Exposure & Shutter Speed
In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance. In photography, shutter speed is the length of time a shutter is open; the total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor.
Correct exposure
The "correct" exposure for a photograph is determined by the sensitivity of the medium used. For photographic film, sensitivity is referred to as film speed and is measured on a scale published by the International Organization for Standardization (ISO). Faster film requires less exposure and has a higher ISO rating. Exposure is a combination of the length of time and the level of illumination received by the photosensitive material. Exposure time is controlled in a camera by shutter speed and the illumination level by the lens aperture. Slower shutter speeds (exposing the medium for a longer period of time) and greater lens apertures (admitting more light) produce greater exposures. An approximately correct exposure will be obtained on a sunny day using ISO 100 film, an aperture of f/16 and a shutter speed of 1/100th of a second. This is called the sunny 16 rule: at an aperture of f/16 on a sunny day, a suitable shutter speed will be one over the film speed (or closest equivalent). Ultimately there is no such thing as "correct exposure", as a scene can be exposed in many ways, depending on the desired effect a photographer wishes to convey. ReciprocityAn important principle of exposure is reciprocity.
If one exposes the film or sensor for a longer period, a reciprocally
smaller aperture is required to reduce the amount of light hitting the
film to obtain the same exposure. For example, the photographer may
prefer to make his sunny-16 shot at an aperture of f/5.6 (to obtain a shallow depth of field). As f/5.6 is 3 stops 'faster' than f/16,
with each stop meaning double the amount of light, a new shutter speed
of (1/125)/(2·2·2) = 1/1000 is needed. Once the photographer has
determined the exposure, aperture stops can be traded for halvings or
doublings of speed, within limits. 
The true characteristic of most photographic emulsions is not actually linear, (see sensitometry) but it is close enough over the exposure range of about one second to 1/1000th of a second. Outside of this range, it becomes necessary to increase the exposure from the calculated value to account for this characteristic of the emulsion. This characteristic is known as reciprocity failure. The film manufacturer's data sheets should be consulted to arrive at the correction required as different emulsions have different characteristics. Digital camera image sensors can also be subject to a form of reciprocity failure.[1] Determining exposure
The zone system is another method of determining exposure and development combinations to achieve a greater tonality range over conventional methods by varying the contrast of the 'film' to fit the print contrast capability. Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying only the shutter speeds) made in quick succession. Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-in light meter, or multiple point meters interpreted by a built-in computer, see metering mode. Negative/Print film tends to bias for exposing for the shadow areas (film dislikes being starved of light), with digital favouring exposure for highlights. See latitude below. LatitudeLatitude is the degree by which you can over, or under expose an image, and still recover an acceptable level of quality from an exposure. Typically negative film has a better ability to record a range of brightness than slide/transparency film or digital. Digital should be considered to be the reverse of print film, with a good latitude in the shadow range, and a narrow one in the highlight area; in contrast to film's large highlight latitude, and narrow shadow latitude. Slide/Transparency film has a narrow latitude in both highlight and shadow areas, requiring greater exposure accuracy. Negative film's latitude increases somewhat with high ISO material, in contrast digital tends to narrow on latitude with high ISO settings. Highlights
Areas of a photo where information is lost due to extreme brightness are described as having "blown-out highlights" or "flared highlights". In digital images this information loss is often irreversible, though small problems can be made less noticeable using photo manipulation software. Recording to RAW format can ameliorate this problem to some degree, as can using a digital camera with a better sensor. Film can often have areas of extreme overexposure but still record detail in those areas. This information is usually somewhat recoverable when printing or transferring to digital. A loss of highlights in a photograph is usually undesirable, but in some cases can be considered to "enhance" appeal. Examples include black-and-white photography and portraits with an out-of-focus background. BlacksAreas of a photo where information is lost due to extreme darkness are described as "crushed blacks". Digital capture tends to be more tolerant of underexposure, allowing better recovery of shadow detail, than same-ISO negative print film. Crushed blacks cause loss of detail, but can be used for artistic effect. See also
References
Shutter speed can have a dramatic impact on
the appearance of moving objects. Changes in background blurring are
apparent from the need to adjust the aperture size to achieve proper exposure.
In photography, shutter speed is the length of time a shutter is open; the total exposure is proportional to this exposure time, or duration of light reaching the film or image sensor.
IntroductionFactors that affect the total exposure of a photograph include the scene luminance, the aperture size (f-number), and the exposure time (shutter speed); photographers can trade off shutter speed and aperture by using units of stops. A stop up and down on each will halve or double the amount of light regulated by each; exposures of equal exposure value can be easily calculated and selected. For any given total exposure, or exposure value, a fast shutter speed requires a larger aperture (smaller f-number). Similarly, a slow shutter speed, a longer length of time, can be compensated by a smaller aperture (larger f-number). Slow shutter speeds are often used in low light conditions, extending the time until the shutter closes, and increasing the amount of light gathered. This basic principle of photography, the exposure, is used in film and digital cameras, the image sensor effectively acting like film when exposed by the shutter. Shutter speed is measured in seconds. A typical shutter speed for photographs taken in sunlight is 1/125th of a second. In addition to its effect on exposure, shutter speed changes the way movement appears in the picture. Very short shutter speeds are used to freeze fast-moving subjects, for example at sporting events. Very long shutter speeds are used to intentionally blur a moving subject for artistic effect.[1] Adjustment to the aperture controls the depth of field, the distance range over which objects are acceptably sharp; such adjustments generally need to be compensated by changes in the shutter speed. In early days of photography, available shutter speeds were somewhat ad hoc.[2] Following the adoption of a standardized way of representing aperture so that each major step exactly doubled or halved the amount of light entering the camera (f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.), a standardized 2:1 scale was adopted for shutter speed so that opening one aperture stop and reducing the shutter speed by one step resulted in the identical exposure. The agreed standards for shutter speeds are:[3]
This scale can be extended at either end in specialist cameras. Some older cameras use the 2:1 ratio at slightly different values, such as 1/100 s and 1/50 s, although mechanical shutter mechanisms were rarely precise enough for the difference to have any significance. The term "speed" is used in reference to short exposure times as fast, and long exposure times as slow. Shutter speeds are often designated by the reciprocal time, for example 60 for 1/60 s. Camera shutters often include one or two other settings for making very long exposures:
The ability of the photographer to take images without noticeable blurring by camera movement is an important parameter in the choice of slowest possible shutter speed for a handheld camera. The rough guide used by most 35 mm photographers is that the slowest shutter speed that can be used easily without much blur due to camera shake is the shutter speed numerically closest to the lens focal length. For example, for handheld use of a 35 mm camera with a 50 mm normal lens, the closest shutter speed is 1/60 s. This rule can be augmented with knowledge of the intended application for the photograph, an image intended for significant enlargement and closeup viewing would require faster shutter speeds to avoid obvious blur. Through practice and special techniques such as bracing the camera, arms, or body to minimize camera movement longer shutter speeds can be used without blur. If a shutter speed is too slow for hand holding, a camera support — usually a tripod — must be used. Image stabilization can often permit the use of shutter speeds 3-4 stops slower (exposures 8-16 times longer). Shutter priority refers to a shooting mode used in semi-automatic cameras. It allows the photographer to choose a shutter speed setting and allow the camera to decide the correct aperture. This is sometimes referred to as Shutter Speed Priority Auto Exposure, or Tv (time value) mode. Creative utility in photographyWhile shutter speed is used to control the amount of light recorded by the camera's digital sensor or film, it can also be used to manipulate the visual effects of the final image well beyond how light or dark it is. Slower shutter speeds are often selected to suggest movement in a still photograph of a moving subject. When an excessively fast shutter speed is selected, a moving subject can often appear unnaturally frozen. For instance, a running person may be caught with both feet in the air with all indication of movement lost in the frozen moment. When a slower shutter speed is selected, a longer time passes from the moment the shutter opens till the moment it closes. More time is available for movement in the subject to be recorded by the camera. A slightly slower shutter speed will allow the photographer to introduce an element of blur, either in the subject, where, in our example, the feet, which are the fastest moving element in the frame, might be blurred while the rest remains sharp; or if the camera is panned to follow a moving subject, the background is blurred while the subject remains sharp. The exact point at which the background or subject will start to blur depends on the rate at which the object is moving, the distance it is from the camera and the focal length of the lens in relation to the size of the digital sensor or film. When slower shutter speeds, in excess of about half a second, are used on running water, the photo will have a ghostly white appearance, with a reminiscence of fog. This effect can be used in landscape photography. Zoom burst is a technique which entails the variation of the focal length of a zoom lens during a longer exposure. In the moment that the shutter is opened, the lens is zoomed in, changing the focal length during the exposure. The center of the image remains sharp, while the details away from the center form a radial blur, which causes a strong visual effect, forcing the eye into the center of the image.[4] Cinematographic shutter formulaIn cinematography, shutter speed is a function of the frame rate and shutter angle. Most motion picture film cameras use a rotating shutter with a shutter angle of 165° or 180°, which leaves the film exposed for about 1/48 or 1/50 second at a standard 24 frame/s.[5] Where E = Exposure, F = Frames per second, and S = Shutter angle:[5] References
See alsoThis article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Exposure" |
