Well I can only speak for one type of radar, but the radar doesn't really matter. What actually matters is the officer's observation. You'd be very surprised how easy it is to estimate how fast a car is traveling with very little special training. I look at each car, make an estimate, and then point the radar at the ones I think are speeding. Legally we have to be within +/- 5mph, but I can usually get within 2mph, depending on the speed (I am actually better with higher speeds). So I know exactly which car I am pointing at because I already know they are speeding.
Furthermore, I don't need the radar to write the summons because of that training. I can write on estimation alone, but have only done it once and only because the guy was speeding while doing something else (which I intended to write him for), but since he cursed me out, I wrote the estimation also (I won both).
Just a little tip by the way (which you all already know, but its worth repeating)... every car stop for me is potentially my last, so I don't make them for no reason. If you have my lights in your rearview mirror (or those of most other cops), 99% of the time, you did exactly what the cop is about to accuse you of and 2-3 more things, so unless you want all 3-4 summonses, relax, take your ONE summonses and try to drive SAFER next time.