A modern TTL flash can take all the guesswork out of proper exposure.
The TTL in camera flash TTL stands for "Through The Lens." What this means is that the light from the flash is metered through the lens just like any other light source.
How It Works
When the photographer presses the shutter with the flash in TTL mode, the light that comes from the flash bounces off the subject and returns through the lens, where it is metered along with the ambient light to set the correct exposure and turn off the flash at the same time. This all happens in thousandths of a second.
Why It Is Used
A flash provides very bright light, and if not used correctly, will overexpose the image. Many people expect a flash to do magic and underexpose due to distance or shutter speed. A flash set for TTL will always provide the correct exposure regardless of f-stop or shutter speed, as long as the flash is powerful enough to reach the subject.
Flash Speed vs. Shutter Speed With TTL
Because of the nature of TTL flash, the speed of the flash will always be much faster than the shutter speed. What this means is that within a fairly wide range, it doesn't matter what the shutter speed is--the exposure is set by the TTL flash. This gives the photographer the greatest leeway in the shutter speed and aperture.
Tags: shutter speed, correct exposure, flash will, flash will always, from flash
