How much do you know about film speed measurement systems?

Film Speed Measurement Systems

• Film speed is the measure of a photographic film's sensitivity to light.

• Relatively insensitive film, with a lower speed index, requires more exposure to light to produce the same image density as a more sensitive film, and is commonly termed slow film.

• Highly sensitive films are correspondingly termed fast films.

• The higher the sensitivity, the grainier the image will be. Ultimately sensitivity is limited by the quantum efficiency of the film or sensor.

• The first known practical sensitometer was invented by the Polish engineer Leon Warnerke in 1880.

• Hurter and Driffield developed another early practical system for measuring the sensitivity of an emulsion in 1890.

• The Scheinergrade system was devised by the German astronomer Julius Scheiner in 1894.

• The DIN system, officially DIN standard 4512, was published in January 1934, and was inspired by Scheiner's system.

• Before the ASA system, the system of Weston film speed ratings was introduced by Edward Faraday Weston and his father Dr. Edward Weston in August 1932.

• General Electric developed its own rating system of so-called General Electric film values around 1937.

• The American Standards Association defined a new method to determine and specify film speeds of black-and-white negative films in 1943.

• The ASA system was superseded by the ISO film speed system between 1982 and 1987.Understanding Film Speed: From GOST to ISO

• GOST 2817-50 was a film speed standard used in the former Soviet Union from 1951, replacing the Hurter & Driffield (H&D) numbers.

• On 1 January 1987, the GOST scale was realigned to the ISO scale with GOST 10691–84.

• The current International Standard for measuring the speed of colour negative film is ISO 5800:2001, which defines scales for speeds of black-and-white negative film and colour reversal film as well.

• Conversion from arithmetic speed S to logarithmic speed S° is given by S° = log2(S/3.125), while conversion from logarithmic speed to arithmetic speed is given by S = 3.125 × 2^S°.

• Film speed is found from a plot of optical density vs. log of exposure for the film, known as the D–log H curve or Hurter–Driffield curve.

• Film speed is used in the exposure equations to find the appropriate exposure parameters, including lighting, film speed, f-number, and shutter speed.

• Exposure index (EI) refers to speed rating assigned to a particular film and shooting situation in variance to the film's actual speed.

• Film sensitivity is related to granularity because larger grains give film greater sensitivity to light.

• Some high-speed black-and-white films, such as Ilford Delta 3200 and T-MAX P3200, are marketed with film speeds in excess of their true ISO speed as determined using the ISO testing method.

• In digital camera systems, an arbitrary relationship between exposure and sensor data values can be achieved by setting the signal gain of the sensor.

• The ISO 12232:2019 standard specifies the measurement of light sensitivity of the entire digital camera system and not of individual components such as digital sensors.

• The Recommended Exposure Index (REI) technique allows the manufacturer to specify a camera model's EI choices arbitrarily.ISO 12232 is an international standard for measuring the sensitivity of digital cameras. The standard defines three different methods for measuring camera sensitivity: saturation-based speed, noise-based speed, and standard output sensitivity (SOS). The saturation-based speed is the maximum possible exposure that does not lead to a clipped or bloomed camera output. The noise-based speed is the exposure that will lead to a given signal-to-noise ratio on individual pixels. The standard output sensitivity (SOS) relates the exposure to the digital pixel values in the output image. The ISO speed ratings are based on the properties of the sensor and the image processing done in the camera, and are expressed in terms of the luminous exposure H (in lux seconds) arriving at the sensor. Cameras typically do not clearly indicate whether the user "ISO" setting refers to the noise-based speed, saturation-based speed, or the specified output sensitivity, or even some made-up number for marketing purposes. Following the publication of CIPA DC-004 in 2006, Japanese manufacturers of digital still cameras are required to specify whether a sensitivity rating is REI or SOS. APS- and 35 mm-sized digital image sensors, both CMOS and CCD based, do not produce significant noise until about ISO 1600.