TTL Dedicated Flash
Get your camera and flashgun "talking"!
TTL simply means “through the lens” and is a term used for both normal camera metering and flash metering. When the speedlight is on the camera, the flash fires out, reflects from the surface of the subject, bounces back and in through the camera’s lens.
The amount of light returning is recorded by the camera which tells the flash to stop the output resulting in a perfect exposure (dedicated).
Dedicated means that the camera and flashgun speak the same language making exposure settings and results that much better.
If the speedlight is off the camera, it is difficult for the camera to judge the correct exposure because camera and flash are at different locations and distances.
Therefore, you would need to go manual with the flash and practice until the exposure is correct…all good fun!