So you've dropped a few thousand on your HD video camera and you're doing fine with the onboard monitor for the small productions that represent the bulk of your business. Why do you need a meter? First things first: what types of meters are available to you?
Ambient: Your meter should have incident capability, that is, readings taken from the subject via the meter's 180° dome. (Most brands also offer a flat diffuser for metering individual sources.) The meter should read out in frames per second. If you're shooting video, you'll probably be using 24 or 30 fps.
Cine: A real cine meter is capable of displaying frame rates and shutter angles in addition to foot-candles and lux. Others also offer contrast ratio measurement and averaging modes. Many offer a spot attachment as an accessory. A good example is the Spectra IV-A. Those with deeper pockets and more varied needs might opt for a super-meter like the Sekonic L758Cine that adds a zoom-spot viewfinder (1-10 degrees) and most of the above light measurement modes. It also reads flash and accepts radio triggering modules. And despite its many features and relatively high cost, it's far from the most expensive meter out there!
Spot: Sometimes incorporated into an incident or cine meter (as above). Spot meters sight like a scope and are usually of the "pistol grip" type. Usually defined by 1° reading capability (vs. 180°), spot meters are made for reading light reflected from small or distant areas of the set.
Color Temperature: You're shooting a scene in an office space with the main subject under fluorescents, cubicles lit with a variety of small halogen desk lamps, with north light streaming in from floor-to-ceiling windows at the far end of the room and it looks… like a mess. You need a color meter.
Although some color meters now include such features as illuminance measurement for ambient readings and flash capability, their primary function is the evaluation of the color of light displayed in the following ways:
1) Color temperature display, in degrees Kelvin (K) in the red-blue spectrum from approximately 2000-20,000˚ K
2) LB (light-balancing) Index Display for Amber and Blue filter corrections
3) CC (color correction) display for magenta and green corrective filter values
Properly used, the above-mentioned scene will read as a natural transition from artificial light to daylight and not a riot of burning coals and screaming blue.
10 Reasons to Use a Meter
Common language: On set the need for a universal mode of technical communication is paramount (especially on international sets) and the language of light fills the bill. DP, grip, and cameraman, for instance, don't have to know each other's craft but all can achieve the desired quantifiable result. Depending on the set that language might be lux, foot candles or f- stops. Foot/candle readings in the U.S. (and lux in most other countries) are absolute values that, unlike f-stops, exist independently from film or video sensitivity ratings. Foot-Candles translate exactly into f-stops and shutter speeds when needed.
Unusual exposure situations: Out-of-range highlights, large flat values and silhouettes, for instance, demand strict control of foreground or background lighting within the capabilities of the medium that might not be accurately displayed in the camera's monitor.
Unreliability or inconsistency of in-camera monitors: In-camera (LCD) monitors may be compromised by the ambient light level or by the angle of view of the videographer, causing anomalies in color saturation, contrast, and brightness. Compare the view on a laptop, which varies in intensity relative to the angle of the screen. For one reason or another, the monitor's gain may have been boosted or reduced (and by user error-left in that position) resulting in improper exposure for the final product. These on- board monitors should be used for framing, not strictly for exposure. Separate reference or field monitors need to be specifically calibrated to use for judging exposure. This is now handled on set by a digital imaging technician (DIT) using a waveform monitor and vectorscope to make sure that the display matches the color range of the video medium. A cine meter will ensure that your light sources fit within that range.
Tektronix WFM 7020
Unreliability of the human eye in judging light levels and intensity on set or when scouting locations: The human eye has a dynamic contrast ratio of about 1,000,000:1, or about 20 stops. Paradoxically, because it works so well as an optical instrument, it is a poor judge of light intensity and color temperature for video, which captures only a fraction of that range. The rapid dilations and contractions of the iris, coupled with complex processing in our brains, usher detail into our consciousness under extreme lighting conditions that are impossible to replicate. What we can do is mimic the end result by careful preproduction.
Using the simplest example first: It's more practical to carry around a cine meter than a camera. When scouting a location you obviously don't want to haul around a case or two of heavy gear if you don't have to. "Ball-parking" exposure around the two working frame rates you have available is just not going to cut it, on any kind of budget. In this perspective, a lightweight meter is going to be a negligible expense.
On set, with a properly calibrated monitor and your exposure and color range established, you can easily tweak lights that are too dark or too "hot" for said range and adjust accordingly. For distant or remote lights, you may want to employ a 1-degree spot meter to read the source, possibly changing your overall exposure to accommodate it. Using the pre-established (metered) readings of your on-set sources will allow you to adjust their intensity expeditiously. It will also facilitate the process of adding a fill (light) to reduce contrast between values.
Lastly, a meter is essential for creative "sculpting" of the subject by manipulation of light values, thus leading the eye and directing attention where the script demands.
Accurate, balanced lighting for green or blue screen: Evenness of illumination is essential here and will reduce post-production nightmares. As discussed above, the eye is not a good gauge of relative intensity, especially on glowing featureless surfaces. Besides ensuring uniformity, accurate metering will help regulate overexposure of the background, resulting in blue or green "bounce-back" or fringing on your subject. Of course, once that's done, your subject has to be lit separately, with sources that match the depth, dimension, and quality of your final scene.
Adding and balancing multiple sources: As soon as a second light source is introduced, there is a need for metering. Sure, you can do lots of work with nothing but an on- camera light as your source, especially at night or in dim light, but once your lighting needs gain in complexity, "winging it" is not an option.
For instance, once you've set your key light and taken a reading, you can build the rest of the setup (sidelight, backlight, hair light etc.) around that reading, within the working dynamic range of your medium. A flat diffuser is handy for this. If there are other scenes within the shot they will also have key lights which will have to be metered to fall within the exposure range relative to the main action of the scene. This isn't possible without a meter.
Judging the color temperature of new discharge lamps: This is a situation where the use of a color meter is imperative. Out of the box, most HMI lamps, for example will read over 10,000˚ Kelvin and need to be "burned-in" for several hours down to their nominal value of 5600-6000˚k. After that, the temperature decreases by the hour for their useable life (Approx. 50-750 watts depending on the lamp) making the presence of a color meter essential. Rental equipment even further reinforces this need.
Making sure the light sources fit within the dynamic range of the video camera, film, or media: In general, video has much less dynamic range (exposure latitude) than film stock. Highlights are particularly problematic, with no remedy available once they're blown out. With more productions being wholly or partially shot on video, the need for accurate metering and monitoring has become a necessity. Contrast between the key and other sources, especially fill lights, needs to be lower and flatter to achieve the same result as a film version. Although one of the significant benefits of shooting video is its low light sensitivity, the clear rendition of this additional shadow detail demands accurate metering as well.
Determining filter compensation remotely or before adding it to the light: Knowing in advance how much a particular gel combination is going to reduce your light output can save a lot of time and legwork. This is especially true with effects or creative filter combinations. It's easily done by placing the filters directly over the meter's sensor, pointing it at a uniform source of illumination, and comparing the before-and-after readings. Modern cine meters, which have silicon photo cells that are uniformly sensitive to colors within the visible spectrum, are essential for reading colors accurately.
Flash: While this one is a bit of a stretch, there are occasions on set which incorporate the flash of paparazzi cameras or a bolt of lightning. Luminys (formerly Lightning Strikes) is a company that literally made its name on such controlled effects. Metering these explosive effects is essential to confirm that they fit within the medium's dynamic range.
Despite the availability of high-quality video sensors and video taps in film cameras, there's definitely a need to have a meter on hand to plan your lighting with a common on-set language, and to make certain that a multitude of disparate light sources are harmonized within the dynamic range of your film or video medium.