Originally posted by Blue Only in "equivalency" terms. Otherwise the FA 77/1.8 LTD wouldn't be f1.8 on my MZ-3 and K-5. Physically, the focal length and f numbers are the same.
It is true that the focal length of the lens doesn't change (upon format change). But its FOV does.
It is also true that the f-ratio of a lens doesn't change (upon format change). But the total amount of light collected does.
Only FOV and total amount of light collected matter for the image.
Even on a Q, the F77/1.8 would be an f/1.8 lens. But the images you take on a Q with "f/1.8" correspond to "f/10" images on an FF. The DOF and noise of the Q-f/1.8 image and the FF-f/10 image would be the same.
One can call a Q-f/1.8 image an "f/1.8" exposure and one can call the Pentax 16-50/2.8 an f/2.8 lens. There is nothing wrong about this. Except that one has to bear the format in mind, according to which these figures are expressed. When expressed in the context of the Q, "f/1.8" means "very, very slow" (not "rather fast"). When expressed in the context of APS-C, "f/2.8" means "a stop slower than f/2.8 on FF". So if a TC converts the 16-50/2.8 into a 24-75/4.2, it doesn't make it slower (on an FF camera). It just translates the APS-C parameters into FF parameters.
I think we agree, but I thought it'd be worth spelling all those things out.