The photometric aperture, also known as “T-stop” (T = transmission), gives the aperture of the lens corrected by its transmission loss (see light transmission definition).
The measurement consists of taking a picture of a uniformly illuminated (+/-1%) opalescent transmission target.
The light source, chosen for its remarkable stability, is exactly the same as for the ISO speed measurement: A halogen lamp filtered to achieve a daylight color temperature of 5500K. This is worth noting because the ISO sensitivity values are used in the T-stop calculation.
We measure the luminance of the diffusing surface (about 140 cd/m²) with a certified luminance-meter .
Knowing the entrance light flux, the sensor response, and the shutter speed, we can calculate the T-stop of the lens for a given focusing distance:
We place the camera at a distance equal to 40 times the focal length of the lens (2 meters for a 50mm lens). We take one picture for each aperture of the lens (using full-stop increments).