To use this Black Cat Decoder first select a ULC letter. A listing of scenes, all requiring the same amount of light, will be displayed below the chart. Any film speed (ISO), shutter speed or f-stop may be selected to display a "good exposure" or under/over exposure. Play with these settings until you have an appropriate combination. You may also use this chart to decode a photograph and find its ULC. This will determine the amount of light that was required to take the photograph. Place the ISO, shutter speed and f-stop used to make the exposure, then select the ULC letter that shows "good exposure"