Originally posted by pentaxus It is my understanding that 1.7 is 1/2 stop slower than 1.4 and 1.2 is 1/2 stop faster with 1.8 being 1/3 stop faster than f/2. That being said, I remember that the SMC Pentax 50mm 1.4 lens was tested slightly faster by transmission (T-stop) than the f/1.2 Olympus lens by Popular Photography sometime over 30 years ago. Way back when I had my first Spotmatic I compared my old Tamron 70-210 f/4 to my new SMC Takumar 85-210 f/4.5 and found the Tak transmitted more light according to my light meter. "f" numbers are mathematically calculated and there are a lot of variables. Film makers have historically used t-stops for more consistent results. TTL lightmeters correct for irregularities but it could very well be that some manufacturers take advantage of that.
Transmission is another thing than apperture really. We assume it is the same for simplicity, but light as to go through the glass, some of it is absorbed, some of it is reflected and this has nothing to do with the physical apperture. The apperture define the maximum light density that can go through the lens, not the effective light density that go through the lens.