Intensity, Exposure, and Time The area of the camera's aperture regulates the intensity. (i.e. cutting the aperture area in half, cuts the intensity in half; doubling the aperture area doubles the intensity). The shutter speed regulates the exposure time. For most films, the relationship E = I x T does not hold when the exposure time is very slow or extremely fast. Typically, most films require more exposure than the E = I x T formula indicates. This is called reciprocity failure. It can cause color distortion since it usually effects each color component differently.

For the WestonMaster series I - IV: Invercone mounted - With these meters it works with reflected or incident light metering. Note: Low (Values 1sec - 1/50th sec.)and high scales (Values 1/25th. sec - 1/1600th sec.)can be used with this method. Take the square root of your film speed: i.e A film of 125 ASA gives the number 11. Your working aperture is f11, adjust your camera accordingly. Take a reading as you would do normally. Note where the meter stops at the cd/ft2 scale. The reciprocal of this value will be the shutter-speed; i.e needle stops at 100 - reciprocal is 1/100. Resume: Exposure will be 1/100 sec at f11.

For the math aficionados....! The Exposure Formula. (Excerpt from WestonMaster.com) A formula exists which, once learned, facilitates rapid exposure calculation mentally, without the need for setting dials on meters. It requires knowing luminance values in candles per square-foot, and unfortunately few, if any meters today are calibrated in these units. Since the early Weston Master meters read directly in Cd/ft2, very rapid exposure determination was possible. Current meters can usually be calibrated in these units to facilitate the use of the Exposure-formula, as described below.(See Converting to candles per square foot above) To use the Exposure Formula, take the film speed number of the film you're using in ISO or ASA and determine its approximate square root. This number is remembered as the Key Stop for that particular E.I. For example, a film rated as ASA 125 has a key stop of f/11. At the key stop, the correct shutter speed in seconds to expose a given luminance on Zone V [Kodak 18 % Neutral Grey Card], is the reciprocal of the luminance expressed in Cd/ft2. An example.... Thus for a surface that measures 60 Cd/ft2, we would use a shutter speed of 1/60 second at the key- stop. In a typical situation, you might visualize a certain area as a Value III in the print and find that its luminance is 30 Cd/ft2. If this value is placed on Zone III, then 60 Cd/ft2 falls on Zone IV and thus 120 Cd/ft2 on Zone V. Your exposure then is 1/125 at the key-stop, or any equivalent combination. Resuming, the Exposure Formula thus permits very rapid calculation of exposure without reference to the meter's rotating dial. Hence, you will have to compute a little more , when you don't have a scale calibrated in Cd/ft2. One more example.... You're using a film rated at 400 ISO, thus the keystop will be f/22, which you will set accordingly to your Camera lens. For a surface that measures 13 (Westonmeters Models V, Weston Euromaster, Euromaster and Euromaster II) and placed on Zone V, this will give you a luminance of 200 Cd/ft2 ( Conversion: 10=25Cd/ft2, 11=50Cd/ft2, 12=100Cd/ft2, 13=200Cd/ft2 Note: Models mentioned above only). Your final exposure time will be thus 1/250 second at the key-stop, which we defined as f/22. If you are familiar with these mental calculations, remember this one: Full Moon is about 250 cd/ft2 Place it in Zone VII and start from there...!

Sekonic Studio -S in action after revision( 6-9-2003)

Measuring Incident light: Checking Speed formula - Simultaneous metering on a sunny cloudy day around 1100 hours - Location Driebergen-R On the right side a spiffy WestonMaster III with Invercone mounted(high scale) & On the left side Sekonic Studio -S with Photosphere (High slide in). As the Westonmaster scale is in cd/ft2 and the Sekonic in Foot-candle this does not influence the outcome.! Note that our formula will work with the WestonMaster in high and low readings as the Sekonic works only in high reading (High slide in). This is due the scalenumbers on the WestonMaster will change accordingly and the Sekonic scale remains static. With this information and a film of 400ASA loaded, what would be the exposure/aperture combination? Tell us Exposure correction for close-ups(Macro) The above formulae are brought to you by JohnDesq - Driebergen-Rijsenburg, the Netherlands. Both formalas were thoroughly tested, if you find any discrepancies, please let us know!

See also the APEX system.

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