Exposure Metering Relating Subject Lighting to Film Exposure

Transcription

1 Expoure Metering Relating Subject Lighting to Film Expoure By Jeff Conrad A photographic expoure meter meaure ubject lighting and indicate camera etting that nominally reult in the bet expoure of the film. The meter calibration etablihe the relationhip between ubject lighting and thoe camera etting; the photographer kill and metering technique determine whether the camera etting ultimately produce a atifactory image. Hitorically, the bet expoure wa determined ubjectively by examining many photograph of different type of cene with different lighting level. Common practice wa to ue wide-angle averaging reflected-light meter, and it wa found that etting the calibration to render the average of cene luminance a a medium tone reulted in the bet expoure for many ituation. Current calibration tandard continue that practice, although wide-angle average metering largely ha given way to other metering technique. In mot cae, an incident-light meter will caue a medium tone to be rendered a a medium tone, and a reflected-light meter will caue whatever i metered to be rendered a a medium tone. What contitute a medium tone depend on many factor, including film proceing, image potproceing, and, when appropriate, the printing proce. More often than not, a medium tone will not exactly match the original medium tone in the ubject. In many cae, an exact match in t neceary unle the original ubject i available for direct comparion, the viewer of the image will be none the wier. It often tated that meter are calibrated to an 18% reflectance, uually without much thought given to what the tatement mean. Will metering an 18% reflectance enure a correct expoure? I an 18% reflectance rendered a an 18% reflectance in a print? How would thi apply to an image that i projected or viewed on a computer monitor rather than printed? In any event, meter calibration ha nothing to do with reflectance. A reflected-light meter i aimed at a target of known luminance and adjuted to give an appropriate reading; imilarly, an incident-light meter i expoed to a point ource of known illuminance and adjuted. 1 Subject Luminance Range Almot any ubject of photographic interet contain element of different luminance; conequently, the expoure actually i many different expoure. The hutter time i the ame for all element, but the image illuminance varie with the luminance of each ubject element. The ubject luminance range (SBR) i the difference, in expoure tep, between the brightet and darket part of the ubject. An SBR of 7 expoure tep uually 3 i conidered normal for a unlit outdoor cene; the SBR depend on two factor: The reflectance of the ubject element. The reflectance of natural object varie from approximately 4% to 90%, a 4.5-tep range. It unuual for a ubject to encompa the entire range, epecially the lower end, o a range of 4 tep i more typical.

2 Expoure Metering Page The ubject illuminance range, typically 4 tep. Illuminance in area of open hade typically i about 3 tep le than in unlight; area in deep hade can be coniderably darker. An illuminance range of 3 tep i conidered normal 4 for a unlit outdoor cene. Becaue different part of a typical ubject receive different illumination, the concept of average ubject reflectance in t alway meaningful. Etimate of the effective average reflectance of a typical unlit outdoor cene (taking varying illumination into account) range from about 1% to 0%, but thi doen t directly relate to the reflectance of any element in the cene. It probably i more intructive to examine the effect of expoure on individual ubject element. If a ubject with a 4.5-tep reflectance range were uniformly illuminated, an element with 90% reflectance would be given.3 tep greater expoure than an element with 18% reflectance. Similarly, an element with 4% reflectance would be given.17 tep le expoure than the 18% reflectance. If a meter reading were made of the 18% reflectance, the overall expoure on mot film probably would be acceptable, although there might be a light lo of highlight detail with color reveral film. If the ame ubject contained hadow area that received.5 tep le illuminance than the unlit area, an element with 9% reflectance in a hadow area would get 3.5 tep le expoure than a unlit element with 18% reflectance, and would record a featurele black. If the expoure were increaed by 1 tep, ome detail might be retained in the 9% hadowed area, but the expoure of the unlit area alo would increae. With negative film, thi expoure probably would retain adequate highlight detail; with blackand-white heet film, the development could be reduced to expand the uable expoure range and perhap tolerate even additional expoure. With color reveral film, however, the expoure could not be increaed without lo of highlight detail. Becaue wahed-out highlight uually are more objectionable than lo of hadow detail, the bet expoure for color reveral film probably would be baed on the unlit 18% reflectance. Subject Luminance Ditribution An averaging reflected-light meter repond only to overall average luminance, and cannot ditinguih between a ubject of uniform luminance and one that conit of light and element. If the part of a cene that i metered include large area of unuually high or low reflectance, or unuually large area of highlight or hadow, the effective average reflectance may differ ubtantially from normal, and the rendering may not be what i deired. In uch a ituation, ome expoure adjutment or alternative metering technique may be required. It not alway obviou what contitute a normal luminance ditribution, even in a imple cae for which the ubject illuminance i uniform. Conider a cheboard for which the white quare have a reflectance of 90% and the black quare a reflectance of 4%; if the illuminance i uniform, the luminance range depend only on reflectance. The midpoint of the 4.5-tep reflectance range would correpond to approximately 19% reflectance: 0.90 Luminance range log log

3 Expoure Metering Page Midpoint reflectance If neceary, the bae- logarithm can be computed from log log a ln a a log ln The reflectance of the white and black quare would be.5 expoure tep above and below the midpoint. A cheboard with a ingle white quare, one with a ingle black quare, and a normal cheboard with equal number of black and white quare all would have the ame luminance range, and would require the ame expoure for proper tonal rendering. An allwhite cheboard would have only a ingle luminance, but till would require the ame expoure a the other board for proper rendering of the white. Thi requirement become epecially obviou if all cheboard appear in the ame photograph. A reflected-light reading of an 18% reflectance in the ame poition a any of the cheboard probably would indicate a reaonable expoure, a would an incident-light reading. However, wide-angle average reflected-light reading of each of the cheboard would indicate different expoure, and none of the indicated expoure likely would be correct. For the all-white board, a reflected-light reading would indicate an expoure that would render the board a middle gray, the ame a would happen with a board of midpoint reflectance, depite the.5-tep difference in reflectance. It often thought that a normal cene contain an equal number of light and dark area. Thi imply in t true if the light and dark area contitute a ignificant fraction of the ubject: a 50% reduction in luminance i one expoure tep. The average reflectance for the normal cheboard would be The difference in reflectance, in expoure tep, from that of the white quare would be 0.47 log log The indicated expoure would render the average reflectance of 47% a a medium gray, even though no element in the image actually would be medium gray. That expoure would be approximately one tep greater than that for the all-white board, and would render the white quare a light gray and the black quare a nearly black. The fraction w of white quare that would give an average reflectance equivalent to the 19% midpoint reflectance can be found from 0.90w0.04(1 w) w w or white quare. It hould be obviou that light area dominate wideangle reflected light meaurement.

4 Expoure Metering Page 4 Dependence on normal luminance ditribution can be avoided by making reflectedlight meaurement of the individual white and black quare. Thi tak i eaier when uing a narrow-angle reflected-light meter, commonly known a a potmeter. Averaging individual reflected-light meaurement work well when the area meaured are approximately equally brighter and darker than a medium tone, a would be the cae with a cheboard, but le well otherwie. Moreover, rendering the average luminance a a medium tone doe not enure that the lightet and darket element of the ubject will be within the film expoure range. Many camera incorporate multi-egmented metering that make eparate meaurement of different part of the ubject, and et expoure baed on comparion of luminance ditribution with tatitical data compiled from many type of image. For ubject with unuual luminance ditribution, multi-egmented metering often give better reult than wide-angle averaged metering, but reult depend on how the different metering egment align with the different ubject element. The Two-Minute Zone Sytem In the early 1940, Anel Adam and Fred Archer devied the Zone Sytem, in which meaurement are made of individual ubject element, and expoure i adjuted baed on the photographer knowledge of what i being metered: a photographer know the difference between frehly fallen now and a black hore, while a meter doe not. Volume have been written on the Zone Sytem, but in eence, the concept i very imple render light ubject a light, and dark ubject a dark. The Zone Sytem aign number from 0 through 9 to different brightne value, with 0 repreenting black, 5 middle gray, and 9 white. To make zone eaily ditinguihable from other quantitie, Adam and Archer ued Roman rather than Arabic numeral. Strictly peaking, zone refer to expoure, with a Zone V expoure (the meter indication) reulting in a mid-tone rendering in the final image. Each zone differ from the preceding zone by a factor of two, o that a Zone I expoure i twice that of Zone 0, and o forth. A one-zone change i equal to one expoure tep, correponding to tandard aperture and hutter control on a camera. Many camera incorporate proviion for expoure compenation; the relationhip between expoure zone and expoure compenation i hown in Table 1. Table 1. Expoure Zone and Expoure Compenation Zone Exp. Comp. Characteritic 0 5 Pure black I 4 II 3 III Darket tone with detail IV 1 V +0 Middle gray (meter indication) VI +1 VII + Lightet tone with detail VIII +3 IX +4 Pure white

5 Expoure Metering Page 5 For example, a ubject lightly lighter than normal might be given a Zone VI expoure ( placed on Zone VI), and would be rendered lightly lighter than a middle tone with normal printing. Such an expoure would be equivalent to +1 expoure compenation. Black-and-white film typically maintain detail for hadow placed on Zone III or higher, and for highlight placed on Zone VII or lower. For color reveral film, detail i uually maintained between Zone IV and VII, with the detail becoming faint in Zone VII. With black-and-white heet film, for which the Zone Sytem originally wa devied, each negative can be proceed individually, and development can be adjuted to control the contrat. With roll film, epecially color, contrat control uually in t poible. Accordingly, expoure mut be choen for one ubject element, and the other element then fall where they will. For example, if a riing moon were photographed on color reveral film, and the contrat between the foreground and the moon wa four tep, the expoure would need to favor either the moon or the foreground. If the foreground were placed on Zone IV, the moon would fall on Zone VIII, and would looe nearly all detail. If the moon were placed on Zone VII to retain a hint of detail, the foreground would fall on Zone III, and mot hadow detail would be lot. Adam decribed the Zone Sytem in numerou book, mot recently in The Negative (1981) 5. Some photographer were intimidated by the purported complexity of the Zone Sytem; Fred Picker (1974) 6 decribed very imple method for determining effective film peed and development time. In addition, Picker determined film development time baed on a print tone lightly lighter than pure white (the original Zone Sytem determined development baed on a medium tone), enuring that the brightet and darket element of the ubject would be within the expoure cale of the paper. Phil Davi (1981) 7 decribed ytematic, rigorou teting of paper and film to match film expoure and development to the paper expoure cale. Additionally, Davi included a method to achieve eentially the ame reult with incident-light meaurement. Relationhip of Subject Luminance to Film Expoure For a ubject of uniform luminance, expoure at the film plane of a camera i given by H f E t (1) f where E f = Film plane illuminance in lx H f = Film plane expoure in lx t = Effective hutter time in The hutter time i determined by the hutter peed etting; film plane illuminance i determined by the len aperture, and i given by where E f 4 L f TFV 1 co bl N u 4 N ()

6 Expoure Metering Page 6 b = Contant with unit of lx cd 1 m = Angle between ubject and len axi N = Relative aperture (f-number) of len F = Len flare correction factor f = Focal length of len in m V = Len vignetting factor L = Luminance of ubject in cd m T = Len tranmittance factor u = Subject ditance in m 8 ANSI PH aumed F = 1.03, T = 0.90, and V = 1.0 a repreentative value for len performance. To implify expoure determination for the caual photographer, that tandard alo aumed a ubject 1 from the len axi at a ditance u = 80f, o that and 4 co f u 9 giving a value of b = The correction for the len extenion aumed here i minimal, but in ome ituation, uch a cloe-up photography, the correction can be ubtantial. It may be eaier for the eriou photographer to tart with a ubject on the len axi at infinity, and make correction for ubject poition and len extenion if required. The tarting value then become 4 co 1 f 1 1 u and b = Of coure, a camera with a meter that meaure through the taking len automatically repond to the effect of len extenion, o that no additional correction i needed. Expoure Meter Calibration Reflected-Light Meter A reflected-light expoure meter indicate aperture and hutter peed etting baed upon ubject luminance. The relationhip between indicated camera etting and cene luminance i N L EV S t K (3)

7 Expoure Metering Page 7 where EV i the expoure value, S i the arithmetic ISO peed, and K i the meter calibration contant. The expoure H g that reult from etting the camera according to the meter indication then i bk H EV g Eft bl (4) S Nominal calibration differ lightly from manufacturer to manufacturer. The range of value for K recommended by ANSI/ISO i 10.6 to 13.4; in practice, value of 1.5 (Canon, Nikon, and Sekonic) or 14 (Minolta and Pentax) are common. The difference between the value of 1.5 and 14 i approximately 0.16 tep, which i le than mot meter manufacturer calibration tolerance. If two meter from different manufacturer indicate ubtantially different expoure for the ame ubject, it probably not becaue of difference in the manufacturer calibration. In the late 1970, there wa a brief controvery regarding the ue of calibration contant for expoure meter. Anel Adam lamented 10 that ome manufacturer depart from tandard calibration of their meter by incorporating a K factor, with the reult that if we make a careful reading from a middle-gray urface, the reult will not be exactly a middle gray! Although Adam acknowledged the tendency of average reading of a ubject to produce light underexpoure, he continued,... I find it far preferable to work with what I conider to be the true characteritic of the light and the film. Intelligent ue of the meter eliminate the need for uch artificial aid a the K factor. At that time, the calibration equation often wa given a N LS t 11 The apparent lack of a K factor in the calibration equation aroe from the United State dogged echewal of metric practice: it wa common to expre luminance in cd ft, even though film peed wa expreed in SI (metric) unit. When luminance wa expreed in cd m for conitency with the other variable, the formula became N t LS with the K factor of ariing from the converion of m to ft. It hould be apparent that the K of 10.76, rather than ariing from a natural law, wa an arbitrary choice, determined from what viewer determined to be the bet expoure. The quetion, then, wa not whether to ue a K factor, but rather, what it value hould be. Becaue of light change in the method for determining film peed, ANSI PH increaed the recommended value to 1.16 (or 1.5 when luminance wa expreed in cd m ). The ANSI/ISO recommend a range of 10.6 to The controvery regarding the ue of the K factor appear to have faded, although Adam advocacy of intelligent ue of the meter i a valid now a it wa then. Incident-Light Meter An incident-light meter relate camera etting to ubject illuminance; the relationhip i

8 Expoure Metering Page 8 N E EV S t C With a flat-dic (coine-repone) receptor, the range of value for C recommended by ANSI/ISO i 40 to 400, with 50 a common choice of meter manufacturer. With a hemipherical (cardioid-repone) receptor, the recommended range of value for C i 30 to 540; value toward the lower end of the range are common. Comparion of Reflected- and Incident-Light Meter The luminou exitance M i the luminou flux reflected or emitted by a urface; reflectance i the ratio of luminou exitance to illuminance E : M E (6) For a flat, perfectly diffue, front-lighted ubject, the luminance L i related to the luminou exitance by L M (7) Luminance then i related to illuminance by L E E (8) where i the luminance coefficient. Comparing a reflected-light meter to an incident-light meter, LS ES ES K K C and the reflectance i K (10) C Illuminance i meaured with a flat-dic receptor; an incident-light meter uing a flat-dic receptor with a C of 50 would indicate the ame expoure a a reflected-light meter with a K of 1.5 meauring a flat ubject with a reflectance of 15.7% ( = cd m lx 1 ). Thi might ugget that the meter are calibrated to a 16% reflectance, although thi really decribe only the relationhip between the incident- and reflected-light meter. If the value for C and K were doubled, the meter alo would indicate the ame expoure when reading a flat 16% reflectance, but the indicated expoure would double. ANSI/ISO doe not ugget a relationhip between K and C, and doe not ugget that either relate to any pecific average ubject reflectance, merely directing that The contant K and C hall be choen by tatitical analyi of the reult of a large number of tet carried out to determine the acceptability to a large number of oberver, of a number of photograph, for which the expoure wa known, obtained under variou condition of ubject manner and over a range of luminance. (5) (9)

9 Expoure Metering Page 9 Typical photographic ubject aren t flat, and the light ource often in t directly behind the camera. Experience ha hown that a hemipherical receptor uually give better reult for incident-light reading in practical picture-taking ituation, integrating the effect of lighting from different ource at different angle on three-dimenional object. In mot uch ituation, however, it i difficult to develop a imple relationhip uch a Equation (10) between the reflected- and incident-light meaurement. Effect of Meter Calibration Reflected-Light Meter The effect of mot reflected-light meter calibration i to give whatever i metered an expoure omewhere near the middle of a film expoure range, although it eldom at the exact midpoint. If the area metered i cloe to a medium reflectance, the expoure for that area uually will be atifactory. The greater problem uually i in getting atifactory expoure for light and dark area. The effect of meter calibration on light and dark element of a ubject perhap i bet viualized by uperimpoing thoe expoure on the film characteritic curve. For uer of the Zone Sytem, the expoure given to a ubject element placed on Zone z i H bk S 5 z z (11) The reult of thi are hown for typical black-and-white negative, color negative, and color reveral film. Black-and-White Negative Film 1 The ISO peed for black-and-white negative film i baed on an expoure H m that reult in a net denity in the negative of 0.1, and i given by o that 0.8 S H H g m bk bk H m (1) S 0.8 For an on-axi ubject at infinity focu, with b = 0.78 and K = 1.5, the expoure that reult i bk Hg Hm 11.38H m (13) 0.8 or, in expoure tep, log H log H 3.51 (14) g m ANSI/ISO pecifie a higher contrat than uually i deired for practical photographic ue. Conequently, photographer often ue different proceing, and the effective film peed may be different from the ISO peed. However, if, except for the contrat, the

10 Expoure Metering Page 10 method ued to determine the peed i imilar, the above relation often can be ued if the ISO film peed i replaced with the effective film peed. It common for photographer who ue the Zone Sytem to determine effective film peed by requiring that a Zone I expoure (four tep le than meter indication) produce the ame 0.1 net denity pecified in the ISO tandard. Conequently, effective film peed determined in thi manner will be approximately 0.5 tep le than the ISO peed. If the film i developed to a lower contrat, a often i the cae, the difference between ISO peed and effective peed may be even greater. Figure 1. Characteritic Curve and Expoure Zone for Black-and-White Negative Film The zone expoure are baed on the ISO peed of 30; the 6-minute development time i cloet to normal. If the Zone I expoure were required to yield the threhold denity of 0.1 above film bae + fog, the zone expoure would hift to the right by 0.5 expoure tep (0.16 log H), and development probably would be lightly reduced. Typical of modern black-and-white film, thi film ha coniderable uable expoure range to the right of the indicated (Zone V) expoure, o the bet expoure of a ubject with a wide SBR probably would favor hadow detail, while till allowing acceptable recording of highlight detail. Tone reproduction i only one element of a good image, however. Figure 1 ugget that highlight detail would be maintained even if expoure were increaed by 3 tep, but increaed expoure increae grain and lightly reduce harpne. Moreover, a dener negative uually i more difficult to print. The bet expoure uually i the minimum expoure that give adequate hadow detail.

11 Expoure Metering Page 11 Color Negative Film 13 The ISO peed for color negative film i baed on a minimum-denity expoure and i given by o that S H H g m bk Hm (15) With b = 0.78 and K = 1.5, the expoure that reult i then bk Hg Hm 6.43H m (16) or, in expoure tep, log H log H.69 (17) g m The ISO tandard pecifie the film manufacturer normal proceing, o that the effective film peed uually i very cloe to the ISO peed. Figure. Characteritic Curve and Expoure Zone for Color Negative Film Thi film ha greater uable expoure range to the right of the indicated (Zone V) expoure than to the left, o it i quite tolerant of moderate overexpoure. The bet expoure of a ubject with a wide SBR probably would favor hadow detail.

12 Expoure Metering Page 1 Color Reveral Film 14 The ISO peed for color reveral film i baed on a mid-tone expoure and i given by 10 S H m With the ame value for b and K, the reulting expoure i bk Hg Hm 0.91H m (18) 10 or, in expoure tep, log H log H 0.14 (19) g m The expoure will be near the middle of the expoure range for mot color reveral film. The ISO tandard pecifie the film manufacturer normal proceing, o that the effective peed for mot color reveral film i very cloe to the ISO peed. Figure 3. Characteritic Curve and Expoure Zone for Color Reveral Film Typical of color reveral film, the expoure range i limited, epecially to the right of the indicated (Zone V) expoure. Detail may be lot in any area given expoure at Zone VII or above, o the bet expoure normally hould favor the highlight. Subtitute metering of an 18% neutral tet card in a ubject with many light area may reult in lo of highlight detail unle the indicated expoure i reduced by 0.5 to 1 tep a indicated in the Kodak intruction.

13 Expoure Metering Page 13 Uable contrat extend to Zone II, but under traditional viewing condition, detail may be difficult to dicern becaue of the high denity. With contrat making, however, ome of thi detail can be recovered. The advent of digital potproceing greatly implifie uch a tak, and give color photographer ome of the flexibility previouly available only to large-format black-and-white photographer.

14 Expoure Metering Page 14 Note 1 ANSI/ISO (R1994), General Purpoe Photographic Expoure Meter (Photoelectric Type) Guide to Product Specification (New York: American National Standard Intitute, 1994). Originally, ubject brightne range. Brightne, however, properly refer to the ubjective impreion of luminance. Although ued by ome author, SLR invite confuion with ingle len reflex, o SBR ha perited. 3 Holli Todd and Richard Zakia, Photographic Senitometry: The Study of Tone Reproduction, 7 (Dobb Ferry, NY: Morgan and Morgan, Inc, 1969), ugget a range of 160:1, or about 7.3 expoure tep. 4 Phil Davi, Beyond the Zone Sytem, 4th ed. (Boton: Focal Pre, 1999), ugget a range of two tep, in conjunction with a 5-tep reflectance range. 5 Anel Adam, The Negative (Boton: New York Graphic Society, 1981). 6 Fred Picker, Zone VI Workhop: The Fine Print in Black & White Photography (Garden City, N.Y., Amphoto, 1974). 7 Davi, Beyond the Zone Sytem (New York: Curtin & London, 1981). 8 ANSI PH , after everal reviion, wa redeignated a ANSI/ISO (R1994). Unfortunately, the explanatory appendixe no longer are included. 9 Thi value i ued in everal ISO tandard that relate object luminance to film-plane illuminance. 10 The Negative, Actually, ANSI PH recommended a K of 1.06 when the luminance wa expreed in cd ft. ANSI PH increaed thi value to 1.16 to accompany change in method for determining film peed. 1 See ANSI/ISO , Photography Black-and-White Pictorial Still Camera Negative Film/Proce Sytem. 13 See ANSI PH , Method for Determining the Speed of Color Negative Film for Still Photography. Note: thi tandard ha been upereded by ANSI/ISO (R1994), Photography Color Negative Film for Still Photography Determination of ISO Speed. 14 ANSI PH , Method for Determining the Speed of Color Reveral Film for Still Photography). Note: thi tandard ha been upereded by ANSI/ISO , Photography Color Reveral Camera Film Determination of ISO Speed.