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1949 LENS-CALIBRATION REPORT 371
"thirds of a stop," namely, 0.7, 0.8, 0.9, 1.0. 1.13, 1.27, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 0.S, 3.2, 3.6, 4-0, 4.5, 5.0, 5.0, 6.3, 7.1, 8.0, 9.0, 10.0, JJ.3, 12.7, 14.2, 16, 18, 20, 0S, 25, 28, 32.0,
The reason for dividing each stop interval into three parts is so that the lens apertures will agree with the exposure-meter markings stated in American Standard Z52. 12-1944, page 5. The same cube-root-oftwo series is used for the Exposure Index of a film, see American Standard Z38. 2. 1-1947, page 11. One third of a stop represents a logarithmic illumination ratio equal to 0.1, which is the transmission of a neutral density of 0.1. It is worth noting that \/2 is equal to tyW, or 1.260. The ratio of successive circular stop diameters is equal to \/2 = 1.123.
It is contemplated that a lens will be engraved with either the / numbers or the T numbers, but not with both sets of markings.
VII. SYMBOL
Lenses calibrated on the/ system should bear the designation// or /: followed by the numerals (see American Standard Z38.4. 7-1943).
Lenses calibrated on the T-stop system should bear the designation T or T followed by the numerals.
VIII. ACCURACY OF MARKING (/SYSTEM)
The maximum opening of a lens on the / system shall be marked with an accuracy of ± 10 per cent of area, or ±5 per cent of diameter.* Since blanket calibrations are generally used for / apertures, the smaller openings may be in error by ±25 per cent of area, or ±12 per cent of diameter (one third of a stop) . These figures are based on the assumption that the iris will always be closed down to the desired aperture and not opened up from a smaller aperture, to eliminate backlash effects.
* In Z38.4.4-1942 the engraved focal length of lenses for still picture photography must be within ±4 per cent of its true value, and in Z38.4.7-1943 the measured diameter of the maximum entering beam shall be at least 95 per cent of the quotient obtained by dividing the engraved focal length by the engraved / number. Thus by combining these tolerances we find that the diameter of the maximum lens aperture may be in error by as much as 9 per cent. This represents an error in area of 18 per cent, or one quarter of a stop, which is felt to be unnecessarily large for the maximum aperture. The proposed tolerance on aperture marking for motion picture objective lenses allows less latitude than that provided for still picture camera lenses by Sectional Committee Z38 (Photography), because of the stricter requirements in cinematography for uniform density of successive scenes photographed on the same continuous length of film using different lenses.