There are many types of flash triggers available with a variety of capabilities and price ranges. A good overview is at: http://dpanswers.com/content/radio_flash.php#ttlpt. That article mentions TTL Pass-through, among other topics, and often I get questions about this such as: “Is TTL pass-through important?” As is usually the case, the correct answer requires a lot more detail. To start, let’s define ‘advanced TTL’ to mean Camera controlled flash functions including TTL, HSS/AutoFP, exposure values, lens focal length, focus beam, and other information sent from camera to flash via digital signals.
Passing these ‘advanced TTL’ signals directly to the on-camera flash via the camera mounted trigger is referred to as ‘TTL pass-through’. This usually means the camera shoe signals are blindly passed from camera, through the trigger, to the flash. Unfortunately, for the photographer, TTL pass-through doesn’t do anything to provide ‘advanced TTL’ to the slaves, which is a significant omission since that is exactly what triggers are used for: controlling slaves! Yet, this doesn’t mean such devices are not capable. It simply means you should not expect TTL Off-Camera at the slaves.
