Exposure Language

Different DPs have different methods of calculating exposure and communicating with the gaffer and assistant cameraperson who usually sets the stop on the lens. Here are some of the systems. In the motion picture business most lenses are calibrated in "T" stops which are calibrations for the actual amount of light a lens transmits.

Some DP's use a BASIC STOP which is related to ASA and scene brightness or an incident light reading. A SHOOTING STOP takes into consideration an increase or decrease in exposure dictated by "effects" filtration, shutter angle, and camera speed.

The Basic Stop is determined by the film's rated speed, possible modified development (pushing), "flashing" and/or a color correction filter if used, such as an 85. The DP may also choose to under or over expose the negative. The assistant does not make these decisions. Any filters, ND's, shutter angle changes or speed changes are taken into account by the assistant, who makes the exposure calculations. (Hopefully the DP also passed through the assistant stage and also knows how to make these calculations.)

Calculating the Shooting Stop from the Basic Stop is sometimes the assistant's job. There are meters that make all the calculations, but I think that is dangerous. I found that if both the assistant AND I made the calculations, on paper if necessary, we rarely made a mistake.

Once all the corrections are understood, a meter, calculator or charts can help, but roughly checking the plusses and minuses of each exposure factor can prevent mistakes.

Example: Basic stop is f 16, at 96 FPS we loose 2 stops to f 8.0, for a strobe effect and sharper image the shutter is closed to 22 degrees, (3 stops less) gives f 2.8. We have an f 4.0 zoom so either faster film is required or a forced development to raise the ASA by one stop to f 4.0.

Filtration, in almost all cases, reduces exposure. Fog filters on high key scenes and "flashing" can increase exposure.

Shutter angle: a 180 degree shutter gives a 1/48 shutter speed, usually called a 1/50. Many Panavision cameras have a 210 degree shutter that allows 1/5 stop more light.

To calculate shutter speed: a 180 degrees shutter means the film is exposed 1/2 of the time (the film is moved to the next frame during the other 180 degrees.) At 24 frames the film receives a 1/2 of 24 = 1/48 second of exposure.

Camera Speed changes (FPS) can translate into + or - stops.

24 FPS = "0" correction. 12 FPS adds one stop exposure (Stop down lens = go to next higher F number.) 48 FPS reduces exposure one stop. Open the lens one stop, one less F number.

If the DP uses a graduated neutral density filter to darken a sky or part of a scene, this is not usually entered into the exposure calculation.


For many years in Hollywood, each film had a foot candle rating for a certain F Stop. The Spectra Incident Light Meter was calibrated in foot candles and F stops. For F STOP READINGS, a slide for each different ASA adjusted the INCIDENT light hitting the photo cell so the needle read directly in F stops. This of course assumed the scene was of average brightness and lit by the same light the reading was taken in.

For FOOT CANDLE READINGS, the DP reads the key light, and incident reading which is his BASIC exposure. All other readings were related as ratios to the key light. The system worked well and still does especially for KEY, FILL and KICKER lighting of average interior sets. Outside most scenes are average brightness, in the same light and can be exposed only considering incident light from the sun, sky and reflections from objects. Problems arise when the scene is a source of light, such as a sunset where only a reflected light reading will work.

With the advent of spot meters, designed for still use some DPs consider both incident and reflected light.

Some DPs use the Zone System. (See "Zone system.")

Some DPs using the Minolta meters read stops in numbers followed by tenths. For example 8 "and" 5 tenths is half way between 8 and 11. They always say "and" between the stop and fraction to avoid problems with "two eight" being 2.8 and not 2 and 8 tenths, which is almost f 2.8.

Other DP's round out the difference between stops in 1/3's, which accurate enough. A DP also needs control of other variables such as actual film ASA, film development, and light meter calibration to be this accurate.

Some DP's rely on the Spectra IVa. With a flip of some switches, it will measure stops, footcandles, or lux. It indicates shutter speeds as FPS or fractions of a second. Best of all, all the information is displayed on a backlit LCD screen at one time. This can be a time saver, but I suggest that both the DP and the assistant understand how to make the calculations by hand. If all meters fail and it's sunny, the "Sunny 16" rule could be a basis for keeping shooting.

Knowing something about all these systems will prepare the assistant for whatever system a DP uses.

It is very important to establish what a DP means when they say "give me a little under 8". He could mean F 7 if he meant a lower F number which is more exposure OR... he could mean F 9 (a smaller F number) which is less exposure.

There are also corrections necessary for dark objects such as a black cat in a coal bin. If an incident reading is taken with no correction there will be little detail. Most DPs know to give a dark scene more exposure with an incident reading. For a reflected reading he wants a black cat and not a gray one which the reflected reading would give and would under expose.

For a white cat in the snow it is the opposite. For an incident reading less exposure is needed to retain some detail in the scene. For a reflected reading more exposure is needed so the snow and cat records white and not neutral gray.

Thanks to Jon Fauer and Bill Bennett for contributions to this piece. Check out Jon's books and tapes on Arriflex cameras which include crucial information on assisting and shooting at www.fauer.com.


© Copyright 1999-2004 Ron Dexter. All Rights Reserved