Voltage Adapter Converts 24V AC to 12V DCThis is the answer for long power runs. A long run somewhat depends on the camera you are using.
12 volt DC voltage experiences a voltage drop because it is direct current. For example, a 12-volt power source on 22-gauge wire running 500 ft with a camera consuming .5amps (500ma) will only put out 8.5 volts at the camera. This is not good, it will produce a bad picture and reduce the life of the camera.
Another example is all the same as the above example except using 18-gauge wire, will only have a voltage drop of 1.4 volts--this is normally within the tolerance limits of most cameras.
Ideal For Long RunsOn the other hand, AC voltage or alternating current does not experience a voltage drop for all practical purposes. This means a person can begin the power run with a 24 VAC low-voltage power supply and run the 24 volts AC all the way to the camera distance. At that point you connect the Voltage Adapter and convert the 24VAC to 12 VDC. Now you have a full 12 volts DC at the camera regardless whether your camera is 300, 500, or even 800 ft away from the power source.
This entire process is done best when utilizing a 500 ft or 1000 ft roll of Siamese cable for your long-run installations.
This is the smartest, safest, and most efficient way to power a 12 volt DC camera when you have a long run.