Difference between revisions of "Exposure meter"

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Revision as of 14:04, 16 March 2012

Glossary Terms

At best, photographic film or digital sensors are only able to record a range of scene brightness of roughly 1000:1 (10 stops)—significantly poorer than human vision. Thus, a camera's exposure settings must be adjusted so that no significant highlight or shadow detail falls outside the range that can be recorded.

A light meter (or exposure meter, short form meter) is a device that measures light to determine the proper exposure settings for a particular scene and ISO sensitivity.

Whilst the two terms are often used interchangeably, there is a difference:

  • Light meters display lux or footcandles and are used to measure absolute light intensity, such as assessing proper lighting levels in a work environment (photometry). Measurement is always incident using a flat diffuser.
  • Exposure meters have calculators to display computed exposure settings for a camera, taking film / sensor sensitivity into account. Measurement can be reflected, or incident using a domed diffuser.

Many professional meters can make both types of measurement.

Photoelectric cells

Analog measuring instruments with a selenium cell as voltage source were the most common early type of photoelectric light meter. Multiple "insect eye" lenses covering the photocell are very characteristic of a selenium meter. Like a small solar panel, the selenium cell generates its own electric current, which (typically) deflects an indicator needle by varying amounts.

Later, more light-sensitive meters were made, with Cadmium Sulfide (CdS) photo resistors or silicon photodiodes as sensors (SBC). SBC sensors respond quickly enough to evaluate flash intensity. Selenium meters have not disappeared entirely as they are the only type not requiring a battery.


Early light meters

Actinometers

Actinometers were the first light meters. They typically had the shape of a pocket watch and used light sensitive photo print paper as means of measuring. The time to darken a piece of such paper until it matches a standard tint is the input value for the scales on which an appropriate shutter-speed/aperture combination for the light situation can be found.

Extinction Meters

Another type of early meter, the extinction meter, depended on eyesight: the user looks through the meter at a row of numbers, each behind a celluloid window of different opacity, the highest or lowest visible number determining which light situation is given. Other extinction meters have a pattern visible through the eyepiece, and a control varies the amount of light allowed into the device until the pattern can only just be seen; the position on the control then indicates the exposure.


Light meter makers

and many others; see Category: Meter makers.

Light meters in cameras

The first camera to include a built-in meter was the 1935 Zeiss Ikon Contaflex (TLR), using a selenium cell. To give sufficient sensitivity, a selenium cell requires a certain surface area (roughly postage-stamp sized). When covered with "insect eye" lenses to give directivity, they dominated the appearance of many mid-century cameras. In the early 1960s, makers of cameras and light meters adopted the battery-powered CdS photoresistor as a sensor element, which could be much more compact. Finally, gallium arsenide and silicon photodiodes were adopted, for their faster response rate and greater sensitivity.

The earliest built-in meters had no coupling to the camera's shutter or aperture settings. The photographer was expected to set both manually, based on the reading given. Later meters were "semi-coupled": meter readings were adjusted according to one of the camera's settings, and the readout indicated how the user should set the other. Finally, both aperture and shutter speed settings were coupled

Meter Limitations and Workarounds

A hand-held meter may be brought very close to a subject, or used to measure the light falling onto it, for a very accurate reading. But a meter viewing the light from the entire scene can easily be tricked into giving an incorrect exposure. The most common problem is a bright background behind the subject such as a bright sky, or bright light reflecting off snow, sand or water, leading to underexposure of the desired subject. Unusually dark backgrounds (e.g. a coal mine) can also fool the meter.

Built-in meters may use a metering pattern strongly weighted towards the center of the frame (or even just a small spot) to help with this problem. Over time, camera-makers have added increasingly complicated multiple-zone metering, designed to identify typical "tricky light" situations and compensate accordingly.

For manual exposures, a solution is to walk closer to the main subject or zoom in with the lens, take a meter reading, and then recompose. The correct exposure value can also be read off a mid-toned 18-percent gray card held in front of the subject (or even the palm of the hand, then adjusting upwards by 1 stop). Like using an incident meter, this ensures that the light falling on the subject is measured, rather than the variable amounts reflected back towards the camera by a white wedding dress or a black cat.

Many advanced cameras with autoexposure offer some kind of Auto Exposure Lock (aka AE Lock, Exposure Memory Lock or EV Lock). Using AEL, it's possible to selective take a meter reading just of the desired area of the subject (for instance the bride's face). With that value now locked in, the complete scene can be photographed at the locked exposure.

As well, the technique of exposure bracketing can be used. Bracketing (Exposure & Exposure Bracketing) is the technique of taking multiple photos at different f-stops to ensure at least one of them will be correctly exposed.

Professional Meters

Certain types of light meters are typical professional photographic tools:

Links