The transmission result from the DxOMark set of metrics. Transmission refers to the amount of light that reaches the sensor through all of the glass elements of a lens, with a lower TStop signifying more light. This is important as less light reaching the sensor can lead to the requirement of higher ISOs or slower shutter speeds. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.

Transmission: is this important to you?