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Exposure

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The amount of light reaching light sensitive material such as film, printing paper or a CCD. Exposure is controlled by the shutter speed (time) and aperture (intensity).

Related Terms

This is how the camera adjusts the shutter speed and aperture to ensure the right amount of light reaches the film or CCD. Early cameras only had a manual mode (M) where the user had to select the aperture and shutter speed manually to ensure the correct exposure. Over the years cameras have become more sophisticated and now offer several automated modes including Program (P) - a fully automatic exposure mode that sets the aperture and shutter speed; Aperture priority mode (AP) where the user selects the aperture and the camera sets the necessary shutter speed; and shutter priority (SP, or Tv on some cameras) where the user selects the shutter speed and the camera sets the necessary aperture. Auto bracketing (AB) takes a pre-selected number of photographs, one at the suggested exposure and one to either side, so you can be sure of one accurate result. There are also several subject-based program modes that we haven't listed here that tailor the camera for particular subjects such as sports (action), landscapes, portraits, or flowers (close-ups). Some digital cameras have black & white and sepia modes. Buying advice A full auto program mode is ideal for point-and-shoot photography, but it's also useful to have some control over the exposure. The beauty with digital is that you can see whether the camera has got the shot right by previewing the image on the LCD. If not, you try again. If there is no manual control you can often preset the exposure using an auto-exposure lock or exposure compensation. The subject based program modes are often a waste of time and don't really bring much to the package. Special effects modes on digital cameras are also throw-away because all these can be created using the computer later.
An auto-exposure mode where you select the required lens aperture and the camera sets the necessary shutter speed, to give the correct exposure based on the auto meter reading. This mode is ideal for landscape and still-life photography where maximum depth-of-field is required. It's either indicated on the camera as AP (aperture priority) or AV (aperture value).
Dial or buttons on a camera that allow the user to override the automatic exposure mode. Most cameras have +/-2 stops control that is enough to compensate in situations where the camera's system would normally be fooled. On digital compacts it's invaluable in adjusting exposure. It can also be used to modify contrast when a film is to be pushed or pulled.
This is an indication of the film's tolerance to exposure. A film with a wide exposure latitude will still produce acceptable results when the film has been under or overexposed by several stops.
An automatic exposure mode that sets the camera's aperture and shutter speed often referred to on camera dials and panels as P. What sets it apart from Auto mode, is that all the other functions, including exposure compensation, can still be adjusted.
An auto exposure mode where the user selects the shutter speed and the camera sets the necessary aperture to give correct exposure.
Taking several versions of the same photograph using different exposure settings to ensure one accurate result. Some cameras have an auto bracketing mode which will have the camera automatically fire off several frames of varying exposure in succession.
Where the camera measures the amount of light required for a scene and adjusts the exposure to suit.
Used in the zone system to segment the exposure range into one-stop intervals from O to IX.
An exposure mode where you set the shutter speed and aperture using either a hand held or built in meter, or knowledge, to help you determine what values to set.
A technique where several pictures are exposed on one frame of film. This can be used for special effects such as shooting the same person so they appear twice in the same photo. Its also useful for shooting, say, a photo of the moon using a long lens and taking a second shot of a landscape with a wide angle. The two combined will look surreal. You normally have to do this manually and on cameras with automatic film advance it can be very tricky to achieve. Fortunately some cameras have an automatic multiple exposure mode.
Reciprocity law states that as you increase the intensity of light reaching the film you also need to decrease the speed it reaches the film by the equivalent amount. Most films work quite happily between exposures of 1/2sec and 1/1000sec, but go beyond these extremes with a very low intensity of light and a long exposure or a very high intensity of light and a correspondingly short exposure and the law fails an exposure increase may be required when the shutter speed is beyond these limits. At these extremes the law fails. Compensation is required to adjust for this, but there is no strict rule to correct the error. Most film and paper manufacturers provide technical details on request with a rough guide to exposure adjustments. As a rough guide for an exposure of one second you would increase the speed to two seconds, or open the aperture by one stop. A speed of 10sec would need to be increased to about 50sec or open the aperture up two stops. With black & white film you only have to worry about this exposure correction, but with colour film does not only suffer from exposure problems but also colour casts. A colour film is made up of three individual colour layers, each layer suffers from reciprocity failure at different levels. On an uncorrected film the shadows may have a magenta colour cast but the highlights may suffer from a cyan cast. To correct the cast not only would a longer exposure be needed but also the inclusion of a colour correction filter of a low value, care would have to be taken in choosing the correct filter otherwise an over corrected result may appear.
A button or lever on the camera or exposure meter used to lock the automatically measured light reading into a memory while you recompose.This is ideal for backlit subjects because you can move closer to the subject, take a reading without any background in the view, lock the exposure and move back to the original position to take the photo.It's also fine for landscapes when there's an expanse of bright sky. Point the camera down to the ground, lock the exposure, recompose and shoot.Use this to bias the exposure to one area of the scene or to maintain a consistent reading when lighting conditions are variable.
To make an exposure, a photographer can, for example, choose a combination of a small aperture and a slow shutter speed or a large aperture and a fast shutter speed. Each movement of the aperture is classed as one stop – go from f/8 to f/11 and you close the aperture by one stop. Similarly by adjusting the shutter speed from 1/125sec to 1/500sec you reduce the exposure by one stop. If you adjust the aperture by a stop and counteract this by also adjusting the shutter speed by one stop you will produce the same exposure value. Therefore an exposure of f/8 at 1/60th could be changed to any of the following combinations: f/11 at 1/30th, f/16 at 1/15th, f/5.6 at 1/125th or f/4 at 1/250th. This is called the law of reciprocity (if one value increases the other will decrease proportionally and visa versa).
An automatic exposure mode which is specifically designed for the correct exposure of bright scenes. On cameras without an automatic mode for this, but with a manual mode, correct exposure can be achieved by overexposing by about two stops
A Global shutter exposes the entire imager simultaneously. The entire frame is exposed and begins gathering light; when the predetermined “shutter speed” has elapsed, the sensor stops gathering light and turns its current exposure into an electronic image.

Used in many CCD sensors. Global shutters are immune to effects like skew, wobble, and partial exposure.

Some CMOS sensors are configured to use global shutters - in which case they do not suffer from vertical smear, skew, wobble or partial exposure.
Override of the camera's auto-exposure setting that increases the exposure by between one and two stops. Use this mode to prevent a silhouette when taking photos while the light is behind the subject.
A display for modes and features in the viewfinder or top-plate of many cameras. Most display the number of photos you have left along with the battery condition and exposure mode. More advanced cameras also have the metering mode, exposure mode and flash mode displayed.
A meter reading taken from the image that appears on the film surface during exposure. Its main use is for through-the-lens flash exposure readings.
A lighting control that lets you fine tune the output of flash. This comes in useful when space is limited and you can't move lights back and forth or you want to adjust the exposure of one units when used in a set to balance the exposure.
A metering system that uses several sensors to take readings from various sections around the image and calculates an exposure based on these values. This type of metering, also known as matrix or segment, is better for difficult lighting situations, such as backlit subjects, as it can determine when several sections are brighter than the area that you have focused on and adjust to compensate. Some of the more sophisticated versions couple up to the focusing system to offer even more precise exposure. Basic systems have as few as two metering zones where advanced ones can have eight or more segments.
A flash technique where you fire the flash several times to increase the exposure allowing a smaller aperture to be selected. It can also be used for special effects; the most common being strobe like effect, following the swing of a golfer or the flapping wings of a nocturnal bird. It can also be used to light the same subject in several positions in the frame - to allow a multiple exposure effect.
The "brown" in the name of this process gives you some idea of the colour of the finished print -although tones tend to be delicate rather than heavy - so clear, graphic images tend to work best.The process involves combining iron salts with silver nitrate and either oxalic acid for a warm-brown colour or tartaric acid for brown-black result. Tightly-woven fabrics can be a better base than paper, as the finished "print" must be washed in water. Exposure is by contact printing, but, because of the relatively high sensitivity of the medium to light, you should work in dim conditions, and give an exposure of around 1 minute in sunlight.Develop the image in a 1% borax solution, fix it in traditional hypo, and give a final wash of at least 30 minutes.
An automatic exposure mode which chooses a small aperture to increase the distance in focus and a long shutter speed to help capture detail in dim lighting conditions. Using some form of camera support is normally recommended when using this scene mode, as the longer exposure duration can cause images to be blurred through camera movement.
A mechanical lever or electronic button used to close the lens aperture down to the actual exposure setting so depth of field can be previewed.