The film speed number indicates how sensitive the film is to light.
High-numbered film speed is more sensitive to light and therefore the film is
faster so that the resulting image does not become overexposed or too bright. On
the contrary, low-numbered film speed is less sensitive to light and needs more
time to absorb enough light to make an image that isn’t underexposed or too
dark. Over the years, There have been many systems of denoting film sensitivity.
Current ISO system
The current International Standard for measuring the speed of color negative film is called ISO 5800:1987 from the International Organization for Standardization (ISO). Related standards ISO 6:1993 and ISO 2240:2003 define scales for speeds of black-and-white negative film and color reversal film. This system defines both an arithmetic and a logarithmic scale, combining the previously separate ASA and DIN systems.
In the ISO arithmetic scale, corresponding to the ASA system, a doubling of the sensitivity of a film requires a doubling of the numerical film speed value. In the ISO logarithmic scale, which corresponds to the DIN scale, adding 3° to the numerical value that designates the film speed constitutes a doubling of that value. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.
Commonly, the logarithmic speed is omitted, and only the arithmetic speed is given; for example, “ISO 100”.
GOST (Russian: is an arithmetic scale which was used in the former Soviet Union before 1997. It is almost identical to the ASA standard, having been based on a speed point at a density 0.2 above base plus fog, as opposed to the ASA's 0.1. After 1987, the GOST scale was aligned to the ISO scale. GOST markings are only found on pre-1987 photographic equipment (film, cameras, lightmeters, etc.) of Soviet Union manufacture.
Conversion between current scales
Conversion from the logarithmic DIN speed S° to the arithmetic ASA speed S, as given by requires the following calculation:
and rounding to the nearest standard arithmetic speed in the table below. By simple rearrangement, conversion from arithmetic speed to logarithmic speed is given by
and rounding to the nearest integer. Here the log function is base 10.
Comparison of current scales
The following table shows a comparison of various film speed scales:
ISO arithmetic scale
ISO log scale
Example of film stock
with this nominal speed
Kodachrome 8 mm film
Gevacolor 8 mm reversal film
Agfacolor 8 mm reversal film
Adox CMS 20
old Agfacolor, Kodachrome 25
Kodachrome 40 (movie)
Fuji RVP (Velvia)
Kodachrome 64, Ektachrome-X
Ilford Commercial Ortho
Kodacolor Gold, Kodak T-Max (TMX), Provia
Ilford FP4+, Kodak Plus-X Pan
Fujicolor Pro 160C/S, Kodak High-Speed Ektachrome
Fujicolor Superia 200
Kodak Tri-X Pan Professional (TXP)
Kodak T-Max (TMY), Tri-X 400, Ilford HP5+
Fuji Pro 800Z
Kodak P3200 TMAX, Ilford Delta 3200 (see text below)
Kodak T-Max (TMZ)
Determining film speed
ISO 6:1993 method of determining speed for black-and-white film.
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. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For black and white negative film, the “speed point” m is the point on the curve where density exceeds the base + fog density by 0.1 when the negative is developed so that a point n where the log of exposure is 1.3 units greater than the exposure at point m has a density 0.8 greater than the density at point m. The exposure Hm, in lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed then is
Determining speed for color negative film is similar in concept but more complex because it involves separate curves for blue, green, and red. The film is processed according to the film manufacturer’s recommendations rather than to a specified contrast. ISO speed for color reversal film is determined from the middle rather than the threshold of the curve; it again involves separate curves for blue, green, and red, and the film is processed according to the film manufacturer’s recommendations.
Applying film speed
Film speed is used in the exposure equations to find the appropriate exposure parameters. Four variables are available to the photographer to obtain the desired effect: lighting, film speed,
f-number (aperture size), and shutter speed (exposure time). The equation may be expressed as ratios, or, by taking the logarithm (base 2) of both sides, by addition, using the APEX system, in which every increment of 1 is a doubling of exposure, known as a "stop". The effective f-number is proportional to the ratio between the lens focal length and aperture diameter, which is proportional to the square root of the aperture area. As a result, a lens set to f/1.4 allows twice as much light to strike the focal plane as a lens set to f/2. Therefore, each f-number factor of the square root of two (approximately 1.4) is also a stop, so lenses are typically marked in that progression: f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc.
Exposure index, or EI, refers to speed rating assigned to a particular film and shooting situation in variance to the film's actual speed. It is used to compensate for equipment calibration inaccuracies or process variables, or to achieve certain effects. The exposure index may simply be called the speed setting, as compared to the speed rating.
For example, a photographer may rate an ISO 400 film at EI 800 and then use push processing to obtain printable negatives in low-light conditions. The film has been exposed at EI 800.
Another example occurs where a camera's shutter is miscalibrated and consistently overexposes or underexposes the film; similarly, a light meter may be inaccurate. One may adjust the EI rating accordingly in order to compensate for these defects and consistently produce correctly exposed negatives.
Upon exposure, the amount of light energy that reaches the film determines the effect upon the emulsion. If the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor by varying the camera's shutter speed and aperture, so that the energy received is the same, the film will be developed to the same density. This rule is called reciprocity. The systems for determining the sensitivity for an emulsion are possible because reciprocity holds. In practice, reciprocity works reasonably well for normal photographic films for the range of exposures between 1/1000 second to 1/2 second. However, this relationship breaks down outside these limits, a phenomenon known as reciprocity failure.
Film sensitivity and grain
Grainy high speed B/W film negativeFilm speed is roughly related to granularity, the size of the grains of silver halide in the emulsion, since larger grains give film a greater sensitivity to light. Fine-grain stock, such as portrait film or those used for the intermediate stages of copying original camera negatives, is "slow", meaning that the amount of light used to expose it must be high or the shutter must be open longer. Fast films, used for shooting in poor light or for shooting fast motion, produce a grainier image. Each grain of silver halide develops in an all-or-nothing way into dark silver or nothing. As a result, each grain is a threshold detector; in aggregate, their effect can be thought of as a noisy nonlinear analog light detector.
Kodak has defined a "Print Grain Index" (PGI) to characterize film grain (color negative films only), based on perceptual just noticeable difference of graininess in prints. They also define "granularity", a measurement of grain using an RMS measurement of density fluctuations in uniformly-exposed film, measured with a microdensitometer with 48 micrometre aperture. Granularity varies with exposure — underexposed film looks grainier than overexposed film.
Use of grain
In advertising, music videos, and some drama, mismatches of grain, color cast, and so forth between shots are often deliberate and added in post-production.
Some high-speed black-and-white films, such as Ilford Delta 3200 and Kodak T-MAX P3200, are marketed with film speeds in excess of their true ISO speed as determined using the ISO testing method. For example, the Ilford product is actually an ISO 1000 film, according to its data sheet.The manufacturers do not indicate that the 3200 number is an ISO rating on their packaging. These films can be successfully exposed at EI 3200 (or any of several other speeds) through the use of push processing.
Digital camera ISO speed and exposure index
A CCD image sensor, 2/3 inch size.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 relationship between the sensor data values and the lightness of the finished image is also arbitrary, depending on the parameters chosen for the interpretation of the sensor data into an image color space such as sRGB.
For digital photo cameras ("digital still cameras"), an exposure index (EI) rating—commonly called ISO setting—is specified by the manufacturer such that the sRGB image files produced by the camera will have a lightness similar to what would be obtained with film of the same EI rating at the same exposure. The usual design is that the camera's parameters for interpreting the sensor data values into sRGB values are fixed, and a number of different EI choices are accommodated by varying the sensor's signal gain in the analog realm, prior to conversion to digital. Some camera designs provide at least some EI choices by adjusting the sensor's signal gain in the digital realm. A few camera designs also provide EI adjustment through a choice of lightness parameters for the interpretation of sensor data values into sRGB; this variation allows different tradeoffs between the range of highlights that can be captured and the amount of noise introduced into the shadow areas of the photo.
The ISO 12232:2006 standard
The ISO standard 12232:2006 gives digital still camera manufacturers a choice of five different techniques for determining the exposure index rating at each sensitivity setting provided by a particular camera model. Three of the techniques in ISO 12232:2006 are carried over from the 1998 version of the standard, while two new techniques allowing for measurement of JPEG output files are introduced from CIPA DC-004. Depending on the technique selected, the exposure index rating can depend on the sensor sensitivity, the sensor noise, and the appearance of the resulting image. The standard specifies the measurement of light sensitivity of the entire digital camera system and not of individual components such as digital sensors, although Kodak has reported. using a variation to characterize the sensitivity of two of their sensors in 2001.
The Recommended Exposure Index (REI) technique, new in the 2006 version of the standard, allows the manufacturer to specify a camera model’s EI choices arbitrarily. The choices are based solely on the manufacturer’s opinion of what EI values produce well-exposed sRGB images at the various sensor sensitivity settings. This is the only technique available under the standard for output formats that are not in the sRGB color space. This is also the only technique available under the standard when multi-zone metering (also called pattern metering) is used.
The Standard Output Specification (SOS) technique, also new in the 2006 version of the standard, effectively specifies that the average level in the sRGB image must be 18% gray plus or minus 1/3 stop when exposed per the EI with no exposure compensation. Because the output level is measured in the sRGB output from the camera, it is only applicable to sRGB images—typically JPEG—and not to output files in raw image format. It is not applicable when multi-zone metering is used.
The CIPA DC-004 standard requires that Japanese manufacturers of digital still cameras use either the REI or SOS techniques. Consequently, the three EI techniques carried over from ISO 12232:1998 are not widely used in recent camera models (approximately 2007 and later). As those earlier techniques did not allow for measurement from images produced with lossy compression, they cannot be used at all on cameras that produce images only in JPEG format.
The saturation-based technique is closely related to the SOS technique, with the sRGB output level being measured at 100% white rather than 18% gray. The saturation-based value is effectively 0.704 times the SOS value. Because the output level is measured in the sRGB output from the camera, it is only applicable to sRGB images—typically TIFF—and not to output files in raw image format. It is not applicable when multi-zone metering is used.
The two noise-based techniques have rarely been used for consumer digital still cameras. These techniques specify the highest EI that can be used while still providing either an “excellent” picture or a “usable” picture depending on the technique chosen.
Measurements and calculations
ISO speed ratings of a digital camera 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. For a typical camera lens with an effective focal length f that is much smaller than the distance between the camera and the photographed scene, H is given by
where L is the luminance of the scene (in candela per m², t is the exposure time (in seconds), N is the aperture f-number, and
is a factor depending on the transmittance T of the lens, the vignetting factor v(θ), and the angle θ relative to the axis of the lens. A typical value is q = 0.65, based on θ = 10°, T = 0.9, and v = 0.98.
The saturation-based speed is defined as
where Hsat is the maximum possible exposure that does not lead to a clipped or bloomed camera output. Typically, the lower limit of the saturation speed is determined by the sensor itself, but with the gain of the amplifier between the sensor and the A/D-converter, the saturation speed can be increased. The factor 78 is chosen such that exposure settings based on a standard light meter and an 18-percent reflective surface will result in an image with a grey level of 18%/√2 = 12.7% of saturation. The factor √2 indicates that there is half a stop of headroom to deal with specular reflections that would appear brighter than a 100% reflecting white surface.
The noise-based speed is defined as the exposure that will lead to a given signal-to-noise ratio on individual pixels. Two ratios are used, the 40:1 ("excellent image quality") and the 10:1 ("acceptable image quality") ratio. These ratios have been subjectively determined based on a resolution of 70 pixels per cm (180 DPI) when viewed at 25 cm (10 inch) distance. The signal-to-noise ratio is defined as the standard deviation of a weighted average of the luminance (overall brightness) and color of individual pixels. The noise-based speed is mostly determined by the properties of the sensor and somewhat affected by the noise in the electronic gain and AD converter.
In addition to the above speed ratings, the standard also defines the standard output sensitivity (SOS), how the exposure is related to the digital pixel values in the output image. It is defined as
where Hsos is the exposure that will lead to values of 118 in 8-bit pixels, which is 18 percent of the saturation value in images encoded as sRGB or with gamma = 2.2.
The standard specifies how speed ratings should be reported by the camera. If the noise-based speed (40:1) is higher than the saturation-based speed, the noise-based speed should be reported, rounded downwards to a standard value (e.g. 200, 250, 320, or 400). The rationale is that exposure according to the lower saturation-based speed would not result in a visibly better image. In addition, an exposure latitude can be specified, ranging from the saturation-based speed to the 10:1 noise-based speed. If the noise-based speed (40:1) is lower than the saturation-based speed, or undefined because of high noise, the saturation-based speed is specified, rounded upwards to a standard value, because using the noise-based speed would lead to overexposed images. The camera may also report the SOS-based speed (explicitly as being an SOS speed), rounded to the nearest standard speed rating.
For example, a camera sensor may have the following properties: S40:1 = 107, S10:1 = 1688, and Ssat = 49. According to the standard, the camera should report its sensitivity as
ISO 100 (daylight)
ISO speed latitude 50–1600
ISO 100 (SOS, daylight).
The SOS rating could be user controlled. For a different camera with a noisier sensor, the properties might be S40:1 = 40, S10:1 = 800, and Ssat = 200. In this case, the camera should report
ISO 200 (daylight),
as well as a user-adjustable SOS value. In all cases, the camera should indicate for the white balance setting for which the speed rating applies, such as daylight or tungsten (incandescent light).
Despite these detailed standard definitions, 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. Because the 1998 version of ISO 12232 did not permit measurement of camera output that had lossy compression, it was not possible to correctly apply any of those measurements to cameras that did not produce sRGB files in an uncompressed format such as TIFF. 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.
As should be clear from the above, a greater SOS setting for a given sensor comes with some loss of image quality, just like with analog film. However, this loss is visible as image noise rather than grain. Current (April 2009) APS and 35mm sized digital image sensors, both CMOS and CCD based, do not produce significant noise until about ISO 800.