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Film speed

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Used to indicate the light sensitivity of a film as ISO. Digital cameras also use the ISO rating to indicate the CCD sensitivity.

Related Terms

Most cameras now set the speed film automatically using DX coded film cassettes, but if there's a dial you can usually override. This can be used to uprate a film or adjust to suit your shooting preferences or to bring an out of calibration exposure meter back in line.
The range of films that can be used by the camera. Most modern cameras set this speed automatically using DX coding, some have manual settings that have a slightly wider acceptance range. The range quoted is for the maximum that can be used.
A technique used to gain speed with a film. You manually override its ISO speed, using the film speed dial or exposure compensation dial, to make the film more sensitive. To film would then be given longer in the developer.
A pattern on the base of APS film cartridges that tells the camera the film speed and number of exposures, so when loading a film you don't have to worry about getting it wrong. Or, in a more general sense, a data disc can be any disc with electronic files on it.
German-based film speed rating used in Europe before ISO became the norm. A three DIN increase doubles the films light sensitivity.
A technique used to reduce subject contrast and film speed by overexposing and underdeveloping.
A technique used to increase contrast and film speed by underexposure and overdevelopment. Also known as uprating.
The range, usually indicated as a range of EVs (exposure values), that the meter can work in. The larger the range the more versatile the meter. An ISO film speed is usually quoted to indicate what the measurement scale is based on.
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.
Cameras have various methods of blocking the light from reaching the film or CCD. When a picture is taken this barrier, known as a shutter, will open and allow light to reach the film or CCD. Most cameras have a way of controlling how long the shutter stays open and this duration is known as the shutter speed. More sophisticated cameras can adjust from several seconds to speeds as rapid as 1/10,000sec. Cameras with wider ranges are usually more versatile.Buying advice: Look for a camera that has speeds way below 1/30sec. The longer the speed the lower the light you will be able to take pictures in without having to use flash. As an indication, a speed of around one sec is required to take street lit scenes and a speed of over 30 seconds for moonlit shots. Buy a model with a B setting if you're considering taking extremely long exposures.
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.
Responsible for supplying the speed rating of photographic film. Doubling the ASA number indicates twice the light sensitivity. ASA was replaced with ISO in the 1980s.
A graph used to show a film emulsion or developer's limits of tonal reproduction, relative speed and fog level.
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).
Used to indicate the light sensitivity of a film. Digital cameras also use the ISO rating to indicate the CCD sensitivity. The standard rating is ISO100 and as this is increased it means that faster shutter speeds can be used. When the ISO is doubled, it doubles the available shutter speed. The drawback for digital cameras is that increasing the ISO increases the amount of digital noise in an image.