Film speed

Film speed is the measure of a photographic film's sensitivity to light. Film with lower sensitivity (lower ISO speed) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed) can shoot the same scene with a shorter exposure and is called a fast film.

In the first approximation the amount of light energy which reaches the film determines the effect on the emulsion, so that if the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor so that the energy received is the same, the film will be exposed to the same density; this rule is called reciprocity, and the concept of a unique speed for an emulsion is possible because reciprocity holds. In practice this holds reasonably well for normal photographic films for the range of exposures usually used, say 1/1000 sec to 1 sec, but longer exposures, different for different films, are required outside these limits, a phenomenon known as reciprocity failure.

Technical information

ISO film speed scales

International standard ISO 5800:1987 from the International Organization for Standardization (ISO) defines both an arithmetic scale and a logarithmic scale for measuring color-negative film speed. Related standards ISO 6:1993 and ISO 2240:2003 define scales for speeds of black-and-white negative film and color reversal film.

In the ISO arithmetic 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 speed is omitted, and only the arithmetic speed is given (e.g., “ISO 100”). In such cases, the quoted “ISO” speed is essentially the same as the older “ASA” speed.

GOST (Russian: ГОСТ) is a pre-1987 arithmetic 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 speeds are 25/15°, 50/18°, 100/21°, 200/24°, 400/27°, 800/30°, 1600/33°, and 3200/36°. Consumer film speeds are generally between 100/21° and 800/30°, inclusive.

Conversion from the logarithmic speed S° to the arithmetic speed S is given by

$S=10^{\left({S^{\circ }-1}\right)/10}$

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

$S^{\circ }=10\log S+1$

and rounding to the nearest integer.

The following table shows the correspondence among the various speed scales:

ISO arithmetic 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°		Kodachrome 8 mm film
12	12°	11	Gevacolor 8 mm reversal film
16	13°	11	Agfacolor 8 mm reversal film
20	14°	16	Adox CMS 20
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 Pro 160C/S, 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), Tri-X 400, Ilford HP5
500	28°	350
640	29°	560	Polaroid 600
800	30°	700	Fuji Pro 800Z
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	Kodak P3200 TMAX
4000	37°
5000	38°
6400	39°

Determining film speed

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 H_m, in lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed then is

$S={\frac {0.8}{H_{\mathrm {m} }}}$ .

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. 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.

Exposure index

Exposure index, or EI, refers to speed rating assigned to a particular film and shooting situation, and used in the exposure meter or equation, to compensate for equipment calibration inaccuracies or process variables, or to achieve certain effects. Exposure index may or may not be the same as manufacturer's film speed rating for that particular film.

The exposure index is sometimes called the speed setting, as opposed to the speed rating.

For example, a photographer may choose to rate a 400 ISO speed film at 800 and then use push processing in order to get printable negatives from low-light conditions. In this case it is said that film has been shot at EI 800.

Another example of a situation when exposure index would differ from film manufacturer's rating is when a camera shutter is known to be miscalibrated and consistently overexposes or underexposes the film; similarly, a light meter can be known to understate or overstate lighting conditions. In such cases one could adjust EI rating accordingly in order to compensate for these effects and consistently produce correctly exposed negatives.

Film grain

Main article: Film grain

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 grainier image. Each grain of silver halide develops in an all-or-nothing way into dark silver or nothing. Thus, 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.

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 (PDF). 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

A CCD image sensor, 2/3 inch size.

In digital camera systems, an arbitrary relationship between exposure and finished image lightness can be achieved by setting the system's signal gain. This gain is not exactly the same as a sensitivity, which is why the relationship to an ISO speed is more complicated. On a camera, however, setting an ISO speed and setting the exposure accordingly, whether automatically or manually with the help of an exposure meter, should result in a photo of appropriate lightness just as with film cameras.

For digital photo cameras ("digital still cameras"), the ISO standard 12232:2006 specifies several definitions of the speed rating depending on the sensor sensitivity, the sensor noise, and the appearance of the resulting image. The digital ISO speed ratings are related to the conventional film-speed ratings in how a standard 18 percent reflective surface would appear in an image under given lighting conditions.

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

$H={\frac {qLt}{N^{2}}},$

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

$q={\frac {\pi }{4}}T\,v(\theta )\,\cos ^{4}\theta$

is a factor depending on the transmittance T of the lens, the vignetting factor $v(\theta )$, 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

$S_{\mathrm {sat} }={\frac {78}{H_{\mathrm {sat} }}},$

where $H_{\mathrm {sat} }$ 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

$S_{\mathrm {sos} }={\frac {10}{H_{\mathrm {sos} }}},$

where $H_{\mathrm {sos} }$ 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: $S_{40:1}=107$, $S_{10:1}=1688$, and $S_{\mathrm {sat} }=49$. 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 $S_{40:1}=40$, $S_{10:1}=800$, and $S_{\mathrm {sat} }=200$. 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.

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. The best digital cameras as of 2008 exhibit no perceptible noise at ISO 200 sensitivity, and some produce usable results up to ISO 25,600.

References

• Leslie Stroebel, John Compton, Ira Current, and Richard Zakia. Basic Photographic Materials and Processes, second edition. Boston: Focal Press, 2000. ISBN 0-240-80405-8.

See also

• APEX system
• Lens speed

External links

• The official ISO 6:1993 (black-and-white negative films; paid download).
• The official ISO 2240:2003 (color reversal films; paid download).
• The official ISO 5800:1987 (color negative films; paid download).
• The official ISO 12232:2006 (digital still cameras; paid download).
• What is the meaning of ISO for digital cameras? Digital Photography FAQ