ISO in Photography

The standard known as ISO 5800:1987 from the International Organization for Standardization (ISO) defines both a linear scale and a logarithmic scale for measuring film speed.

In the ISO linear scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed. In the ISO logarithmic scale, which corresponds to the older DIN scale, doubling the speed of a film implies adding 3° to the numeric value that designates the film speed. For example, a film rated ISO 200/24° is twice as sensitive as a film rated ISO 100/21°.

Commonly, the logarithmic (DIN) component is omitted from film speed ratings, and only the linear component is given (e.g. “ISO 100”). In such cases, the quoted “ISO” rating is in effect synonymous with the older ASA standard.

GOST (Russian: ГОСТ) is a pre-1987 linear standard used in the former Eastern Bloc. It was almost, but not quite identical to the ASA standard. 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 Eastern Bloc manufacture.

The most common ISO film ratings are 25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°. Consumer films are generally rated between 100/21° and 800/30°, inclusive.

A film speed is converted from the linear scale to the logarithmic scale by this formula (plus rounding to the nearest integer):

\mbox{log-scale speed} = 3\,\log_2\left(\frac{128}{100} \,\mbox{linear speed}\right) \!

Conversion from the logarithmic scale to the linear scale is analogous, except that results must be rounded to the conventional values of the linear scale listed in the table below.

\mbox{linear speed} = \frac{100}{128}\,2^\left(\frac{\mbox{log-scale speed}}{\mbox{3}}\right) \!

The following table shows the correspondence between these scales:

ISO linear scale
(old ASA scale)
ISO log scale
(old DIN scale)
GOST
(Soviet pre-1987)
Example of film stock
with this nominal speed
6 9°   original Kodachrome
8 10°    
10 11°    
12 12° 11  
16 13° 11  
20 14° 16  
25 15° 22 old Agfacolor, Kodachrome 25
32 16° 22 Kodak Panatomic-X
40 17° 32 Kodachrome 40 (movie)
50 18° 45 Fuji RVP (Velvia)
64 19° 45 Kodachrome 64, Ektachrome-X
80 20° 65 Ilford Commercial Ortho
100 21° 90 Kodacolor Gold, Kodak T-Max (TMX)
125 22° 90 Ilford FP4, Kodak Plus-X Pan
160 23° 130 Fuji NPS, Kodak High-Speed Ektachrome
200 24° 180 Fujicolor Superia 200
250 25° 180  
320 26° 250 Kodak Tri-X Pan Professional (TXP)
400 27° 350 Kodak T-Max (TMY)
500 28° 350  
640 29° 560 Polaroid 600
800 30° 700 Fuji NPZ
1000 31° 700 Ilford Delta 3200 (see text below)
1250 32°    
1600 33° 1400–1440 Fujicolor 1600
2000 34°    
2500 35°    
3200 36° 2800–2880 old Konica 3200
4000 37°    
5000 38°    
6400 39°    

Determining film speed

Film speed is found by referencing the Hurter–Driffield curve, or D–logE curve, for the film. This is a plot of optical density vs. log of exposure (lux-s). There are typically five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. Following the curve to the point where density exceeds the base + fog by 0.1, find the corresponding exposure. Dividing 0.8 by that exposure yields the linear ISO speed rating.

Applying film speed

Film speed is used in the exposure equation 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 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. Thus, 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.

Film grain

Grainy high speed B/W film negative

Grainy high speed B/W film negative

Film 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 grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way. Thus, film is a threshold detector rather than a linear detector. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. An accumulation of such areas breaks up the visible contours of the object, the effect known as graininess (or grain). Fast films are also relatively contrasty, for the same reason. That is, an area of the image will consist of bright areas and dark ones with few transitional areas of midtones.

Kodak used to use a Granularity Index (GI) to characterize film grain. Alternating images of the film under test and a standard grain were shown to test subjects who indicated when they perceived a match. The standard grain samples were the index. More recently, Kodak switched to a measurement of grain using an RMS measurement. Granularity varies with exposure — underexposed film looks grainier than overexposed film.

Improvements in film

In the early 1980s, there were some radical improvements in film stock. It became possible to shoot color film in very low light and produce a fine-grained image with a good range of midtones.

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.

Altering film speed

Certain high-speed black-and-white films, such as Ilford Delta 3200 and Kodak T-Max P3200 (TMZ), are marketed with higher speeds on the box than their true ISO speed (determined using the ISO testing methodology). For example, the Ilford product is actually an ISO 1000 film, according to its data sheet. The manufacturers are careful not to refer to the 3200 number as an ISO speed on the packaging. These films can be successfully exposed at EI 3200 (or any of several other speeds) through the use of push processing. The most sensitive sensor common in commercial photography may be the Silicon Intensified Target vidicon, at ASA 200,000, used in TV cameras.

Digital camera ISO speed and exposure index

The International Organization for Standardization (ISO) has a performance-based ISO speed standard for digital cameras, just as they have for film. ISO Standard 12232:2006 (“Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index”) defines ISO speed in terms of the amount of light needed to achieve a certain “quality” in the sense of a per-pixel signal-to-noise ratio.

However, this standard ISO speed “rating” for a digital camera is not necessarily very related to the ISO “setting” or “exposure index” used on the camera.

The image sensors in digital cameras can be adjusted, or can have their outputs adjusted, in sensitivity to function with metering at any given ISO setting. This is usually done by simply amplifying the output of the image sensor, which increases image noise, sometimes beyond the level that the ISO standard says is acceptable.

Just as in photographic film, greater sensitivity comes with some loss of image quality, though this is visible as noise rather than grain. The best digital cameras as of 2006 exhibit no perceptible noise at ISO 200 sensitivity, and some produce useable results up to ISO 3200.

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