While doing some low-light tests, I was surprised by the difference here: Canon FD 135mm f/2.8 Canon FD 50mm f/2.8 (cropped) Both images were shot manually with identical settings on an NEX-6, attached to a Lens Turbo II. Some slight post-processing but exposure levels are identical. Both lenses are of FDn type, with S.S.C. coating. Is this purely because of a relatively poor T-stop performance for the 135mm?