known as 8X filters because you reduce the light to half, three times, 2x2x2 = 8).

As we like it (one should think), those ND-filters are handmade and hard to get. But get them anyways, and don't buy 64X filters or any other silly filters some photo stores have in stock. Outside the Leica ranks where few lenses are used wide open, ND-filters are mostly used to create stop-motion series. Hence the very dark 64X ND-filters are in stock for such purposes.

You want 3-stop 8X ND-filters.