A diffuser takes a narrow, concentrated beam of light and distributes that light across a larger area. We have to adjust for this by increasing the light output of the flash. Some automatic modes will make this adjustment correctly but we recommend using flash exposure compensation (FEC) to fine tune it. When firing the flash straight towards your subject, the camera’s TTL calculations are based off that concentrated beam of light and does not take the dispersed light fall off into consideration. A simple FEC adjustment can bump up the flash power to properly light your subject. By adjusting FEC you have more control over the exposure of your pictures.
On the other hand, when using a flash in bounce mode through a diffuser, the TTL anticipates the light loss and compensates. The diffuser makes the camera’s TTL think that the walls and ceiling are farther away than they really are and the flash automatically increases the flash power.
For Graslon Flash Diffusers we recommend adding 1 2/3 to 2 stops of FEC in normal portrait distances.
To see how to change FEC on your camera or flash, check out these tutorials that show how easily and quickly FEC can be changed on Canon and Nikon bodies, as well as on the flash.