A Tale of Two Meters

If I could only afford to buy one light meter it would be a spot meter.

An incident meter reads light falling on the subject.  Hold this meter in front of the subject and point the spherical dome toward the camera. A reflective or spot meter reads the light reflected off of the subject and requires an understanding of the Zone System to determine an F Stop. Simply put, a spot meter wants to put everything it reads at ZONE 5 or 18% neutral grey.  Any other value requires an exposure compensation to achieve proper exposure. Here’s a very common example: Spot reading faces. Generally speaking, Caucasian faces are Zone 6 or one stop brighter than Zone 5. Since the spot meter sees everything at Zone 5, your reading of that typical face will give you an F stop that is one stop under proper exposure. So after a face reading, always open up one stop to achieve full exposure. For clouds – open up 2 or more stops, for snow on a sunny day – open up 3 or more stops. For Zones 0-4 you must stop down to achieve the proper value as the spot meter brings those darker values UP to Zone 5. DSLR cameras are equipped with reflective metering systems.

A spot meter evaluates a small “spot” in your view. Whereas, an incident meter evaluates all of the “incidental” light falling on the subject.  An incident meter fitted with a spherical dome samples light from every direction and is influenced by the brightest source. That’s why on cloudy days you’ll often see a DP shield the top of the dome to minimize the strong top light. I always use a flat disk on my incident meter so that the reading I get is true to the direct light hitting the front of the subject.

If you spot read an 18% grey card and the meter says F4 that’s what your exposure should be, no compensation. Next, put your incident meter in front of that same grey card and take a reading. If the set up is the same as your spot meter – ISO and Shutter Speed - it should also read F4. This is the only situation where the readings coincide. And in fact, this is how I verify that my meters are calibrated correctly to each other.

Determining proper exposure is a critical part of our job as cinematographers. Choosing an F Stop is a two part evaluative process. It’s technical and creative. On the technical side, there are three factors required for determining an exposure setting. The ISO, that’s the sensor or emulsion sensitivity, the Shutter Speed and the Frame Rate of the camera. The F Stop is a numerical value that refers to the precise diameter or opening of the aperture in a lens.
By the way, here’s a handy thing to know: How to calculate shutter speed. The arbitrary number here is 180, I chose it because it is a standard shutter angle for most movie camera's. Put 45 degrees in and see what you get!

As cinematographers, we use light meters to determine the amount of light that hits the emulsion or sensor at the “film plane” for proper exposure. This is achieved by adjusting the aperture in the lens or “setting the stop.” It is cinematographer alone who has the responsibility for calling the stop.
A sunny day will give you dramatically different readings depending on where the camera is pointed relative to the sun. I find that an incident meter with the dome attachment works well on sunny days. The illuminated dome gives you the truest angle of where the sun is relative to the subject. Day exterior readings are pretty straightforward and in most situations I’ll go with the displayed F number while shooting wide exteriors.

You can use an incident meter for backlight readings, but in my experience, a spot meter is the better tool to arrive at a precise exposure that protects delicate density values. Values that could be unprintable or possibly lost entirely, if improperly exposed while using a digital camera system. If it’s hazy day I’ll use a spot meter to read in the backlight direction. Haze, smoke and fog are accentuated when lit from behind. an incident meter doesn’t account for that overexposed “look”. Identify a mid tone value in the shot and expose for that in the middle of the gray scale. Adding a haze filter helps as well. If your schedule permits, shoot your wide shots when the light looks best, first thing in the morning or towards the end of the day. The metering is straightforward and worry free. Be alert to wispy clouds as they can effect your exposure. And don’t forget to compensate for color correction, neutral density and polarizing filters.

The above photo includes all 10 zones of Ansel Adams remarkable exposure system.

I primarily use a spot meter now, and not just because I’m comfortable with the zone system. Certain photographic events aren’t measurable with an incident meter. Examples: Aerial photography, explosions, blue screen and green screen photography, smoke, moonrises, monitors, underwater work and sunsets. These examples all require a reflected spot meter reading.

For more information about light meters and exposure, go to the Tutorials page on this website and watch the video version of this written piece. (the video has many helpful examples.)        "A Tale of Two Meters".

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