Avoiding DC voltage drop for surveillance cameras over long distances
Wire has resistance. According to Ohms law, V=IR. Assuming your cameras use the maximum current allowable by the supply, I = 1 Ampere. Your wire resistance will depend on the gauge and metal used. Assuming good quality copper wire at 20AWG, you are looking at 10.15 Ohms per 1000ft. Lets assume you use 1600ft exactly - 800 there, 800 back. This is approx 16 Ohms.

V = 1 Amp x 16 Ohms = 16V.

Oh dear, you have dropped more voltage than you can supply!

Increasing the wire gauge to 10AWG brings the resistance down to ~1 Ohm per 1000ft, ergo

V = 1 x 1.6 = 1.6V 

But wait, what if you bundle 10 seperate 20AWG wires?

V = 1 x (16/10) = 1.6V

If you have 16000ft of wire, that is.

The cameras may not run at a voltage lower than 12V, so even this could fail. However, they could also draw significantly less current than 1A, which would mean less voltage drop.

Some testing on your part would be advisable.

Edit: A simpler solution would be to use a mains extension cable and bring the 12V adapter closer to the cameras.

Wire gauge source: http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/wirega.html