Though-the-lens (TTL) flash metering works as the light from the flash bounces off the subject and reflects back through the camera's lens. The sensor inside the camera determines the amount of light needed to properly illuminate the photographic subject.

History

Prior to the TTL flash system, the process of calculating the correct amount of flash for any photo setting had to be manually gauged. According to Chris George, author of "Mastering Digital Flash Photography," "This was a laborious procedure, requiring you to know the flash-to-subject distance, the ISO, and the flash unit's guide number."

Types

A TTL flash unit may be built into the camera or it may be external. External flashes are typically mounted in the camera's hot shoe.

Advantages

Because the camera's sensor automatically does all of the work, using a TTL flash makes shooting spontaneous. In addition, an external TTL flash unit may be used off of the camera, giving the photographer greater lighting flexibility.

Disadvantages

Very bright subjects in an otherwise poorly lit scene can result in an inaccurate sensor reading. The significant amount of light reflecting off of a bright surface might cause the flash to improperly illuminate the entire setting.

Review photos

Whenever you shoot a series of photos, it's advisable to constantly review your shots to check for under- or overexposure. Most digital single-lens reflexes include a mode allowing you to compensate for an inaccurate TTL flash exposure.

Source:

"Mastering Digital Flash Photography;" Chris George; 2007

Moose Peterson Wildlife Photography: The TTL Flash System

More Information:

All Things Photography: Flash Photography Can Be a Bit Hit or Miss