Photographic lenses are made of several elements of very pure
glass, each element being carefully coated to limit reflection.
However, a portion of the light is lost in the optical system
due to residual reflection and absorption. The transmission
factor (the proportion of light that actually makes its way to
the sensor) depends on the optical formula (the number of
lenses, the glass and coating formulas) and to a lesser degree
on the shooting parameters. As a measure of transmission,
DxOMark reports the T-stop, defined as the f-number of a lens
with 100% perfect transmission, and resulting in the same
illumination of the sensor at the image center as the lens to be
measured. DxOMark displays T-stop at full aperture function of
focal length, and transmission loss at full aperture function of
focal length.