![]() ![]() There are bright spots and dark spots within any image, so the single camera exposure must be chosen with care, ideally to show bright things bright and dark things dark. The proper exposure of a lighted subject is not affected by the background being bright or dark (but it probably affects a light meter anyway). Exposure is about the light per unit area (and is NOT about the total amount of light on the sensor). Luminance can be thought of as similar effect as brightness, except brightness is relative and has no units and is not measured. Light meters measure Luminance, which is the luminous intensity per unit of area. 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). 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. 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. ![]()
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