The reason why different focal lengths are different exposures (unless both are set to the same f/stop number) is about the magnification of the focal length. And note that a hand-held light meter produces exposure settings that work on any camera. So what photographers need to know is that the purpose of using the f/stop numbering system is so that the same numerical f/stop on any lens will produce the same exposure, on any camera. However (the main point), if the same f/stop number were used on both lenses, then both are the same exposure, in any lens on any camera. But a 25 mm aperture on a 200 mm lens is f/8, much slower, four stops slower. A 50 mm lens with a 25 mm opening diameter is a fast f/2 lens. So f/8 is an aperture diameter dimensioned as literally a fraction of the focal length. See special cases of telephoto and wide angle lens diagrams below. In a 200 mm lens, a f/2.8 aperture is 70.7 mm diameter (2.78 inches), and f/32 aperture is 6.25 mm diameter (1/4 inch).
DEFINITION OF F STOP IN PHOTOGRAPHY FULL
The maximum aperture generally uses the full diameter of the glass lens, and stopped down is a smaller diameter. The physical aperture inside the lens is designed to that frontal definition, which itself will measure differently to produce the correct entrance pupil. The optical lens elements usually magnify its apparent size somewhat. That's the size of the "effective" aperture opening as seen from the front of the lens. Note that the "effective" aperture diameter is the frontal dimension “D” in the diagram above, called “entrance pupil” ( Wikipedia). The nomenclature is f/ denoting focal length and the division. The f/stop numbers are fractions of the focal length. The number for lens f/stop in photography (for example, f/8) is the ratio of lens focal length divided by the effective lens aperture. The choice and balance of these three factors is all important to photography.Ĭamera design involves math and physics, but still the very useful purpose of the f/stop numbering system is the grand concept that the same f/stop, like say f/8, will be the same exposure in any lens or camera, regardless of focal length, or physical lens size or construction.
DEFINITION OF F STOP IN PHOTOGRAPHY ISO
A higher ISO is more sensitive to light, helpful in dim light or at extremes of shutter speed or f/stop, but which after a point, adds greater digital noise.
ISO is the sensitivity to light, how well the light registers on the sensor.A faster shutter stops motion better, but which reduces the exposure, which can be compensated by opening aperture or increasing ISO. Shutter speed is the time duration while the shutter is open, letting light in.However a larger f/stop Number creates more depth of field and, up to a point, can be generally sharper. The smaller the f/stop Number, the larger the aperture diameter, passing more light. The f/stop Number is about the diameter of the lens aperture, which is how much light it lets in.Then the third page shows charts of the actual precise goal values of the settings the camera actually uses. The Camera Numbers: Thia page is about "What are those numbers?" There is a How Many Stops? Calculator on the Next following page to help with the numbers.
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