 BetterPhoto Member |
Lens Focal Length Dear John, I'm having a standing argument with my lab about the change in exposure due to the changing of focal length, eg. in a studio situation using 2 lights, the light meter reading is f8/125. Now if I change the focal length of the lens (from say 75 to 210) to zoom in on the subject, does the exposure change and if so by how much? Regards, Garry
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John A. Lind |
Garry, In general it does not change (see special situation in last paragraph). f/8 _is_ f/8 on every lens, and you get the same volume of light if you keep the shutter speed the same. Indeed, that's why f-numbers are used for apertures, so it _will_ mean the same thing at every focal length! The f-number is defined theoretically as the focal length divided by the diameter of the aperture. As you increase focal length, the aperture diameter must increase to get the same volume of light through the lens. f/8 has about a 9.4mm aperture diameter on a 75mm lens and about a 26.3mm aperture diameter on a 210mm lens. This how using aperture f-numbers automagically compensates for different focal lengths to maintain the same volume of light. Note that this is the _effective_ diameter. In real life lens design it's adjusted slightly for absorption and reflection by the glass. It also may not be what you see if you try to measure it from outside the lens (front or back). I made an assumption in the first paragraph that the brightness level of highlight and shadow, and the balance between the two hasn't changed much when you shift focal length from 75mm to 210mm. When switching to the longer lens, if you are now framing on what was in in a highlight or shadow region when you used the shorter length, the exposure can shift. How much would be impossible to say. It depends on how much of the highlight or shadow you are omitting with the longer lens. -- John
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