 Josh |
Lens Question Hi again, I hear all this talk about 20 -310mm f2 (or something along those lines) about lenses. I have figured out the mm part is the zoom... (I think) but somehow I still can't manage to figure out what the f thing is. Does it have something to do with the depth of field? Thanks for your help.
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Jon Close |
The f-number, also called f-stop, refers to the maximum aperture of the lens. It is expressed as a ratio to the focal length, so f/2 means an maximum aperture with diameter equal to 1/2 the focal length, f/5.6 is 1/5.6 of the focal length, etc. The larger the maximum aperture (smaller number), the more light the lens lets in. The f-numbers are standardized to ease the calculation of proper exposure. Longer focal length lenses let in less light than shorter focal length lenses, but if two lenses are both set to the same aperture value, say f/4, they let in the same amount of light. Because the area of a circle (the aperture) is calculated as a function of the square of its diameter, doubling or halving the area is a function of the square root of 2, approximately 1.4. A "full stop" change in exposure is 1/2 or double the amount of light on the film, so the progression of f-numbers for aperture settings is 1 1.4 2 2.8 4 5.6 8 11 16 etc. f/4 lets in twice as much light as f/5.6 and 1/2 as much as f/2.8. Apertures have an effect on depth of field (the depth of a scene that is in apparent focus). Smaller apertures (larger f-numbers) give greater depth of field, larger apertures (smaller f-numbers) give shallow depth of field.
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