Ralph E Jacobson e« Sidney F Ray
`Geoffrey G Attridge » Norman RAxford
`
`THE MANUAL OF
`
`Photography’
`Photographic and Digital Imaging©
`
`TESLA,INC.
`
`-
`
`NINTH EDITION
`
`
`Ex.1040 / Page 1 of 27Ex.1040 / Page 1 of 27
`Ex.1040 / Page 1 of 27
`
`TESLA, INC.TESLA, INC.
`
`

`

`19 Camera exposure determination
`
`Camera exposure
`Other chapters have explained image formation by a
`lens andthe responseto light of various photosensors
`such as film. After due consideration of subject
`composition, perspective and sharpness, the photog-
`rapher judges the best moment to make the exposure.
`An optical
`image of appropriate intensity is trans-
`mitted by the lens and reaches the photoplane for an
`appropriate length of time. This camera exposure is
`controlled by the combination of lens aperture (NV)
`and shutter speed (rt). More generally,
`the photo-
`graphic result of exposure (H)is the productof image
`illuminance (£) and exposure duration (f), so that
`H = Et
`(1)
`
`Determination of the optimum camera exposure is
`important for any photographicsituation 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 producedis predicted by the
`
`characteristic curve of the film, which depends on the
`type of film and the developmentconditions useq
`given the positioning ofthe 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)
`jn
`Chapter5, including subject luminance,lensaperture
`and image magnification, with others such aslens
`transmittance (including the necessary exposurefac-
`tor for any filter used) and vignetting, both optical
`and mechanical. Camera exposure maybe determined
`by practical
`trial, e.g. by an estimate based on
`previous experience, followed by assessmentof the
`resultant image after processing, especially if using a
`self-developing material. However, scene measure-
`mentusing a lightmeteris preferable. Essentially this
`may be done by measurement of the Juminance of
`selected areas (zones) of the subject, which is the
`resultant of the illuminance on the subject andits
`reflectance. Allowanceis then made for other factors
`affecting camera exposure,
`including the effective
`film speed, which dependsonfilm 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
`
`Film speed
`
`400 ISO
`Areas = 1 unit
`
`4
`
`1.500
`
`;
`5.6 (__] a
`
`61
`
`25
`
`1[|e
`200 ISO
`at
`5.
`
`6 (mers|125 Areas = 2 units
`Aperture
`
`(f-number)as125
`56 ere
`8
`60
`————————— x30
`Time ——
`s
`~ Shutter speed (seconds)
`tra
`es
`d ape"
`:
`Figure
`ionshi
`19.
`xposure relationships. The diagram shows
`apertu
`saztite (ge equivalent expomures (shown kesh: eee the relationships between film speed, exposure duration an
`i
`ion
`ecli
`lies a reduce
`of rectangles). A long exposure duration imp
`oe
`and a slowfilm a larger rectangle, for a subject of g
`tven luminance
`
`Light
`
`ee
`Areas = 4 units
`
`eas
`
`310
`
`
`Ex.1040 / Page 2 of 27Ex.1040 / Page 2 of 27
`Ex.1040 / Page 2 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA,INC.
`
`

`

`aperture and shutter speed. As shownin 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 subjectitself
`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 tungstenillumina-
`tion or electronic flash allows such variations in
`practice.
`
`Camera exposure determination
`
`311
`
`Otherquantitative criteria for optimum exposure of
`negative materials relate to the systems used for
`determining film speed.
`In the case of black-and-
`white 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).
`
`Exposurelatitude
`
`Optimum exposurecriteria
`Before considering methods of measuring subject
`luminance or illumination to estimate camera expo-
`the
`From the characteristic curve of a material,
`sure, it is useful to consider the criteria by which the
`separation on the log exposure axis between the
`correctness of exposure is judged. For black-and-
`maximum and minimum useful density points is
`white or colour negatives, judgementrests finally on
`termed the useful
`log exposure range. As stated
`printing quality. So a correctly exposed negative is
`above,
`the subject
`luminance ratio to be recorded
`defined as a negative that will give an excellent
`determines the log exposure range given to the
`print with least difficulty. Colour reversal material
`material. This may be less than, equal to or greater
`will be considered later. The visual judgement of
`than the useful log exposure range the material can
`black-and-white negative quality requires consider-
`accommodate,related to its intrinsic contrast proper-
`able experience due to related printing circum-
`ties and processing treatment. If the log exposure
`stances such as
`the print material used and the
`range is less than the useful log exposure range of the
`illumination system of the enlarger,
`together with
`material, as may often be the case, then the exposure
`personal preferences for the overall density level of
`latitude of the material
`is the factor by which the
`a negative. A long-standing empirical quality guide
`minimum camera exposure necessary to give ade-
`is that a correctly exposed and processed negative
`quate shadow detail
`in the case of negatives (or
`madeonpictorial film will usually have some detail
`highlight detail in the case of reversal materials) may
`in the shadows (low-density regions) and the high-
`be multiplied, without correspondingloss of highlight
`lights (high-density regions) will just permit text to
`detail (or shadow detail) (see Figure 19.2), Detail
`be read through them if the negative is laid on a
`may be defined as discernible density differences in
`Page in good light. For images on line film the
`adjacent tones corresponding to resolved regions of
`Correct exposure is judged from the appearance of
`the subject.
`the low-density fine line detail, which should be
`In the case of colour-negative materials, exposure
`completely transparent and sharp-edged, without any
`latitude also depends on use with an illuminant of
`filling-in,
`and the highlights
`should be
`totally
`suitable colour temperature. For example, an excess
`Opaque.
`(or deficiency) of the blue componentofthe illumi-
`to judge
`are more difficult
`Colour negatives
`nant will give the blue-record layer more (orless)
`Visually because of the presence of coloured dye
`exposurerelative to the green- and red-record layers,
`masks. Again,
`the presence of shadow detail
`is a
`and place the blue exposure range on a different
`useful clue, but these regions are heavily masked. A
`region ofthe characteristic curve relative to the other
`colour densitometer can provide a quantitative guide
`two. This reduces exposurelatitude, as any exposure
`'0 optimum exposure. For example, witharedfilter,
`outside the region thatis linear for all three emulsions
`readings of 0.7-0.9, 1.15-1.35 and 1.10-1.30 in
`simultaneously will result in coloured highlights or
`a of mid grey tone, highlight and diffuse
`shadows.
`ighlight respectively can indicate correct exposure.
`
`
`Ex.1040 / Page 3 of 27Ex.1040 / Page 3 of 27
`Ex.1040 / Page 3 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA, INC.
`
`

`

`312 Camera exposure determination
`
`Density——
`
`ee
`
`Log exposure —>
`
`128 :1
`
`-1 EV may beused. The primary useis fo, aut
`exposure metering; for example, if the cameravate
`on aperture-priority mode, and the Overall xae
`luminanceis so high that the aperture chosen reno
`an exposure duration shorterthan that available
`the shutter speed range,
`then if the resultant 9 .
`exposure does not exceedthelatitude ofthe Didier
`the shutter will be released; otherwise it Could be x
`to lock.
`"
`Most colour-negative materials have @ Usefy|
`exposurelatitude of up to +3 EV (factor X8) to Over.
`exposure, and —1 EV (factor x0.5) 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 neg-
`ative 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 luminanceratio, plus a range
`of paper grades for printing.
`
`Subject luminanceratio
`
`To assist exposure determination a subject can be
`classified by its luminanceratio, 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 whoselightest
`and darkest zones havereflectances 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 minimumillumination levels,
`has a subject luminance range of (10 x 5):1, i.e. 50:1.
`A high luminance ratio denotes a subject of high
`contrast. Luminanceratios range from as low as 2:
`
`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 shownrelative to an average
`subject luminance range. The exposurelatitude K is a
`multiple of the minimumuseful exposure
`
`The high contrast of colour reversal material,
`typically equivalent to a gammavalue 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,
`exposurelatitude is expressed as tolerance in stops or
`exposure values (EV)relative to the optimum expo-
`sure. Colourreversal materials usually havea latitude
`of + O.5EV or
`less, where + 1EV 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 DXfilm speed coding system used on 35mm
`film cassettes can input the metering system with film
`exposurelatitude value. Depending onfilm type, four
`alternative ranges of + 0.5, +1, +2, -1 and +3, or
`
`Table 19.1 Subject classification by luminance range or ratio
`
`Subject luminance
`ratioclassification
`
`Luminance
`ratio
`
`EV
`range
`
`Average luminance as
`percentage of illumination
`
`.
`Weston Master series
`exposure meter index
`
`e
`;
`:
`Approximate exposul'
`correctionfactor
`
`Very high
`
`2048:1 (2!!:1)
`
`High
`
`Average
`
`Low
`
`Very low
`
`512:1 (29:1)
`
`128:1 (27:1)
`
`32:1 (251)
`
`8:1 (2:1)
`
`I]
`
`9
`
`T
`
`3
`
`3
`
`4]
`
`Cc
`
`arrow
`
`A
`
`+1 EV
`
`+0.5 EV
`
`0
`
`-0.5EV
`
`-1 EV
`
`
`Ex.1040 / Page 4 of 27Ex.1040 / Page 4 of 27
`Ex.1040 / Page 4 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA,INC.
`
`

`

`individual
`Integration of
`to 1000000:1 (10®:1).
`juminance levels gives a value for
`the average
`reflectance ofa scene, which depends on the subject
`juminanceratio. Based upon appropriate scene meas-
`urements,
`it
`is usually assumed that an average
`outdoor subject has a luminance ratio of 128:1 or
`77:1, corresponding to a seven-stop (or 7 EV)range.
`As well as luminanceratio, the integrated lumi-
`nanceofall tones in a scene maybe 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 coloursalso integrate to a mid-grey tone,
`usually called ‘integration to grey’. Indoor studio
`subjects may be closer to 18 per centreflectance.
`Neutral grey cards of 18 per centreflectance are often
`used as substitute ‘average’ subjects for exposure
`determination using reflected light measurement. For
`meter calibration purposes, a subject with a reflec-
`tance ratio of 2048:1 (2'':1) has a mean reflectance of
`some 11 per cent; with 8:1 (27:1) 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 ofadditionalillumination, 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 luminanceratio of zones in the scene, such as by
`measuring the central area and the background
`separately. Supplementary fill-in light from an inte-
`gral flash unit (to reduce subject contrast) may be
`provided automatically over a limited range of flash-
`to-subject distances. The required amountof flash is
`monitored by off-the-film measurement.
`
`Developmentvariations
`For a givensetof 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 determinesthe 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 prinung
`characteristics, of course: some cannot adequately be
`matchedto the available grades of printing pape!» and
`others do not have a suitable overall density level.
`
`Camera exposure determination
`
`313
`
`For example, an under-exposed and under-devel-
`oped 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, over-
`developed 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 developmenttime changeseffective film
`speed,
`too;
`in particular, prolonged development
`results in a higher effective exposure index for the
`material, producing what amountsto 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-developmentis a loss ofeffective
`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 altera-
`tion of
`first development
`time, contrast
`is also
`affected,
`though the exact nature of the variation
`depends on the material.
`
`Exposure determination
`
`Early experience of developmentby inspection led to
`an empirical
`rule of exposure:
`‘expose for
`the
`shadowsand develop for the highlights’. At a later
`stage, development by time-and-temperature meth-
`ods, 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 adviceis still useful for black-
`and-white negatives.
`Colour negatives can be exposedfor shadowdetail,
`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
`
`
`Ex.1040 / Page 5 of 27Ex.1040 / Page 5 of 27
`Ex.1040 / Page 5 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA, INC.
`
`

`

`314 Cameraexposure determination
`
`(d)
`
`
`
`
` log H
`
`K-A—+
`
`slog
`
`(c)
`
`
`A—“1—logH kA
`
`(9)
`(h)
`
`
`
`
`at
`
`log H
`
`(i)
`Figure 19.3 Summary ofthe effects of exposure and developmentvariations on negative density range and density values.
`(a) Under-exposure and prolonged development. (b) Correct exposure and prolonged development. (c) Over-exposure and
`;
`prolonged development. (d) Under-exposure and normal development. (e) Correct exposure and normal development.
`(f) Over-exposure and normal development. (g) Under-exposure and curtailed development. (h) Correct exposure and curtailed
`development. (i) Over-exposure and curtailed development. Key: A, normal subject log luminance range; d, negative density
`range, to print on normalgrade paper; B, low subject log luminance range; C, high subject log luminance range
`
`acceptable throughoutthe range of densities. A colour
`print cannotreproducethe large tone range shownin
`a black-and-white print, as tonal distortions accept-
`able in the latter are unacceptablein colourprints due
`to the colour-balanceerrors in the deepest shadows,
`and the low tolerance of most viewers to such
`‘skewed’ colour balance. Such local colour imbal-
`ances are not correctable at
`the printing stage, as
`corrective filtration is only effective for overall
`colourcasts.
`the method of camera
`With reversal materials,
`exposure determination by measurement of both
`shadow and highlight luminancesis equally valid, the
`shadowsandhighlights being suitably located on the
`characteristic curve. This time the highlights are
`recorded as lowdensities, as described above.Soitis
`best
`to base exposure on subject highlights, and let
`the shadows‘take care of themselves’. This arrange-
`
`for high-key
`ment works for most subjects, but
`subjects (i.e. with no important dark areas), exposure
`is best increased by +4 to +1 EV to avoid a ‘washed-
`out’ appearance. Likewise, a low-key subject
`(1.¢.
`with minimal highlight detail) needs a reduction in
`exposure of —% to -1 EV.
`It may not be convenient or easy to measure
`directly the highlight luminance of a subject, but an
`incident-light measurement gives a good approxima-
`tion,since the highlights of most subjects are white or
`near-white, with a high reflectance, so an illumination
`measurement
`is similar to a highlight
`luminance
`measurement.
`Anartificial highlight may be provided in a scene
`for easy measurementofreflectance by making the
`exposure measurement on a matt white card. If the
`assumedreflectance ofthis is 90 per cent,i.e. 5 times
`more than an assumed average scene reflectance of
`
`
`Ex.1040 / Page 6 of 27Ex.1040 / Page 6 of 27
`Ex.1040 / Page 6 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA,INC.
`
`

`

`18 per cent, and the meter usedis calibrated in the
`normal way, for 18 per centreflectance, the indicated
`exposure should be multiplied by 5 to providecorrect
`exposure. Theartificial highlight method is useful in
`copying work, and is particularly worthwhile when
`colour slides are being exposed of subjects with
`unusual
`reflectance properties (snow scene, dark
`forest glades).
`
`Practical exposure tests
`
`If the material has no film speed rating, optimum
`exposure can be determined bypracticaltests, as for
`materials intended for copying, photolithography and
`similar tasks. The instruction sheet supplied with the
`material may suggest starting points for exposure
`level with particular illuminants,filters, development
`and subjects. Such techniquesare suited to the studio
`and photographic laboratory, as processing and evalu-
`ation are carried out
`immediately after exposure.
`Whenusing a large-format studio camera, for exam-
`ple,
`the dark-slide of the filmholder may be pro-
`gressively inserted to allow a doubling sequence of
`timed exposures to be given. Thus a sequenceof1, 2,
`4, 8 and 16 seconds exposure is given by consecutive
`additive exposures of 1,
`1, 2, 4 and 8 seconds
`respectively. The camera must
`remain stationary
`during the tests, of course. For small- and medium-
`format cameras a similar test series is given by
`exposing a sequenceofindividual frames. Either the
`shutter speed can be varied while the aperture is held
`constant or vice versa. Of these choices, the shutter-
`speed variation gives a constant image luminancebut
`may causereciprocity-failure effects, and the depth of
`field is constant. If aperture variation is used, two
`series of exposures
`should be made, one with
`increasing and one with decreasing aperture size. This
`is because many iris diaphragm systems have upto a
`quarter of a stop backlash. Aperture variation 1s
`mandatoryif flash illumination is being used.
`Many cameras with integral power-wind offer an
`‘automatic bracketing’
`feature in some automatic
`exposure modes, so that on setting a choice of up to
`+3EV in YEV increments, a range of exposures 1s
`given with exposure variations to bracket the expo-
`sure in cases of doubt or where the necessity for a
`good result overrides the cost of the film. Either
`shutter speed or aperture may be varied in such a
`series,
`The use of self-developing materials, usually
`knownas a ‘Polaroid test’, with material of a known
`€xposure
`index, gives a means of determining oF
`verifying exposure, and may be used on location.
`Suitable back is used on the camera, and the results
`examined after exposure. Differences In exposure
`indexes between the self-developing film and the
`(colour) material can be allowed for by simple
`calculation or the use of a neutral density filter.
`
`Camera exposure determination
`
`315
`
`Composition, positioning of subject and props,light-
`ing balance and the presence of any undesirable
`reflections mayalso be checked.
`
`Light measurement
`
`Optimum exposure may be determined by measure-
`ment ofthe luminanceof the subject (or zonesofit),
`or, alternatively, by measuring the illumination on the
`subject
`(incident
`light),
`followed by location of
`the log luminanceratio in a suitable position on the
`characteristic curve of the material.
`Ideally,
`the
`luminanceratio itself should be determined; but this
`requires experience in locating suitable zones,and is
`by no meanseasy to measure accurately. This method
`is usually too complicated for everyday use. Simpler
`methods are preferred in practice, even though they
`may, in theory, be less accurate.
`
`Measurement of luminance of darkest
`shadow
`
`This method locates this tone at a fixed point on the
`toe of the characteristic curve, giving the correct
`density to the deepest shadow that is to be recorded.
`Practical difficulties include the necessary metering
`sensitivity, the effects of flare light on measurements
`and the effect of highlights in the field of view of the
`meter. Some meters have an indicator
`for
`this
`measurement; for example, Weston exposure meters
`had a ‘U’ mark which wasset to the darkest-shadow
`reading.
`
`Measurementof luminanceof lightest
`highlight
`
`This locates the highlight at a fixed point with the
`highlight detail and othertonesas differing densities.
`Shadow detail is usually adequately recorded, but a
`subject of average contrast may receive more expo-
`sure than necessary. The original Weston exposure
`meter had an ‘O” setting for this method; otherwise
`with other meters the indicated exposure should be
`divided by 5. This method is similar to the artificial
`highlight method, which works well for colour-slide
`material. Note that whenusingthis method, measure-
`ments must not be made on specular highlights.
`
`Measurementof a key tone
`A subject zone of intermediate luminance,i.e. a mid-
`tone, is selected and located at a fixed point on the
`characteristic curve. Typically an exposure 10 to 16
`times morethan that for the shadow orspeed pointis
`given. This key tone may be a Caucasian flesh tone
`
`
`Ex.1040 / Page 7 of 27Ex.1040 / Page 7 of 27
`Ex.1040 / Page 7 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA, INC.
`
`

`

`316 Camera exposure determination
`
`(about 25 per cent reflectance). Control of scene
`illumination allows the shadows andhighlights to be
`suitably located too. A substitution method may also
`be used: a medium grey card (18 per centreflectance)
`is held so as to receive the sameillumination as the
`subjectit is replacing. The light meter is used close up
`so that
`the card fills the acceptance angle of the
`photocell. The ‘arrow’ setting of the original Weston
`meter wascalibrated for such a mid-tone, as are most
`metering systems.
`Cameras with automatic exposure control may
`offer an ‘exposure memory lock’ feature to allow the
`metering system to measure andset an exposure fora
`key or substitute tone, after which the picture is
`recomposed and the stored exposure given. This
`memory may be self-cancelling after exposure, or
`may require manual cancellation.
`
`than the mid-tones) on the characteris;
`(rather
`curve, and is thusparticularly suitable for use with
`colour-reversal materials. No information jg give
`with regard to the shadows, and exposure Citi,
`tions may have to be made if the Subject
`js Very
`dark or back-lit.
`
`Exposure metercalibration
`
`LxaT cos* 0
`Ey =———__—
`4N?
`
`Equation (2) may be rewritten as
`
`(2)
`
`qL
`Er = N2
`
`where q is a constant derived from known (or
`assumed) values of the quantities above.
`So from equation (1), the exposure H, at the focal
`plane is given by
`Ltam
`
`(4)
`
`A hand-held light meter, commonly called an expo-
`sure meter, may be used to measure either Subject
`luminance or subject
`illumination.
`It
`is a form of
`photometer not dissimilar to those used by lighting
`engineers, with its light value readings convertible
`into camera exposure settings. The usual
`form of
`meter calibration and its
`relationship to subject,
`camera and film speed parameters are shown in
`Figure 19.4, whereareflected light readingis being
`Measurementof total scene luminance
`taken.
`Consider a small off-axis area of the subject of
`This ‘integration’ method accepts all the light flux
`luminanceLthat is imaged in the focal planeofthe
`from the chosen area of the subject
`to be photo-
`camera with image illuminance E;. The values of L
`graphed. Such a reflected light measurementis widely
`and Ey are related by equation (16) in Chapter5, so
`used, as it
`is simple to carry out and requireslittle
`thatif the lens has a transmittance 7, and f-numberN,
`selective judgement.It is used in hand-held meters,in
`and the region is at an angle @ from the optical
`flash meters and for in-camera metering systems for
`axis,
`both ambientandflash illumination.Its high success
`rate is largely becausethe useful log exposure range
`of negative materials is greater than the log lumi-
`nance range of the average subject.
`A correction is usually necessary to the indicated
`values of exposure settings for subjects of very high
`or very low contrast, or for a subject of non-typical
`tone distribution, such as a high-key or low-key
`subject. Typical corrections necessary are a doubling
`of indicated exposure for high-contrast or high-key
`subject matter, and a halving of indicated exposure
`for
`low-contrast or
`low-key subject matter. The
`original Weston exposure meter had two exposure
`index marks denoted ‘C’ and ‘A’ to cope with high-
`and low-contrast (or high- and low-key) subjects.
`These settings give corrective alterations of +1 EV
`and —1 EV respectively.
`Most cameras with an automatic-exposure mode
`have an associated exposure compensation control
`whichoffers manually-set increases or decreases of
`up to +3 EV or morein increments of % or %EV to
`cope
`with
`atypical
`subjects
`or
`luminance
`distributions.
`
`
`
`N-
`
`Hy can be replaced bya sensitometrically determined
`speed numberS for the film used where
`
`H
`pe
`H,m
`
`(5)
`
`Measurementof incident light
`inci-
`The incident
`light method measures the light
`dent on the subject rather than the light reflected
`fromit. It
`is an easy measurement to make with a
`hand-held light meter, but
`is not suitable for an in-
`camera metering system.
`It positions the highlights
`
`where Hois a constant and H,,, is the speed point or
`minimum useful exposure. Now,
`(6)
`Hr = kD
`Where k is constant and a multiple of H,, therefore.
`ee
`)
`
`S
`
`
`Ex.1040 / Page 8 of 27Ex.1040 / Page 8 of 27
`Ex.1040 / Page 8 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA,INC.
`
`

`

`
`
`E-= LnT cos‘ 0
`4n2
`
`log H
`(H = Et)
`
`Camera exposure determination
`
`317
`
`H
`
`He
`
`minated by light (2) and imagedbythe lens (3) onto the
`Thecalibration of a light meter. The subject (1) is illu
`Figure 19.4
`Im speed setting control (6). The luminance ofthe scene is
`film plane (4). The camera has a shutter speed setting dial (5) and fi
`ed by the characteristic curve (8) of the film. (The other
`measured by a li
`speed and exposure are relat
`f
`th
`shepeice
`c
`d from the value H,, and the exposure meter indicated an exposure
`values are referred to in the text.) Film speed is determine
`Corresponding to He
`
`So, by substituting for Hp in equation (4) and
`rearranging, we have
`56 kKHoN?
`
`qLs
`Choosing a new constant K, usually called the
`Calibration constant for luminance, gives
`Lts
`ae
`N2
`
`(9)
`
`(8)
`
`light reading when the
`for an incident
`Similarly,
`illuminance Eg of the subject is measured,
`
`=
`
`EtS
`
`N2
`
`(10)
`
`where C is the calibration constant for illuminance.
`The calibration procedures are detailed in a number
`of national and international standards such as those
`formerly published by ANSI, BS, DIN andcurrently
`by ISO.
`
`
`Ex.1040 / Page 9 of 27Ex.1040 / Page 9 of 27
`Ex.1040 / Page 9 of 27
`
`TESLA, INC.TESLA, INC.
`TESLA, INC.
`
`

`

`or
`
`N2
`EV = 3.32 log ral
`
`(14)
`
`318 Camera exposure determination
`
`For black-and-white materials, the exposure for a
`mid-tone of some 18 percentreflectance is located at
`approximately 16 times that required for the speed
`point,
`to distribute the subject
`tones along the
`characteristic curve. The value of k in equation (6)
`would be 16 in this case. Typical values of the
`constants K and C for a mid-grey (18 per cent) tone
`are 3.33 and 20.83 respectively; more precise values
`may be given in the instruction manual
`for a
`particular exposure meter. For a subject of 18 per cent
`reflectance L = 0.18E;. From equations (9) and (10),
`t is proportional to N?, so that
`
`Given an appropriate film speed and scene lumi-
`nanceorillumination value, the calculator dials of an
`exposure meter will solve equation (11) for a wide
`range of equivalent pairs of shutter speed (t) and len