NSL Photography's™ Glossary of Photographic Terms - E

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ED
Extra Low dispersion - usually refer to glass type. Glass with ED properties indicates special rare earth glass or special formulated glass that limits or corrects light rays passing through the lens elements to achieve all spectrum of colors to fall on the same plane of focus, especially the red and blue spectrum, and is usually more often used for longer focal length lenses where the problem is more serious. ED was first used by Nikon's Nikkor lens line. Canon's version is called "LD." while Minolta uses APO (apodchhromatic).

Effective Aperture
The diameter of the bundle of light rays striking the first lens element that actually pass through the lens at any given diaphragm setting.

Electronic Flash
Light source based on electrical discharge across two electrodes in a gas-filled tube. Usually designed to provide light approximating to daylight. Electronic flashs require a high voltage, usually obtained from batteries through a voltage-multiplying circuit. It discharges a brief, intense burst of light, usually used where the lighting on the scene is inadequate for picture-taking.

Electronic Flash Preflash
The barely visible flashes which are sent out by an electronic flash prior to the main flash which lights the photograph, primarily when in an Automatic Balanced Fill-Flash, TTL, or iTTL modes, or when in a multi-flash system to communicate between flash units to enable the flash and the camera to analyze the correct amount of light to be sent by the flash(es) and the correct exposure by the camera itself.

Element
Single lens used in association with others to form a compound construction.

Emulsion
Micro-thin layers of gelatin on film in which light-sensitive ingredients are suspended and triggered by light to create a chemical reaction resulting in a photographic image.

Entrance Pupil
In an optical system, the entrance pupil is a virtual aperture that defines the area at the entrance of the optical system that can accept light. Rays that pass through the pupil are able to enter the optical system and pass through it to the exit (neglecting vignetting). In a camera, the entrance pupil is at the aperture stop in the lens optics. The aperture stop is the diaphragm aperture in the camera that the photographer uses to adjust and control how much light reaches the digital sensor or film.

EV
Exposure value is a method of quantifying scene brightness. The camera may be used only within the EV range of the exposure meter. 

EXIF
Exchangeable Image File: the file format used by most digital cameras. For example, when a typical camera is set to record an image in a JPEG formatted file, it’s actually recording an EXIF file that uses JPEG compression to compress the photo data within the file. The exif data of each photograph includes information about the camera and lens used to make the photograph, as well as various settings used for the photo such as exposure settings, plus the date and time the image was made.

Existing Light
Available light. Strictly speaking, existing light covers all natural lighting from moonlight to sunshine, but for photographic purposes, existing light is the light that is already on the scene or project and includes room lamps, fluorescent lamps, spotlights, neon signs, candles, daylight through windows, outdoor scenes at twilight or in moonlight, and scenes artificially illuminated after dark.

Exposure
The quantity of light allowed to act on the film or sensor in the camera. It is a product of the intensity (controlled by the lens opening) and the duration (controlled by the shutter speed or enlarging time) of light striking the film sensor.

Exposure Bracketing
Taking a series of photographs of the same subject at different exposures to insure the "correct" exposure; useful when shooting in situations where a normal metering reading is difficult to obtain. It is taking additional pictures of the subject through a range of exposures-both lighter and darker-when unsure of the correct exposure. Some upper end cameras have the ability to do automatic bracketing. Bracketing is an important method for creating an HDR (High Dynamic Range) photograph, by creating individual images exposed for different portions of the photographic composition, to be merged into the HDR image. Some cameras provide Auto Exposure Bracketing and Flash Exposure Bracketing.

Exposure Latitude
The range of camera exposures from underexposure to overexposure that will produce acceptable pictures from a specific film.

Exposure Meter
An instrument with a light-sensitive cell that measures the light reflected from or falling on a subject, used as an aid for selecting the exposure setting. The same as a light meter.

Extension Bellows
Device used to provide additional separation between lens and film or sensor for close-up photography. Consists of extendable bellows and mounting plates at front and rear to fit the lens and camera body respectively.

Extension Tubes
Metal tubes used to obtain the additional separation between lens and film or sensor for close-up photography. They are fitted with screw thread or bayonet mounts to suit various lens mounts.

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