I come from an autofocus camera background. What is the best way to get good focus on an M camera?
The M is a rangefinder camera, the central patch in the viewfinder is your focusing tool.
It is important to look through the viewfinder in the optical axis. Looking into the camera skewed will result in inaccurate focus.
The first thing to do is to ascertain that you can see the rangefinder patch properly. A correct match between the rangefinder and your eye is even more important than it is using an SLR.
Leica sells corrective diopter lenses. Determining which one you need - if any- can be done by going to your optician and holding his try-out lenses between your eye and the viewfinder. The one that allows you to see the rangefinder patch and framelines sharply is the correct one. Order the nearest value from Leica. In a pinch you can use over-the-counter reading glasses for this test. If your eyes need special corrections, you can use your spectacles, provided you can see clearly at 2 meters distance ( the virtual distance of the rangefinder patch). Note that the background will be at background distance,so your eye should ideally be able to accommodate over the distance differential. However, there is some tolerance here.
Note that the -0.5 diopter value that is built into the viewfinder is there for this reason. For an ideal eye it compensates for the virtual distance of framelines and focus patch @ 2 m.
It is possible to calculate the diopter value, but that should be left to an expert.
If there is astigmatism present the diopter value can only be corrected for one camera position. However Walter Leica provides an intelligent solution:
For special cases there are viewfinder magnifiers. (for the time being they are only available in the old diopter size and must be used with the diopter adapter on the M10) They can help, especially with longer and fast lenses and they can give confidence, but they can also be not very useful; they cannot correct errors in the focusing mechanism or your eye, in fact they magnify them.
Also, one loses contrast and brightness.
Leica offers a 1.25x one and a 1.4x. These need diopter correction like the camera, but often of a different value than the camera viewfinder.
There are also third-party magnifiers, sold by Japan Exposures, that include a variable diopter correction. 1.15x and 1.35x. For patent reasons they cannot be sold in the USA and Germany for use on a Leica camera, but they can be purchased for use on for instance a rifle scope.
Basically, for an experienced user, magnifiers are not needed and will only lower contrast and brightness, but many users do like and use them.
Once the viewfinder is corrected optimally, there are three methods of focusing, in ascending order of difficulty aka training.
1. The broken line method. Look for a vertical line in the image and bring it together in the rangefinder patch to be continuous.
2. The coincidence method. Look for a pattern in the image and bring it together to coincide. This may lead to errors with repeating patterns.
3. The contrast method. Once you have focus by method 1. or 2. a small adjustment will cause the rangefinder patch to "jump" into optimum contrast. At that point you have the most precise focussing adjustment.
If you try focusing on a subject emitting polarized light like a reflection it may happen that the polarizing effect of the prism system in the rangefinder will blot out the contrast in the rangefinder patch, making focusing difficult. In that case rotate the camera 90 degrees to focus.