Many people do not fully understand the law as it pertains to traffic signals, and to make matters worse that law also varies considerable from state to state. The distinction is known as either being a "permissive" or "restrictive" yellow light state.
The majority of states are "permissive", which means that they follow the Manual on Uniform Traffic Control Devices (MUTCD) and the Uniform Vehicle Code (UVC), which says that it is legal to enter any intersection under a green or yellow light, and continue through if the light turns red while actually in the intersection. It is (always) illegal to enter an intersection when the light has already turned red, except for when making a right turn in the "Right Turn On Red" states. So, in permissive states the red light camera would have to capture photo evidence of the vehicle crossing the stop line with the light already red, and not making a right turn, to get a ticket.
Restrictive states are those that are not in compliance with MUTCD and UVC, where you are expected to stop on any yellow if possible, and need to fully clear the intersection before the light turns red. In those states all you would need is a photo of the vehicle anywhere in the intersection on a red light to get ticketed. But enforcement of the law would be more troublesome for turning traffic in these states. For instance, left turners may be stuck in the intersection waiting for oncoming traffic until after a light turns red.
The problem with red light cameras in general is that, while it reduces the number of red light runners, and thereby reduces the number of side collisions in the intersection, it has coincidentally been found to increase the number of rear end collisions from people stopping short to avoid the possibility of a ticket. Which is why use of photo enforcement is on the decline in many of the areas that have tried it in the past.
