Confused on Lens Speeds

Started Apr 8, 2012 | Discussions thread
Jay A Senior Member • Posts: 1,672
Re: Confused on Lens Speeds - mult by 1.5 and bob's your uncle

You aren't saying that a 75mm lens shot at f3 on an NEX-7 will result in the SAME EXPOSURE as a 50mm shot at f2 on an A900 are you? DOF, FOV, framing yes, but exposure? No way!

I assume you are not, but if you are then a hand held light meter would have to be recallibrated for different lenses and different fil formats (or sensor sizes).

Fact is a 75mm lens shot at any given f stop will result in exactly the same exposure as a 50mm (or ANY lens for that matter) shot at the same f stop. The important thing to remember is that the given f stop is not going to be the same sized diameter in that lens as it will be in another lens of a different focal length. F stop is derived by dividing the diameter of the opening into the focal length of the lens. Hence a 100mm lens will have a diameter of lens opening exactly twice as big as the same f stop on a 50mm lens. So the diameter for f2 on that 100mm lens will be 50mm and the diameter of that same f2 on the 50mm lens will be 25mm. This results in the same exposure on the sensor from either lens at that same f2 stop.

Again, I assume you were not saying otherwise, it just wan't quite clear in your explanation.

What I was trying to say was that if I were to take a A900 and a NEX-7, put a 75mm f/3 lens (if I could find one or stop down the aperture to f/3 on a 75mm lens), and take a NEX-7, and put a 50mm f/2 lens on it. Focus on the exact same spot in the exact same frame with both cameras, the photographs would look identical in their DOF, FOV, and framing/composition. Thus, equivalency.

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