When shooting street, kids, or anything that doesn't stay still to wait for you to focus, you will have to rely on pre-focusing techniques so you can simply point-and-shoot. This also allow you to shoot from the waist, or from anywhere, if you don't want to draw attentions to yourself by holding the camera up to your eyes.
Depth of Field (DOF)
Rangefinder cameras don't have auto-focus, so all the magic is achieved by understanding the "depth of field" and its relation to the lens aperture. At any given aperture, there is a range of distance that things will appear to be in focus. I won't explain the physics and math behind it, but put it simply, the more open the aperture, the shorter the DOF, and as you stop-down or close up the aperture, the range of DOF is increased. Since we'll want a large DOF zone, we'll probably be setting the aperture at f/8 up to f/16 or higher. All rangefinder lenses will have their DOF scale engraved on the barrel, however, the scale is not 100% accurate, especially with digital sensors, so use it only as a suggestive value.
Using the image on the left, you can see my aperture is set at f/8, and looking at the DOF scale, you'll see there's an 8 on both sides of the arrow. If you follow the line of the left-sided 8, it points to about 2 meters, and you if follow the line of the right-sided 8, it points to about 3 meters. So, that's my DOF zone of about 1 meter, and if I shoot with the camera as is, everything that's 2 to 3 meters away from me will be in focus. Similarly, if I change my aperture to f/4, I will decrease my DOF zone down to about 1/2 meter (somewhere between 2.3 meters to 2.8 meters away from me), which means it's more likely to mis-focus. If I change my aperture to f/16, my DOF zone will increase to over 3 meters (somewhere between 1.7 meters to 5 meters away from me). How is this applicable? With this DOF zone, you can practice by shooting things that are 2-3 meters away, which is about 2-3 arm's length away from you. Once you get used to that distance, you will feel comfortable to shoot people and things that are in that general zone, all without focusing!
This is similar to zone-focusing, except we push the maximum distance of the DOF zone to infinity, and the minimum distance will become the hyperfocal distance. By turning the focus ring so that the infinity mark lines up with my aperture number on the right side of the arrow, you'll see something like the image on the right. Looking at the left side of the arrow, you can see my hyperfocal distance is about 5 meters. So if I was to take a shot, everything beyond the hyperfocal distance of 5 meters will be in focus. This technique is less useful than zone-focusing because it requires you to be much further away from the subjects.
In this post, I'm using my Zeiss 50mm C-Sonnar lens as an example. If you go with a lens of a shorter focal length, like 35mm, then the DOF zone will also increase in size. For example, looking at my 35mm Summicron version 4 lens, at f/8, I can have a zone of 2 to 10 meters, and the hyperfocal distance is about 2.3 meters. If I set the 35mm lens aperture to f/4, then I get the similar DOF zones and hyperfocal distance as the 50mm lens at f/8.
The range of the zone also decrease rapidly as you move it closer to the camera. You can see how the numbers of meters on my ZM50 lens' DOF scale are fairly evenly spaced, almost perfect for lining up the zone focus for f/8. So if I turn the focus ring for closer focus, you'll see my DOF zone range goes from 2 to 3 meters (actual range of 1 meter), then 1.5 to 2 meters (actual range of 0.5 meter), and 1.2 to 1.5 meters (actual range of 0.3 meter). Looking at my 35mm Summicron lens, I see the DOF scale is more optimized for reading the zones for f/4. Perhaps, that should be a hint for us to know which aperture to use when zone-focusing on a lens of any particular focal length.