Chapter 18. Speed of materials, sensors and systems

Speed of materials, sensors and systems



Figure 18.1



303



Threshold criterion of speed



imately to the density of the deepest shadow of an

average negative. With high-contrast materials in

which a dense background is required, a density of 1

to 2 is a more useful basis for speed determination,

while for materials used in astronomy, a density of

0.6 has been suggested. It will be apparent that the

exposure corresponding to a specified density can be

more precisely located than the threshold.

A fixed density criterion (D = 0.1 + fog) was

adopted in the first National Standard speed system,

the DIN system, in 1934, and is now employed in all

current systems.



Figure 18.3



Inertia criterion



characteristic curve, i.e. the part of the curve in

which objectively-correct reproduction in the negative is obtained. With short-toe materials, as used

by Hurter and Driffield, this would be an advantage. With modern materials which have a long toe

often with no linear region, the linear portion of

the curve has little relevance.



Minimum useful gradient

Inertia

This was the basis selected by Hurter and Driffield

for their pioneering work on quantifying the photographic process. The inertia point is the point

where an extension of the line representing portion

crosses the line representing the base + fog level

(Figure 18.3). Under the processing conditions

which were prevalent in Hurter and Driffield’s

time, inertia was independent of development and

so offers a fixed point of reference. Further, the

inertia point is related to the linear portion of the



Figure 18.2



Fixed density criterion



Threshold speed systems work at the very bottom of

the toe of the characteristic curve, while systems

based on inertia ignore the toe completely. Neither

system approximates very closely to actual practice,

where a part (but only a part) of the toe is used. It was

at one time suggested that a criterion more closely

related to practice could be obtained from that point

on the toe of the characteristic curve at which a

certain minimum gradient is reached. A value of 0.2

for tan a in Figure 18.4 was proposed.



Figure 18.4



Minimum useful gradient criterion



304 Speed of materials, sensors and systems



negative is to be printed. If the overall contrast of the

negative is such that it needs a hard paper, the contrast

of the negative in the shadows can be lower than with a

negative requiring a soft paper. In other words, the

minimum contrast acceptable in the toe depends upon

the contrast of the negative as a whole. Realization of

this fact led to the conception of the fractional gradient

criterion. The point chosen for this criterion is the point

A in Figure 18.5, where the slope of the tangent to the

curve at A equals a given fraction of the slope of AB,

the line joining the points marking the ends of the

portion of the curve employed. This is usually

expressed by the equation:

Figure 18.5



Fractional gradient criterion



¯

Gmin = K × G



The minimum useful gradient criterion was based

on the idea that loss of tone separation in the shadows

(shadow detail) is the first sign of under-exposure,

and that this in turn is due to unacceptably low

contrast in the portion of the characteristic curve

occupied by the shadows. The minimum useful

gradient criterion did not come into general use but is

of interest because it led to the more fundamental

fractional gradient criterion.



¯

where Gmin = tan a and G = tan b (provided the

density and log exposure axes are equally scaled), and

K is a constant determined empirically.

Practical tests by L.A. Jones showed that a value

for K of 0.3 gave results corresponding very well with

the minimum exposure required to give a negative

from which an ‘excellent’ (as opposed to merely

‘acceptable’) print could be made. In Jones’s work,

the fractional gradient point A was located by the

equation:

¯

Gmin = 0.3 × G(1.5)



Fractional gradient

The main argument against the minimum useful

gradient criterion is that the minimum value of contrast

acceptable in the shadows is not a constant but depends

upon the contrast grade of the paper on which the



¯

where G(1.5) means the average gradient over a logexposure range of 1.5, a value which has been shown

to be fairly typical for exterior scenes in daylight.

When located in this way, point A is sometimes

referred to as the ‘Jones point’.



Table 18.1 The principal methods of expressing film speed of negative monochrome materials adopted in early standards

System



Date of

introduction



Type of unit



Speed criterion



Development



H and D



1890



Arithmetic



Inertia



Developer to be bromide-free;

development time not important



Scheiner



1894



Logarithmic



Threshold



Not specified



DIN*



1934



Logarithmic



Fixed density (0.1 + fog)



To be continued until maximum

speed is obtained (optimal

development)



BS*

ASA*

BS and ASA

DIN

ASA

DIN

BS

ISO



1941

1943

1947

1957

1960, 1972

1961

1962, 1973

1956, 1962



Logarithmic

Arithmetic

Arithmetic and logarithmic

Logarithmic

Arithmetic

Logarithmic

Arithmetic



Fixed density (0.1 +

Fractional gradient

Fractional gradient

Fixed density (0.1 +

Fixed density (0.1 +

Fixed density (0.1 +

Fixed density (0.1 +

Fixed density (0.1 +



fog)

fog)

fog)

fog)

fog)

fog)



·



To be under carefully controlled

conditions giving a degree of

development comparable with

average photofinishing. See Figure

18.6 and accompanying text



*DIN, ASA and BS stand respectively for Deutsche Industrie Normen, American Standards Association (now American National Standards

Institution or ANSI), and British Standard. ISO stands for International Standards Organization.



Speed of materials, sensors and systems



305



This criterion was employed first by Eastman

Kodak in 1939, and was later adopted by the then

American Standards Association (in 1943) and the

British Standards Institution (in 1947) as the basis for

national standards.



Speed systems and standards

Table 18.1 outlines standards which were adopted by

the various national standards organizations. In

1960–2, the American, British and German Standard

speed systems were brought into line in all respects

except for the type of speed rating employed, the

American and British Standards specifying arithmetic

numbers and the German Standard logarithmic ones.

This agreement was made possible by work which

showed that good correlation exists between speeds

based on a fixed density of 0.1 above fog and the

fractional gradient criterion, for a wide variety of

materials when developed to normal contrast. Speed

in all three systems is therefore now determined with

reference to the exposure required to produce a

density of 0.1 above fog density, this criterion being

much simpler to use in practice than the fractional

gradient criterion.

The American and British standards were further

modified in 1972 and 1973 respectively. Both use the

same speed criterion mentioned below but the

developer solution and the illuminant specified are

slightly different from those specified in the earlier

standards. The common method adopted for determining speed in the three Standards is illustrated in

Figure 18.6. This procedure is used in the current

International Organization for Standardization (ISO)

standard.

The International Organization for Standardization

(ISO) is an international federation of national

standard (member) bodies. Its work is undertaken by

various technical committees on which interested

member bodies have representatives. Draft international standards are adopted when at least 75 per

cent of the member bodies have cast supporting votes.

It is important to note that where ISO speed values

are to be measured, reference must be made to all the

specifications, and definitions which can only be

found by direct reference to the standard.

Figure 18.6 shows the basic principles now used

for the determination of ISO speed. The characteristic

curve of a photographic material is plotted for the

specified developing conditions. Two points are

shown on the curve at M and N. Point N lies 1.3 log

exposure units from point M in the direction of

increasing exposure. The development time of the

negative material is so chosen that point N has a

density 0.80 greater than the density at point M.

When this condition is satisfied, the exposure corresponding to point M represents the criterion from

which speed is calculated. It is for the degree of



Figure 18.6 Principles of the method adopted for

determining speed in current ISO standard



development thus obtained that the correlation

between the fixed density criterion and the fraction

gradient criterion that was referred to above holds

good. In the current standards speeds are determined

from the following formulae:

Arithmetic speed, S =



0.80

Hm



Logarithmic speed, SO = 1 + 10 log10



0.80

Hm



where Hm is the exposure in lux seconds corresponding to the point M in Figure 18.6 and distinctions

between arithmetic and logarithmic speeds are provided in the next section. Table 18.2 shows the

equivalent arithmetic and logarithmic speed numbers.

The range of exposure values for rounding to the

particular speed value are provided in tables of the

ISO Standard.

In Table 18.1 and elsewhere in the text reference has

been made to arithmetic and logarithmic speed ratings.

With arithmetic scales a doubling of film speed is

represented by a doubling of speed number. With

logarithmic speed scales, distinguished by a degree (°)

sign, the progression is based on the common

logarithm (base 10 scale). The common logarithm of 2

is almost exactly 0.3, and logarithmic film speeds are

scaled so that a doubling of film speed is represented

by an increase of 3 in the speed index.

The ISO speed ratings printed on the boxes of films

are daylight ratings represented in the following

ways:

᭹

᭹

᭹



ISO 100, as arithmetic speed

ISO 21°, as logarithmic speed

ISO 100/21°, as both arithmetic and logarithmic

speeds.



Speed ratings are not published for high-contrast

materials, such as those used in the graphic arts or for



306 Speed of materials, sensors and systems

Table 18.2 Conversion table between ISO speed

ratings

ISO speed rating

arithmetic (S)



logarithmic (S°)



3200

2500

2000

1600

1250

1000

800

650

500

400

320

250

200

160

125

100

80

64

50

40

32

25

20

16

12

10

8

6

5

4



36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7



microcopying, nor for special materials such as

astronomical plates, holographic materials and

recording films. As we have already seen, any system

for expressing the speed of a photographic material

must take into account exposure and development

conditions, and must be related to some particular

criterion of correct exposure. The systems in general

use for ordinary photographic materials cannot be

applied to high-contrast or special materials since the

conditions of exposure and development are quite

different. For these applications speed points are

often measured at fixed densities greater than 0.1.

ISO speed ratings should not be confused with

manufacturers’ exposure indexes, which are suggested values for use with exposure meters calibrated

for ISO speeds and have been arrived at by manufacturers’ testing procedures for determining camera

settings. A speed rating preceded by the letters ‘ISO’

implies that this rating was obtained by exposing,

processing and evaluating exactly in accordance with

the ISO standard.



Figure 18.7 Principles of the method for determining speed

of colour negative films (BS 1380: Part 3: 1980, and used

currently in ISO 5800: 1987)



ISO speed ratings for colour

materials

Colour-negative film

The principles of speed determination for colournegative (print) films follow those previously described for monochrome materials. However, speed

determination is complicated by the fact that colour

materials contain layers sensitive to blue, green and

red light and the standards involve averaging the

speeds of the three sensitive layers. The currently

adopted ISO standard (ISO 5800: 1987) uses fixeddensity criteria for locating the speed points of the

three layers, exposed in a defined manner and

processed according to the manufacturer’s recommendations after storage at 23 ± 2 °C for between 5

and 10 days, for general purpose films. Blue, green

and red densities are measured according to defined

ISO measurement standards and the characteristic

curves shown in Figure 18.7 are plotted.

The points B, G and R are located at densities of

0.15 above the minimum densities for the blue, green

and red sensitive layers. The log-exposure values

corresponding to these points are log HB, log HG and

log HR respectively. The mean exposure, Hm, is then

calculated from two of these values (HG and the

slowest layer, usually the red-sensitive layer HR )

according to the following formula:

log10 Hm =



log10 HG + log10 HR



Or,

Hm = ⎯⎯⎯⎯⎯⎯ R

√ HG × H



2



Speed of materials, sensors and systems



307



or Hm = ⎯⎯⎯⎯⎯⎯ T

√ HS × H

from which the speeds may be calculated by analogous procedures to those used for the previously

described ISO standards:

Arithmetic speed, S =



10

Hm



and Logarithmic speed, SO = 1 + 10 log10



10

Hm



All exposure values are in lux seconds.



Speed of digital systems

Figure 18.8 Principles of the method for determining the

speed of colour-reversal (slide) films used in ISO standard

(ISO 2240: 1994(E))



The logarithmic speed is calculated as follows:

Logarithmic speed, SO = 1 + 10 log10



√

⎯2

Hm



where all exposure values are in lux seconds. The

speed values are then determined from a table of log10

Hm provided in the standard.

The arithmetic speed may be calculated from the

value of Hm by the formula:



√

⎯2



Arithmetic speed, S =



Hm



Colour-reversal film

Unlike the previously described standard for colournegative films, that for colour-reversal films involves

measurement of diffuse visual density and the

plotting of a single characteristic curve, shown in

Figure 18.8. Films are exposed in a defined manner

and processed according to the manufacturer’s recommendations. For the determination of speed two

points (T and S) are located on the characteristic

curve of Figure 18.8. Point T is 0.20 above the

minimum density. Point S is that point on the curve at

which the density is 2.0 above the minimum density.

The log exposure values (log HS and log HT )

corresponding to points S and T are read from the

curve and their mean value (Hm ) calculated:

log10 Hm =



log10 HS + log10 HT

2



The ISO standards now becoming adopted for digital

systems arise from the activities of a technical

committee PIMA/IT10. PIMA is the acronym for the

Photographic and Imaging Manufacturers Association which is accredited by the American Standards

Institute (ANSI) and sponsors PIMA/IT10. Members

of PIMA/IT10 are representatives of users which

include: other standards organizations, government

agencies, manufacturers of digital cameras, scanners,

printers, memory cards or components and imaging

software developers. The purpose of PIMA/IT10 is to

establish standards for electronic still-picture

imaging.

The specification of speed for an electronic stillpicture camera poses similar but different problems to

those described earlier for photographic media.

Digital cameras may involve substantial amounts of

on-board data processing, such as data compression,

analogue to digital conversion and the ability to alter

speed settings electronically by increasing the electrical gain. Also most cameras include an IRabsorbing or blocking filter. The standard (ISO

12232:1998) takes many of these factors into

account. Like the standards for photographic media,

those for digital systems specify conditions of

illumination, temperature, relative humidity and

exposure time. In addition there are those variables

specific to digital systems which require specification. These include:

᭹



᭹

᭹

᭹

᭹



,



Adjustment of white balance for the illumination

used, as equal RGB signals according to ISO

14524.

Exposure time (photosite integration time) not

longer than 1/30 s.

IR-absorbing (blocking) filter should be used if

required (ISO 14524).

If the device uses any form of lossy compression

this must be disabled.

If lossy compression systems cannot be disabled

then noise-based values for ISO speed determination should not be reported.



308 Speed of materials, sensors and systems



The standard (ISO 12232) provides two methods

for the determination of speed and has as one of its

aims a correlation between the ISO speed rating of an

electronic camera and that for conventional photographic media. This means that a setting of ISO 200

on an electronic camera requires the same exposure

time and aperture to that of a photographic film with

an ISO speed of 200.

The two methods for determination of ISO speed

are speed based on saturation and speed based on

noise, which is the preferred method.



Speed based on saturation (Ssat )

This is based on the user setting the camera exposure

such that the highlights of the image are just below

the maximum value (saturation) for the signal value,

i.e. before they become clipped.

For focal plane measurement of exposure this is

defined as:

Ssat =



78

Hsat



where Hsat is the minimum exposure level (lx s) that

produces the maximum output signal with no

clipping.

If exposure cannot be measured in the focal plane,

Ssat is calculated as follows:

Ssat =



120A 2

Lsat t



where A is the effective f-number of the lens, Lsat is

minimum luminance (cd m2 ) of a uniform reflecting

medium, and t is the exposure time (s).



Speed based on noise (Snoise )

This is based on the minimum exposure value before

the onset of noise becomes objectionable. This

depends on subjective assessments of judgements and

their correlations with noise to decide on the

acceptability of noise. Two noise-based speeds are

used (Snoise 10 and Snoise 40 ). Where 10 corresponds to

the ‘first acceptable’ image and 40 the ‘first excellent’ image, determined from psychophysical

experiments.

For focal plane measurement of exposure this is

defined as:

Snoisex =



10

HS/Nx



In the above equation HS/Nx, the exposure providing the camera signal-to-noise ratio which must

satisfy a number of criteria specified in the standard,

x has the value of 10 or 40. The signal-to-noise ratio

may in general terms be defined as:



S

N



=



exposure (or luminace) × gain

standard deviation



Exposure is the input photometric exposure (lx s),

luminance is the luminance (cd m2 ), gain is the

incremental signal gain (ratio of rate of output level to

that for the in input level of the monochrome or

luminance channels), standard deviation is that for

monochrome output level or weighted colour output

(for colour cameras) for a 64 × 64 pixel area.

The standard also provides for the situation in

which the focal plane exposure cannot be measured.

The ISO standard (12232) also includes weighting

factors for summing luminances in single exposure

colour cameras in which the weighting factors are not

known, from the individual channels according to

ITU-R BT.709:

Y =



2125

10 000



R+



7154

10 000



G+



721

10 000



B.



This standard provides the complete procedures to be

adopted, together with tables from which ISO speed

values can be read in relation to the determined

values of noise Snoise or Ssat .



Speed ratings in practice

The only useful speed number in practice is one that

adequately represents the speed of a material, or

system, under the conditions in which it is likely to be

used. Published speed values aim at providing

exposures that will secure a printable negative, or an

excellent result from a digital system under a wide

range of conditions. The published rating for a given

material or system cannot therefore be the best to use

under all conditions of use. For critical use it is

always best for the user to determine the appropriate

exposure index to suit the particular equipment and

conditions. Published exposure indexes provide a

valuable starting point, although if you have selected

an exposure index that suits your needs you should

remember that it is no more ‘correct’ than the

published figure, except for your own conditions of

working.



Bibliography

Allbright, G.S. (1991) Emulsion speed rating systems, Journal of Photographic Science, 39, 95–9.

Egglestone, J. (1984) Sensitometry for Photographers. Focal Press, London.

ISO 6: 1993 Photography – Black-and-White Pictorial Still Camera Negative Films/Process Systems–

Determination of ISO Speed.



Speed of materials, sensors and systems



ISO 2240: 1994 Photography – Colour Reversal

Camera Films – Determination of ISO Speed.

ISO 2721: 1982 Photography – Cameras–- Automatic Controls of Exposure.

ISO 5800: 1987 Photography – Colour Negative

Films for Still Photography – Determination of ISO

Speed.

ISO 6846: 1992 Photography – Black-and-White

Continuous Tone Papers – Determination of ISO

Speed and ISO Range for Printing.

ISO 7589: 1984 Photography – Illuminants for

Sensitometry – Specifications for Daylight and

Incandescent Tungsten.



309



ISO 12232: 1998 Photography – Electronic Stillpicture Cameras – Determination of ISO Speed.

ISO 14524: 1999 Photography – Electronic Stillpicture Cameras – Methods for Measuring Optoelectronic Conversion Functions (OECFs).

ITU-R BT.709: 1993 Basic Parameter Values for the

HDTV Standard for the Studio and for International Programme Exchange.

Kriss, M.A. (1998) A Model for Equivalency ISO

CCD Camera Speeds. Proceedings ICPS ‘98,

Antwerp, Belgium, Volume 2, pp. 341–7.

Todd, H.N. and Zakia, R.D. (1974) Photographic

Sensitometry. Morgan and Morgan, New York.



19



Camera exposure determination



Camera exposure

Other chapters have explained image formation by a

lens and the response to light of various photosensors

such as film. After due consideration of subject

composition, perspective and sharpness, the photographer judges the best moment to make the exposure.

An optical image of appropriate intensity is transmitted by the lens and reaches the photoplane for an

appropriate length of time. This camera exposure is

controlled by the combination of lens aperture (N)

and shutter speed (t). More generally, the photographic result of exposure (H) is the product of image

illuminance (E) and exposure duration (t), so that

H = Et



(1)



Determination of the optimum camera exposure is

important for any photographic situation in terms of

the resulting quality of the photographic image. Any

subject contains areas of different luminances giving

the overall subject luminance ratio (or range), so that

image capture in turn gives a range of exposures at

the photoplane, which on processing produces a range

of monochrome or colour densities or values that

form the image. With photographic materials, the

range of image densities produced is predicted by the



characteristic curve of the film, which depends on the

type of film and the development conditions used,

given the positioning of the range of log exposures on

the curve, which depends on camera exposure.

Apart from exposure duration, the components of

camera exposure are given by equation (13) in

Chapter 5, including subject luminance, lens aperture

and image magnification, with others such as lens

transmittance (including the necessary exposure factor for any filter used) and vignetting, both optical

and mechanical. Camera exposure may be determined

by practical trial, e.g. by an estimate based on

previous experience, followed by assessment of the

resultant image after processing, especially if using a

self-developing material. However, scene measurement using a lightmeter is preferable. Essentially this

may be done by measurement of the luminance of

selected areas (zones) of the subject, which is the

resultant of the illuminance on the subject and its

reflectance. Allowance is then made for other factors

affecting camera exposure, including the effective

film speed, which depends on film type and treatment,

the chosen lens aperture or shutter speed, and any

filter factor. These are all known values.

The four principal variables determining camera

exposure are subject luminance, film speed, lens



Figure 19.1 Exposure relationships. The diagram shows the relationships between film speed, exposure duration and aperture

setting to give equivalent exposures (shown here as the areas of rectangles). A long exposure duration implies a reduced aperture

and a slow film a larger rectangle, for a subject of given luminance



310



Camera exposure determination



aperture and shutter speed. As shown in Figure 19.1,

for any particular camera exposure there is a range of

combinations of shutter speed and lens aperture

usable with a given film speed. The selection of a

suitable combination is a primary creative control in

photography and will be dictated by the subject itself

or the treatment required. For example, a moving

subject may require a certain minimum shutter speed

to give a sharp image. Alternatively, the focal length

of lens used or possible camera shake may also

dictate shutter speed as the important factor. In such

cases the aperture must be chosen to suit this

exposure duration. But for subjects requiring specific

depth of field or the optimum performance from a

lens, the choice of aperture is important and the

shutter speed is secondary. The ability to change the

illumination on the subject allows more choice of

both of these variables. The use of tungsten illumination or electronic flash allows such variations in

practice.



311



Other quantitative criteria for optimum exposure of

negative materials relate to the systems used for

determining film speed. In the case of black-andwhite pictorial materials, the sensitivity may be

determined from the minimum exposure necessary to

give a density of 0.1 (above base + fog density),

which ‘pegs’ this density to the image detail of lowest

luminance, i.e. the deepest shadow for which detail is

to be recorded. In the case of colour-negative

material, this minimum density level may be 0.15 or

greater. With colour transparency materials, correct

exposure gives a result that retains details in the

highlights (low-density regions) so that there are no

totally blank (‘burnt-out’) areas. Note that even with

subjects having white highlight areas, the minimum

density above base + fog level, and the shape of the

characteristic curve, are such that a minimum density

value of some 0.2–0.3 is necessary to avoid the

highlight appearing as a ‘hole in the transparency’.

Correct exposure locates the important highlights at

this point on the characteristic curve (which is also

used for speed determination).



Optimum exposure criteria

Before considering methods of measuring subject

luminance or illumination to estimate camera exposure, it is useful to consider the criteria by which the

correctness of exposure is judged. For black-andwhite or colour negatives, judgement rests finally on

printing quality. So a correctly exposed negative is

defined as a negative that will give an excellent

print with least difficulty. Colour reversal material

will be considered later. The visual judgement of

black-and-white negative quality requires considerable experience due to related printing circumstances such as the print material used and the

illumination system of the enlarger, together with

personal preferences for the overall density level of

a negative. A long-standing empirical quality guide

is that a correctly exposed and processed negative

made on pictorial film will usually have some detail

in the shadows (low-density regions) and the highlights (high-density regions) will just permit text to

be read through them if the negative is laid on a

page in good light. For images on line film the

correct exposure is judged from the appearance of

the low-density fine line detail, which should be

completely transparent and sharp-edged, without any

filling-in, and the highlights should be totally

opaque.

Colour negatives are more difficult to judge

visually because of the presence of coloured dye

masks. Again, the presence of shadow detail is a

useful clue, but these regions are heavily masked. A

colour densitometer can provide a quantitative guide

to optimum exposure. For example, with a red filter,

readings of 0.7–0.9, 1.15–1.35 and 1.10–1.30 in

regions of mid grey tone, highlight and diffuse

highlight respectively can indicate correct exposure.



Exposure latitude

From the characteristic curve of a material, the

separation on the log exposure axis between the

maximum and minimum useful density points is

termed the useful log exposure range. As stated

above, the subject luminance ratio to be recorded

determines the log exposure range given to the

material. This may be less than, equal to or greater

than the useful log exposure range the material can

accommodate, related to its intrinsic contrast properties and processing treatment. If the log exposure

range is less than the useful log exposure range of the

material, as may often be the case, then the exposure

latitude of the material is the factor by which the

minimum camera exposure necessary to give adequate shadow detail in the case of negatives (or

highlight detail in the case of reversal materials) may

be multiplied, without corresponding loss of highlight

detail (or shadow detail) (see Figure 19.2). Detail

may be defined as discernible density differences in

adjacent tones corresponding to resolved regions of

the subject.

In the case of colour-negative materials, exposure

latitude also depends on use with an illuminant of

suitable colour temperature. For example, an excess

(or deficiency) of the blue component of the illuminant will give the blue-record layer more (or less)

exposure relative to the green- and red-record layers,

and place the blue exposure range on a different

region of the characteristic curve relative to the other

two. This reduces exposure latitude, as any exposure

outside the region that is linear for all three emulsions

simultaneously will result in coloured highlights or

shadows.



312 Camera exposure determination



–1 EV may be used. The primary use is for automatic

exposure metering; for example, if the camera is set

on aperture-priority mode, and the overall subject

luminance is so high that the aperture chosen requires

an exposure duration shorter than that available from

the shutter speed range, then if the resultant overexposure does not exceed the latitude of the material,

the shutter will be released; otherwise it could be set

to lock.

Most colour-negative materials have a useful

exposure latitude of up to +3 EV (factor ×8) to overexposure, and –1 EV (factor ×0.5) to under-exposure,

and give acceptable prints. Because of the fixed

development conditions of colour-negative material

and the non-scattering properties of the dye images

(so that print contrast is unaffected by the type of

illumination in the enlarger) most colour-print paper

materials are available in only one or two contrast

grades. Alteration of print contrast to suit particular

subjects or negatives is possible by the use of

contrast-altering silver masks. Black-and-white negative material has similar (or greater) exposure

latitude than colour-negative material, but with the

possibility of altering processing conditions to cope

with extremes of subject luminance ratio, plus a range

of paper grades for printing.



Figure 19.2 Latitude to over-exposure for negative

materials. The index, U, O, A and C markings of the Weston

Master exposure meter are shown relative to an average

subject luminance range. The exposure latitude K is a

multiple of the minimum useful exposure



The high contrast of colour reversal material,

typically equivalent to a gamma value of some 1.4,

causes a reduced exposure latitude, even with a

subject whose luminance range is 30:1 or less. The

lower inherent contrast of negative materials gives

more exposure latitude, and the ability to record

subjects of greater luminance range. Conventionally,

exposure latitude is expressed as tolerance in stops or

exposure values (EV) relative to the optimum exposure. Colour reversal materials usually have a latitude

of ± 0.5 EV or less, where ± 1 EV indicates a

permissible error of double or half the optimum

exposure. Note that natural vignetting across an

image field from a wide-angle lens may give an

illumination level that varies as much as this, so a

centre spot filter may be needed. Ideally, reversal

material should be exposed to within 0.3 EV of

optimum.

The DX film speed coding system used on 35 mm

film cassettes can input the metering system with film

exposure latitude value. Depending on film type, four

alternative ranges of ± 0.5, ±1, +2, –1 and +3, or



Subject luminance ratio

To assist exposure determination a subject can be

classified by its luminance ratio, or (incorrectly) the

‘subject luminance range’. This is numerically equal

to the product of the subject reflectance ratio and the

lighting ratio. For example, a subject whose lightest

and darkest zones have reflectances of 0.9 and 0.09

respectively, i.e. a reflectance ratio of 10:1, when

illuminated such that there is a lighting ratio of 5:1

between maximum and minimum illumination levels,

has a subject luminance range of (10 × 5):1, i.e. 50:1.

A high luminance ratio denotes a subject of high

contrast. Luminance ratios range from as low as 2:1



Table 19.1 Subject classification by luminance range or ratio

Subject luminance

ratio classification

Very high



Luminance

ratio



EV

range



Average luminance as

percentage of illumination



Weston Master series

exposure meter index



Approximate exposure

correction factor



2048:1 (211:1)



11



11



C



+1 EV



9



High



512:1 (2 :1)



9



14



Average



128:1 (27:1)



7



18



32:1 (2 1)



5



25



8:1 (2:1)



3



41



Low

Very low



5:



+0.5 EV

arrow



0

–0.5 EV



A



–1 EV



Camera exposure determination



to 1 000 000:1 (106:1). Integration of individual

luminance levels gives a value for the average

reflectance of a scene, which depends on the subject

luminance ratio. Based upon appropriate scene measurements, it is usually assumed that an average

outdoor subject has a luminance ratio of 128:1 or

27:1, corresponding to a seven-stop (or 7 EV) range.

As well as luminance ratio, the integrated luminance of all tones in a scene may be used for exposure

determination purposes. The value for an ‘average’

outdoor scene is taken to be some 12–18 per cent and

the range of colours also integrate to a mid-grey tone,

usually called ‘integration to grey’. Indoor studio

subjects may be closer to 18 per cent reflectance.

Neutral grey cards of 18 per cent reflectance are often

used as substitute ‘average’ subjects for exposure

determination using reflected light measurement. For

meter calibration purposes, a subject with a reflectance ratio of 2048:1 (211:1) has a mean reflectance of

some 11 per cent; with 8:1 (23:1) a reflectance of

some 40 per cent (see Table 19.1). If the exposure

meter is calibrated (as it usually is) for a mean

reflectance of 18 per cent, this could result in

exposure errors respectively of +80 per cent and –60

per cent. It is usually necessary with a subject matter

of excessively high or low contrast to apply a subject

correction factor.

If excessive, the subject luminance ratio may be

reduced by the use of additional illumination, such as

by fill-in flash or synchro-sunlight techniques. In

many cameras with automatic exposure metering, a

segmented photocell monitors the average luminance

and luminance ratio of zones in the scene, such as by

measuring the central area and the background

separately. Supplementary fill-in light from an integral flash unit (to reduce subject contrast) may be

provided automatically over a limited range of flashto-subject distances. The required amount of flash is

monitored by off-the-film measurement.



Development variations

For a given set of processing conditions, a variation in

development time can be used to control the contrast

of black-and-white negatives, as measured by the

negative density range corresponding to the log

exposure range. A change in development time alters

density range (contrast) of a negative, whereas a

change in camera exposure determines the position of

the density range on the characteristic curve. The

effects of exposure and development are summarized

by the graphs in Figure 19.3. By giving normal,

reduced or increased development and exposure in

various combinations, some nine varieties of negative

are possible. Not all of these have ideal printing

characteristics, of course: some cannot adequately be

matched to the available grades of printing paper, and

others do not have a suitable overall density level.



313



For example, an under-exposed and under-developed negative is of low average density and low

density range, and requires a hard grade of paper

(which has a low exposure latitude and emphasizes

graininess). Also, areas of low density show marks

and scuffs to a greater extent than areas of higher

density. On the other hand, an over-exposed, overdeveloped negative is grainy, and requires a long

printing exposure on a soft grade of paper, which

does not in general record shadows and highlights

well owing to the shape of its characteristic curve.

Variation in development time changes effective film

speed, too; in particular, prolonged development

results in a higher effective exposure index for the

material, producing what amounts to an over-exposed

result if the exposure was based on a normal speed

rating. Reduced development, however, can match

the useful log exposure range of a material to a high

subject log luminance range, to yield a negative of

density range printable on a normal grade of paper. A

side effect of under-development is a loss of effective

film speed, so the material must have its camera

exposure adjusted accordingly. This technique of

increased exposure (or down-rating of speed) and

reduced development to deal with high-contrast

scenes works best with materials of nominal speed

ISO 400.

The possibilities of individual development of

exposed material to suit scenes whose luminance

ratios have been measured using a spot metering

system, form the basis of the Zone System of exposure

determination.

Colour-negative material cannot be treated in this

way, but colour-reversal film may have its effective

exposure index varied, (usually increased) by alteration of first development time, contrast is also

affected, though the exact nature of the variation

depends on the material.



Exposure determination

Early experience of development by inspection led to

an empirical rule of exposure: ‘expose for the

shadows and develop for the highlights’. At a later

stage, development by time-and-temperature methods, and use of printing paper with a variety of

contrast grades, meant subjects could be exposed and

negatives processed to a similar contrast to give

acceptable prints. So the rule was modified to ‘expose

for the shadows and let the highlights take care of

themselves’, and this advice is still useful for blackand-white negatives.

Colour negatives can be exposed for shadow detail,

but the minimum exposure should locate shadows on

the linear portion of the characteristic curves and not

on the toe regions. This gives correct reproduction of

tone and colour. Provided the highlights are also on

the linear portion, tone and colour reproduction are