digital filmmakers light meter

Exposure: Why Still Use a Light Meter?

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Photo by Chris Yang. Instagram


Sekonic meterWritten by David Appleby

With digital cameras now equipped with waveform monitors, histograms and programmable zebras, why do you need to use, or even know about, light meters?

A first answer would be that all the built-in features you’re using are light meters. A second answer is that for ease of use, precision, control and portability, nothing beats a good stand-alone meter. And third, if you understand metering, you’ll be better able to decipher and employ the knowledge that the internal features provide.

To begin, you need to be familiar with the concept of “dynamic range” and how the two basic types of meters allow you to navigate within (and outside) that range to create the desired image.

Dynamic Range

Every scene and every sensor that records a scene has a dynamic range – the difference between the darkest and lightest tones available or capable of being recorded, expressed in ƒ-stops – each stop allowing twice or half the light as the next. Understand that the dynamic range of a sensor is fixed and narrower than the range that may exist in nature. The latest CMOS sensors might reach a dynamic range of 13 stops, but the human eye can perceive 20 stops (1,000,000:1) and the actual brightness range on a sunny day can exceed that. The job of the DP is to keep the dynamic range of the shot within what is capable of being captured by the camera. This may mean directing more light into the darker areas, or blocking light from “hot” spots by flagging, using nets or employing ND filters to cut intensity.

So, how do you know if something in your scene falls below or above the recordable range? You might be able to get close reading the image on your LCD viewer, or bringing up a histogram or waveform, but for accuracy, there’s no better way to control the brightness values in a scene than by employing a spot meter plus a gray card or incident meter.

Photographer taking a reflected reading off a gray card. Photo: D. Goodman.

Meet Me in the Middle

 No matter how wide or narrow the dynamic range of your camera, or how many stops difference there is from the darkest to the brightest area in a scene, both have one thing in common – the middle. We refer to this brightness value as middle gray, and it is always the same. A middle gray anything – face, costume or card – reflects 18% of the light that strikes it, placing it in the middle of any sensor’s dynamic range. So, proper exposure* is one that places a middle gray card/object at the midpoint of a camera’s range.

*Note: Proper exposure, as used here, is a technical definition. Creatively, you can expose a shot any way that gives you the desired result.

Incident meterThe Incident Meter

If you have an incident meter, place the white plastic globe in the light where the subject will be (the nice thing about this meter is that you don’t need the subject to be present when taking your readings) facing it toward the key light. (NOTE: We are usually told to face the incident meter toward the camera, but this is only true when the subject is frontally lit.) After setting the ISO and shutter speed, the meter will give you an ƒ-stop reading. Setting your lens to that aperture will ensure that anything middle gray will register in the center of your camera’s dynamic range. Every other reflective value will fall where it normally does in relation to middle gray, i.e., one stop above, three below, etc. Exactly how far above or below middle gray something in your scene is, can be determined with a reflected meter.

If you don’t have an incident meter, taking a reflected reading off a gray card placed in the scene will give you the same reading.

The Reflected Meter

A reflected meter has only one function – to tell you how to represent the tonal value of whatever it is pointed at as middle gray… how to place that reflected value at the center point of your dynamic range. Knowing this, you now have a very precise way to evaluate exposure.

Once you’ve determined the “proper” stop to set your aperture using the incident reading or gray card, you can now use your reflected meter (preferably a spot meter that allows you to be very precise with what you’re metering) to determine exactly how much brighter or darker something is than middle gray and thus, where it will fall within the visible range of your sensor.

Chart 1

Pictured: A shot broken down into zones. Photo by Stephen Leonardi. Follow Instagram @leo_visions.

In the Zone

All the tonal values below and above middle gray can be broken down into zones, each being twice or half the brightness of the adjacent zone. So, each zone is exactly one stop (ƒ-stop) brighter or darker than the next. If zone V is the middle, zone VI is a stop brighter (twice as bright), zone VII four times brighter, etc. The Zones chart (See Chart 1) shows a classic 10 zone range, with 0 being black, V being middle gray and X being pure white.

So, you’ve picked an ƒ-stop that places a middle gray card at the center of your dynamic range (Zone V). Now you can use your spot meter to determine exactly what zone everything in your scene falls in, and then, adjust your lighting (not your aperture) accordingly.

For example: You’ve set your aperture to ƒ-5.6 according to the reading you got from the gray card. Now you point your spot meter at the white shirt worn by an actor and get a reading of ƒ-11. This indicates that the shirt is two stops hotter than the gray card, thus placing it in zone VII. If that’s where you want it, fine. If you want to place it in another zone, add or subtract the light falling on it (or coming through it in the case of a window or other source).

You can test the dynamic range of your camera by shooting a scene with a wide range of light values using the aperture you read from the gray card or incident meter, measuring those values, then viewing the results on a properly set up monitor. Once you’re confident of what you can capture above and below middle gray, you should be able to shoot a scene with no surprises during dailies.

David ApplebyDavid Appleby is an award-winning documentary filmmaker and professor in the Department of Communication and Film at the University of Memphis. His films have aired nationally on PBS, ABC, A&E and Starz.