Liz’s Cinematography 101: Measuring Light

Light must be measured.

Film can only handle a limited range of tones. Therefore we must have a way to measure the light so we know how it needs to be manipulated to get the desired results and fall within the parameters our film stock/sensor.

Incident Meter

An incident meter measures the light falling on the scene. The white plastic hemispherical dome serves to diffuse and average the light that is falling on it. It will provide a reasonable average of all the light sources falling on the subject.

Typically, you point the hemisphere directly at the lens from the subject’s position (some cinematographers say they aim the hemisphere always directly at the light from the subject’s position). Many prefer to shield the meter from the backlights. Because often the backlight is desired to be brighter than the key light, you want to prevent its value from being averaged. The reading that the meter reads out is the ‘averaged’ reading of all of the light sources falling on the subject (unless you have chosen to shield any sources, backlight, etc). This averaged reading will usually determine the aperture setting on the lens.

Another common use of the incident meter is to measure the output of individual light sources (key, fill, side lights, backlights) – in order to determine the lighting ratio.

In order for a meter to give you accurate results, you need to set it for the film speed (or ISO) and shutter speed that you are using. Then, the meter will read out the appropriate f/numbers. Some meters can also read out in footcandles. This is a great feature if you want to know the actual output of a light source (ie: 150fc). [A “footcandle” is the amount of light generated by a single candle at one foot away.]

Reflectance Meter

A reflectance meter measures the luminance of the subject – the light it is reflecting. Different objects will have different reflectivity. For example, you will notice a difference in your comfort when wearing a black t-shirt on a bright, sunny day versus wearing a white shirt. Objects that are black will absorb light…and with it, heat. Objects that are white will reflect more of the light that hits it instead of absorbing it. So, yes, wear lighter colored shirts on those hot, sunny days.

A spot meter is a specialized type of reflectance meter, the most common to be seen on set. This is because they have a smaller acceptance angle (5-10 degrees), and this will allow you to take more precise readings than a regular reflective meter.

Incident meters are pretty straight forward – they give absolute readings. Reflective meters give readings that require interpretation. Typically, you will not set your aperture according to the readout from the spot meter. It is reading reflectivity of an object, but you, the cinematographer, can decide how reflective you want that object to be by interpreting the reading and adjusting your settings accordingly. How does one go about interpreting this reading? Read on.

The Zone System

Reflectance meters are calibrated to give readouts (usually in f/numbers) that will render the subject as an 18% grey tone…aka “middle grey”. For example, say you aim a spot meter at a black backdrop and get a reading of f/5.6. If you set your aperture to f/5.6, then that black backdrop will be rendered as … 18% grey, middle grey, or “Zone V”. Sometimes you will want this effect, but what if you want black to be black? Examine the scale below – this is The Zone System:

The scale gets closer to black three steps – or STOPS – above Zone V. This means if we close down our aperture at least three stops, our black wall will now look black. So, our lens aperture should be set to f/16.

The Zone System is not only intended for black and white photography as the scale may make it appear. It is a way of thinking in tones. It takes practice to look at a colorful world and equate those with the tones on the scale. If you are an aspiring cinematographer, I would encourage you to practice this. Look at the scale again, it provides some common examples you can refer to. Learn to associate these subjects with their zone numbers.

By using this technique, you can control the tonality of your scene, and that is a very powerful tool! Someone told me that this is where you are ‘painting with light’. The incident meter gives you the information to make the broad strokes, and the spot meter gives you further information to make the fine lines.

Remember, everything a spot meter reads will render as grey! Incident meters give absolute readouts of the light falling on the subject.

There is much more to be said about this subject, so for a more in-depth explanation, I highly recommend Cinematography: Theory and Practice by Blaine Brown.

Waveform Monitor

I have heard it said that waveform monitors are the light meters of video. While this is accurate, it does not mean that the traditional light meter is not an effective tool in the video realm. There are certain applications where a waveform monitor will be more practical. One example, a live multi-camera situation.

I agree, it looks awful – so much so that you want to stop reading this – but don’t! Let’s simplify this:

The waveform monitor is an oscilloscope, aka: a device for viewing oscillations of electrical voltage or current. The vertical axis represents the voltage of the video signal. The higher the voltage, the brighter the picture. The horizontal axis represents a sort of timeline of the scene. As the video sensor keeps ‘scanning the scene’ the waveform monitor will update its values to reflect the brightness levels within that scene. It is essentially the same as having a series of spot meters scanning an entire scene and drawing those values on a graph.

The screen is divided into 140 IRE units (Institute of Radio Engineers). A video signal will fall within the range of 0 to 100 IRE units. 100 IRE represents pure white, and 7.5 IRE represents the minimum black level. If a signal exceeds 100 IRE, this means it is overloaded and it will be ‘blown out’. It is good practice to keep your brightest signals under 100 IRE.

If you are interested in unlocking more secrets of the mysterious waveform monitor, pick up a copy of Motion Picture and Video Lighting by Blaine Brown.

Both light meters and waveform monitors are useful tools in measuring light, and in understanding one you can more easily interpret the other.

–EN

Liz’s Lesson:
To effectively “paint with light,” you must
know exactly how much light to use.

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One Response to Liz’s Cinematography 101: Measuring Light

  1. Great read and very well presented.

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