How to Tell if a Light Meter Is Defective

Light meters give a cinematographer an accurate reading on lighting conditions when shooting video or film. Generally, light meters are reliable tools, although the manufacturer must periodically calibrate the meter to ensure it reads light accurately and provides the user with correct "f stop" settings. Here is a quick test that determines whether the meter is running correctly.

Difficulty: Moderate

Instructions

Things You'll Need:

• Light meter
• Paper
• Pencil

Check the Basic Readings

1. Set the ASA or ISO at 50, which is a recommended setting for shooting outdoors in natural light. ASA and ISO represent the speed of the film stock.

2. The f stop reading should be around f/6. The f stop number is the ratio of the distance between the lens and the film to the diameter of the lens's iris. For example, a 50mm lens with the widest opening of the lens measuring 25mm is an f/2 lens.

3. Set the ASA or ISO at 500, the speed most often used when shooting film indoors.
   The f stop reading should be around f/46.

4. Cinematographers can develop a keen eye for the amount of light needed to shoot film or video by following these steps. Each time a light meter reading is taken, record information in a notebook on the basic lighting conditions:
   
   What kind of light is used? A Fresnel, bounce light, white or silver bounce light?
   How far is the light from the subject?

Tips & Warnings

• The film's ASA is determined by the amount of light-absorbing silver nitrate crystals on the film. The higher the film's ASA number, the more densely saturated the film is in silver nitrate.
• If your readings are wildly different from the numbers suggested above, then the meter is likely defective. This could mean that the meter's computer chip, the photo voltaic cell that reads light, is malfunctioning, or that the meter should be returned to the manufacturer for calibration. To learn more about this, go to the manufacturer's Web site.

References

• Stephen Crawford, cinematographer, NYPD Blue, CSI New York, Los Angeles, Calif.
• Cinematography: Second Edition; Kris Malkiewicz; 1992

Resources

• The American Society of Cinematographers