|0||Complete lack of density in the negative image, other than film-base density plus fog. Total black in print.|
|I||Effective threshold. First step above complete black in print. Slight tonality, but no texture.|
|II||First suggestion of texture. Deep tonalities, representing the darkest part of the image in which some detail is required.|
|III||Average dark materials. Low values showing adequate texture.|
|IV||Average dark foliage. Dark stone. Landscape shadow. Recommended shadow value for portraits in sunlight.|
|V||Clear north sky (panchromatic rendering). Dark skin. Gray stone. Average weathered wood. Middle gray (18% reflectance).|
|VI||Average Caucasian skin value in sunlight or artificial light, and in difusse skylight or very soft light. Light stone. Clear north sky (orthochromatic rendering). Shadows on snow in sunlit snowscapes.|
|VII||Very light skin. Light-gray objects. Average snow with acute side lighting.|
|VIII||Whites with textures and delicate values (not blank whites). Snow in full shade. Hightlights on Caucatian skin.|
|IX||Glaring white surfaces. Snow in flat sunlight. White without texture.|
For some time, cameras have been available with shutters and lenses marked in two scales. The first is the familiar fractions of a second, and f/stops. The second is a series of apparently arbitrary numbers for both the speed and aperture; these numbers are so arranged that using any parts of members having the same arithmetic sum, produces the same exposure. On the shutter, the speed varieties are numbered as follows:
SHUTTER SPEED VALUES
Likewise the lens diaphragm carries the following series of numbers:
LENS APERTURE VALUES
Notice that any pair of the above values having the same total will produce the same exposure, thus:
|EV 10 =||1+9||(½ sec at f22)|
|2+8||(¼ sec at f16)|
|3+7||(1/8 sec at f11)|
|4+6||(1/15 sec at f8)|
|5+5||(1/30 sec at f5.6)|
|6+4||(1/60 sec at f4)|
|7+3||(1/125 sec at f2.8)|
|8+2||(1/250 sec at f2)|
|9+1||(1/500 sec at f1.4)|
|10+0||(1/1000 sec at f1.2)|
When these Exposure Values were first applied to cameras, there was no particular method of relating them to the reading of an exposure meter, except by setting the calculator dial to the proper Exposure Index for the film in use, taking a reading, and getting the result from the calculator. Some calculators were latter supplied to read in EV numbers as well as f/stop and shutter speed combinations, but the calculator still had to be used to find the number needed.
ADDITIVE SPEED VALUES AND LIGHT VALUES
The system is now completed by the introduction of Additive Speed Values for the films, and Light Values. Additive Speed Values are derived from the Exposure Index of the film by the following table:
ADDITIVE FILM SPEED VALUES
However, there will be no need to memorize the above, easy as it is, since manufacturers will be publishing their film ratings in this system as well as in the form of Exposure Indexes. These numbers will be distinguished from Exposure Indexes by having a degree mark following, thus: 6°.
The final step in making the whole system practical is to assign a scale of values to the light. This is done according to the following scale:
ADDITIVE LIGHT VALUES
*Incident Light. These values are not the same as the candles-per-square-foot scale on reflected light meters. However, newer light meters will be calibrated directly in Light Values, and eliminate the need for this table. For those who do not wish to use a meter, it will be quite easy to provide tables giving the Light Value for various kinds of illumination. For instance, bright sunshine is about 10, a baseball park under night lighting is about 4, an average living room is about 1, but directly under a 300-watt reading lamp is about 3, and so on.
Using these values, then, is a simple procedure. Assume the meter reading for it given scene is 8, and a film having an additive speed of 6° is in use, all that is necessary is to add 8+6=14, and then any combination of shutter speed and lens opening totaling 14 on the EV scale will provide the correct exposure.
THE SLAT RULE
There is it simple memory device for using this system; the key is the word SLAT. Expressed in usable terms.
|T=Time (Shutter Speed)|
So all you have to do is add the film speed value to the light value as obtained from the meter or a table, and the result will be a number representing the whole range of combinations of shutter time and aperture.
Photo Lab Index, Section 10-03, Supplement No. 137, p. 373.