Sure, this is a technical curiosity question. Does anyone have a simple way to explain how the "speed of a lens" is calculated? I do not mean the basic answer that a f/2.8 is faster than a f/4.o. I know that part. Here is a "for instance": I have binoculars. These are about 350mm focal length, based on the magnification. The exit pupil is about 6mm, meaning these binoculars are about f/52. This is a small aperture. (Good dof but not a lot of light compared to a camera lens.) Still, this does not tell me anything about the speed of the lens. The objective lens(the big one on the front end) is 35mm. This HAS to figure in the speed of the lens. I use the binoculars only as an example. I have a 300mm zoom prime lens for my camera. This is an f/4.0 throughout the zoom range. The objective is 82mm, or close to 3.2 inches. Apertrue is adjustable as in any camera lens. Now, how in the world is the effective "speed of the lens" figured out? I assume the larger the objective lens the faster the lens might be. I assume an f/2.8 300 mm lens would have a larger objective lens...getting really large. Can anyone help me on this question which is almost irrevelant to actually shooting the pic? This is my curiosity working. Thanks. If no replies, I do understand. TomDart.