The M42 "universal screw thread" lens mount was used on film cameras from Pentax, Praktica, and others for decades... so a lot of great old lenses are available at modest prices. A lot of people are buying adapters (usually $5 to $30) that allow M42 lenses to be used on modern DSLRs made by Canon, Olympus, Pentax, Sony, etc.
Physically mounting an M42 lens on your DSLR is easy with an appropriate adapter, and of course the lens will have to be focused manually, but there is also a little matter of how one controls the lens aperture. The aperture is the thing shown in the photo; a diaphragm consisting of a set of blades that allow the amount of light passing through the lens to be reduced to the desired level, which also allows a controlled increase in the depth of field.
There are three basic types of M42 aperture controls , listed here in chronological order. The catch is that all three types can have no response to turning the obviously marked aperture ring -- without being broken. This little instructable is about how to make aperture control (f/ number setting) on each of these types of lenses work on your DSLR.
Step 1: Preset Aperture Control
Before the middle 1960s, M42 lenses typically had preset aperture controls, as does the Soligor 135mm f/2.8 shown here. Preset lenses typically have two separate aperture control rings near the front of the lens.
One of the rings is used to set the smallest aperture (minimum opening and largest f/ number) that you might like to use for taking a photo. This ring is usually marked with f/ numbers and often will have detents that allow it to hold its setting. Turning it usually doesn't get any response from the aperture blades.
The second ring may also be marked with f/ numbers, or sometimes simply with the letters "o" and "c," or it may be entirely unmarked (as in the photo). It usually doesn't have any detents, but turns smoothly. Turning this second ring toward "c" (close) allows the lens aperture to actually be closed to the f/ value set on the first ring, whereas turning toward "o" opens the aperture.
A few preset lenses (e.g., Volna-9) actually use a single ring at the front of the lens for both functions. Pressing that ring toward the camera body allows setting the stop.
Because preset lenses were completely manual, they still work precisely as intended on modern DSLRs. Actually, they are the easiest to use on a DSLR. You focus with the lens wide open and then twist the second ring to stop-down to the desired aperture just before taking the photo.
These lenses often will have the aperture blades far out on the lens body, away from the camera body. More significantly, there are often many aperture blades, giving a very circular openning. Unlike other lenses, a little oil on the blades generally is not a problem for a preset lens. Friction is pretty high, but your hand turning the second ring can provide plenty of force.