The Minolta Auto-Meter III is a digital incident light-meter — a device used by filmmakers to determine correct exposure based on the amount of light falling into the scene to be filmed or recorded. Production Students may add these light-meters to their equipment loan in an on-demand basis, no reservation necessary, and no access restrictions based on course-level.

The Minolta Auto-Meter III is an averaging incident light-meter, so it measures the average amount of light falling on the subject, or portion of the scene being metered — rather than the amount of reflected light coming off of the subject or scene. For comparison, the latter method is typically employed by most cameras and camcorders built-in light-metering functions for determining exposure. Using an incident light-meter is equivalent to using a reflective light-meter in conjunction with an 18% Gray Card.
To use, position the light-sensor of the light meter immediately adjacent the subject to be metered, with the sensor facing the light source (or sources) of interest. Often, taking multiple, isolated readings of each light-source is necessary to determine the full range of the lighting ratio for a given subject.

The readings given by this particular light-meter are limited to exposure settings, of which assume that traditional analog media is the recording format (e.g. 8mm, 16mm, and 35mm celluloid film). Meaning, that for a specified ASA or ISO film-speed, and a given shutter-speed (in fractions of a second, not shutter-angle), the light-meter will indicate the appropriate F-Stop or EV (Exposure Value). It does not give readings in terms of an absolute value (such as foot-candles or lux units).
The F-Stop or EV reading given by the light-meter corresponds to correct exposure for the mid-tones of a given subject. This reading does not ensure that using the exposure settings will guarantee a perfect exposure, as the lighting ratios between this mid-tone exposure setting and that of the highlights and shadow must also be taken into account to render a complete assessment of luminance throughout the scene.

As contemporary digital video camcorders do not typically emulate ASA or ISO ratings (as DSLR still-cameras often do), the utility of the light-meter is somewhat limited. Equivalents for camcorders can be found on the internet, and occasionally manufacturers themselves will specify their recommended ASA/ISO equivalent — however, these equivalents are greatly affected by the settings of the camcorder. As such, great care must be taken to coordinate the settings of the light-meter with that of the digital video camcorder when the two are used in concert.

A more universal application of the Minolta Audo-Meter III is for measuring lighting ratios. Lighting ratios are constant regardless of the ASA/ISO and shutter settings — provided they do not exceed the range of the meter’s sensitivity. Using the light-meter’s marker function, readings from various light sources may be taken and compared to give an idea of the difference between their relative brightnesses. These differences will be registered as F-stop or EV values, but the “distances” between them do indicate an accurate reading of their lighting ratio — where the number of stops between two compared readings designate the difference in brightness by an exponential factor of 2^n.
So a 1-stop difference indicates that one light-source is 2-times as bright as the other. And a 2-stop difference is 4-times as bright. And 3-stops is 8-times as bright. And so on.