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A light meter provides crucial information to photographers and cinematographers for their work; it provides a measurement of the exposure, tells users exactly what shutter speed/aperture combination to use based on the 18% gray standard, and can even provide detailed charts and graphs on the quality and color of the lights one is using. Generally, it will be seen as a tool to check exposure settings for perfectly balanced images. Additionally, some models can provide other exceptionally useful capabilities, such as flash metering, color temperature readings, and more.
Incident versus Reflected Light Readings
Light readings can be taken in two different ways: by reading the light falling on the subject (incident) or the light that is reflected from the subject (reflected). Most meters can take both types of readings, but there are distinct advantages and disadvantages to each.
Incident readings measure the light falling onto the subject, so it requires the meter to be pointed toward the camera from the subject’s position. This method has proven to be very accurate, more so than reflected readings, because the meter will not be fooled by overly reflective or dark surfaces. This can be difficult with fast-moving or faraway subject matter.
Reading the light falling on the subject
Reflected readings are more common, especially since most modern cameras have a built-in reflected light meter. These take an average of the different reflected light, measuring across the entire scene to provide photographers with accurate exposure settings. In areas with a great deal of contrast or many differences in light, these systems can have difficulty providing a proper reading, but it is a very fast and convenient way to acquire initial settings.
Reading the light falling from the subject
Something that can greatly benefit reflected metering is the use of an 18% gray card, which will improve accuracy by providing a consistent surface from which to read light. Other benefits of reflective metering are the ability to provide readings of multiple subjects quickly to gain a better understanding of the dynamic range of the scene as a whole.
Spot meters are dedicated reflective meters that will read a very narrow section of the image area, usually from 1 to 3°, though some models are available with wider 5 to 10° angles of view. These allow users to very specifically meter certain objects in a scene without being thrown off by either extremely dark or bright spots in the image. Examples of this would be backlit subjects or dark backgrounds, where the subject would end up either underexposed or overexposed, respectively, with an averaged reading.
Many handheld meters can be outfitted with add-on accessories that will enable spot readings. Also, many cameras have a spot-metering mode available for use.
While ambient and constant light is simple to meter, flash requires additional connections or modes in order to provide a reading. This is mainly due to the extremely short duration of a flash, usually greater than 1/1000-second. Most flash-capable meters feature a PC terminal or other sync port for directly triggering a flash, though some also pack in a radio transmitter for working wirelessly with strobe units. Another method is a dedicated wireless flash mode, which puts the meter into standby and then captures and saves the measurement of the burst of light when it is triggered via another source.
While photographers are usually fine with basic exposure readings and settings, cinematographers and videographers have some more specific needs and desires when it comes to getting their light just right. Cine-specific models are available with expanded settings for frame rates (fps) along with the ability to display in lux, foot-candles, and shutter angles, which is information geared mainly for film production.
Straying from the common exposure meter, some companies produce color meters or spectrometers that will provide detailed analyses of your lights. The information provided can be as simple as color temperature to spectrum graphs and CRI measurements. These meters make it possible to compare two different light sources and make them match or vary, according to your needs. Another purpose is to check the quality of light, since some light sources can be deficient in certain colors and this can have an adverse effect on your images.
Color temperature can shift (cooler, left; warmer, right) depending on thelight source and ambient conditions.
In-Camera versus Handheld Meters
Modern camera technology provides every user with a built-in TTL metering system for reflected light readings, which can lead some to be skeptical of the need for a dedicated handheld unit. While it is true that with the numerous settings and advancements available, users will be able to take average, spot, and center-weighted readings easily and accurately; some situations will demand a handheld meter.
In-camera systems will not be able to take flash or incident light readings at all, requiring a separate unit for these situations. Also, a camera can be awkward to use as a meter when walking around set or when measuring complex lighting setups. Additionally, built-in memory and comparison capabilities of digital meters enable users to quickly recall and compare readings from other lights or settings to quickly draw conclusions on your light and make adjustments. Handheld meters also feature many other dedicated functions and accessories, including the ability to program for different camera models for increased accuracy.
While many users will not find a need for a handheld light meter, others will find the capabilities of an exposure meter or color meter invaluable. Meters can provide exacting information on your lighting and also simplify things like filtration by providing exact numbers. And, as light is the most important part of photography and cinematography, being able to know more about your setup will greatly help.