28-06-2005, 10:15 PM
referia-me a construçao.
nao ao principio.
reparem q nas digitais funciona por equivalencias. o link do artur mostra isso.
para entenderem melhor o funcionamento generico.
The exposure is the amount of light received by the film or sensor and is determined by how wide you open the lens diaphragm (aperture) and by how long you keep the film or sensor exposed (shutterspeed). The effect an exposure has depends on the sensitivity of the film or sensor.
The exposure generated by an aperture, shutterspeed, and sensitivity combination can be represented by its exposure value "EV". Zero EV is defined by the combination of an aperture of f/1 and a shutterspeed of 1s at ISO 100. Each time you halve the amount of light collected by the sensor (e.g. by doubling shutterspeed or by halving the aperture), the EV will increase by 1. For instance, 6 EV represents half the amount of light as 5 EV. High EVs will be used in bright conditions which require a low amount of light to be collected by the film or sensor to avoid overexposure.
For instance if you are shooting at ISO 100 with an aperture of f/8 and a shutterspeed of 1/125s, doubling the shutterspeed to 1/250 (halving the exposure time) and reducing the f-number one stop to f/5.6 (doubling the aperture) will lead to the same exposure of 13 EV. Or if you double the shutterspeed to 1/250s (halve the exposure time) while keeping the aperture unchanged at f/8, you could double the effect of the incoming light by doubling the sensitivity to ISO 200, thereby keeping the EV constant at 13 EV. Note that doing so will increase noise levels in digital cameras and film grain in conventional cameras.
In automatic mode, the camera determines the optimal combination of aperture, shutterspeed, and sensitivity based on the exposure value determined by the light metering system. A high EV indicates bright conditions, hence the need for high shutterspeeds, high f-numbers, and/or low sensitivities, to avoid overexposure. When you change the aperture in aperture priority mode, the camera will adjust the shutterspeed to keep the EV constant. In shutter priority mode, the camera will adjust the aperture to keep the EV constant.
Aperture refers to the size of the opening in the lens that determines the amount of light falling onto the film or sensor. The size of the opening is controlled by an adjustable diaphragm of overlapping blades similar to the pupils of our eyes. Aperture affects exposure and depth of field.
Just like successive shutterspeeds, successive apertures halve the amount of incoming light. To achieve this, the diaphragm reduces the aperture diameter by a factor 1.4 (square root of 2) so that the aperture surface is halved each successive step.
Because of basic optical principles, the absolute aperture sizes and diameters depend on the focal length. For instance, a 25mm aperture diameter on a 100mm lens has the same effect as a 50mm aperture diameter on a 200mm lens. If you divide the aperture diameter by the focal length, you will arrive at 1/4 in both cases, independent of the focal length. Expressing apertures as fractions of the focal length is more practical for photographers than using absolute aperture sizes. These "relative apertures" are called f-numbers or f-stops. On the lens barrel, the above 1/4 is written as f/4 or F4 or 1:4.
We just learned that the next aperture will have a diameter which is 1.4 times smaller, so the f-stop after f/4 will be f/4 x 1/1.4 or f/5.6. "Stopping down" the lens from f/4 to f/5.6 will halve the amount of incoming light, regardless of the focal length.
Because f-numbers are fractions of the focal length, "higher" f-numbers represent smaller apertures.
Maximum Aperture or Lens Speed
The "maximum aperture" of a lens is also called its "lens speed". Aperture and shutterspeed are interrelated via exposure. A lens with a large maximum aperture (e.g. f/2) is called a "fast" lens because the large aperture allows you to use high (fast) shutterspeeds and still receive sufficient exposure. Such lenses are ideal to shoot moving subjects in low light conditions.
Zoom lenses specify the maximum aperture at both the wide angle and tele ends, e.g. 28-100mm f/3.5-5.6. A specification like 28-100mm f/2.8 implies that the maximum aperture is f/2.8 throughout the zoom range. Such zoom lenses are more expensive and heavy.
Nao sou fotografo profissional, se bem q se ganha muito guitos com casorios, batizados e comunhoes.
nao ao principio.
reparem q nas digitais funciona por equivalencias. o link do artur mostra isso.
para entenderem melhor o funcionamento generico.
The exposure is the amount of light received by the film or sensor and is determined by how wide you open the lens diaphragm (aperture) and by how long you keep the film or sensor exposed (shutterspeed). The effect an exposure has depends on the sensitivity of the film or sensor.
The exposure generated by an aperture, shutterspeed, and sensitivity combination can be represented by its exposure value "EV". Zero EV is defined by the combination of an aperture of f/1 and a shutterspeed of 1s at ISO 100. Each time you halve the amount of light collected by the sensor (e.g. by doubling shutterspeed or by halving the aperture), the EV will increase by 1. For instance, 6 EV represents half the amount of light as 5 EV. High EVs will be used in bright conditions which require a low amount of light to be collected by the film or sensor to avoid overexposure.
For instance if you are shooting at ISO 100 with an aperture of f/8 and a shutterspeed of 1/125s, doubling the shutterspeed to 1/250 (halving the exposure time) and reducing the f-number one stop to f/5.6 (doubling the aperture) will lead to the same exposure of 13 EV. Or if you double the shutterspeed to 1/250s (halve the exposure time) while keeping the aperture unchanged at f/8, you could double the effect of the incoming light by doubling the sensitivity to ISO 200, thereby keeping the EV constant at 13 EV. Note that doing so will increase noise levels in digital cameras and film grain in conventional cameras.
In automatic mode, the camera determines the optimal combination of aperture, shutterspeed, and sensitivity based on the exposure value determined by the light metering system. A high EV indicates bright conditions, hence the need for high shutterspeeds, high f-numbers, and/or low sensitivities, to avoid overexposure. When you change the aperture in aperture priority mode, the camera will adjust the shutterspeed to keep the EV constant. In shutter priority mode, the camera will adjust the aperture to keep the EV constant.
Aperture refers to the size of the opening in the lens that determines the amount of light falling onto the film or sensor. The size of the opening is controlled by an adjustable diaphragm of overlapping blades similar to the pupils of our eyes. Aperture affects exposure and depth of field.
Just like successive shutterspeeds, successive apertures halve the amount of incoming light. To achieve this, the diaphragm reduces the aperture diameter by a factor 1.4 (square root of 2) so that the aperture surface is halved each successive step.
Because of basic optical principles, the absolute aperture sizes and diameters depend on the focal length. For instance, a 25mm aperture diameter on a 100mm lens has the same effect as a 50mm aperture diameter on a 200mm lens. If you divide the aperture diameter by the focal length, you will arrive at 1/4 in both cases, independent of the focal length. Expressing apertures as fractions of the focal length is more practical for photographers than using absolute aperture sizes. These "relative apertures" are called f-numbers or f-stops. On the lens barrel, the above 1/4 is written as f/4 or F4 or 1:4.
We just learned that the next aperture will have a diameter which is 1.4 times smaller, so the f-stop after f/4 will be f/4 x 1/1.4 or f/5.6. "Stopping down" the lens from f/4 to f/5.6 will halve the amount of incoming light, regardless of the focal length.
Because f-numbers are fractions of the focal length, "higher" f-numbers represent smaller apertures.
Maximum Aperture or Lens Speed
The "maximum aperture" of a lens is also called its "lens speed". Aperture and shutterspeed are interrelated via exposure. A lens with a large maximum aperture (e.g. f/2) is called a "fast" lens because the large aperture allows you to use high (fast) shutterspeeds and still receive sufficient exposure. Such lenses are ideal to shoot moving subjects in low light conditions.
Zoom lenses specify the maximum aperture at both the wide angle and tele ends, e.g. 28-100mm f/3.5-5.6. A specification like 28-100mm f/2.8 implies that the maximum aperture is f/2.8 throughout the zoom range. Such zoom lenses are more expensive and heavy.
Nao sou fotografo profissional, se bem q se ganha muito guitos com casorios, batizados e comunhoes.