brightness of fine detail in ground photography

VISIBILITY OF STARS

threshold stellar magnitude at each luminance is alsoindicated. The average luminance of the full night skyis between 0.00002 and 0.00004 c-ft2, and the curveshows that the observer will not encounter luminancescharacteristic of the night sky until he is above 100 km

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

(328 000 ft). However, no account has been taken ofpossible light emission from the upper atmosphereduring the day. The intensity of such emission is notknown, but it may contribute significantly to sky lu-minance at the higher altitudes.

VOLUME 49, NUMBER 6 JUNE, 1959

Brightness of Fine Detail in Ground Photography

P. D. CARMAN AND H. BROWNDivision of Applied Physics, National Research Council, Ottawa, Canada

(Received November 11, 1958)

For a number of typical outdoor photographic scenes on cloudless days, a statistical study has been madeof logarithmic brightnesses and of differences of log brightnesses on adjacent pieces of fine detail, as theywould be seen by a representative camera. Values were obtained photographically on Super XX film andalso by means of a photoelectric telephotometer corrected to Super XX spectral response. The results do notdisagree seriously with accepted values for the photographic brightness range. However the uncertainty inthe meaning of "brightness range" is illustrated and a definition is proposed for a "99% range." For differ-ence of log brightness on fine detail, low values are found to predominate, with two-thirds of the differencesbelow the average value of 0.15.

INTRODUCTION

THE decisive test of any camera comes when it isput into regular use. Consequently information

on the conditions of use is important to those whodesign, manufacture, or test cameras. One of these con-ditions of use is the distribution of radiance over realscenes of the type which will be photographed. Forblack and white photography the weighting of spectralradiances is determined by the spectral sensitivity ofthe emulsion and the spectral transmission of the lens.The distribution of this photographic radiance, or pho-tographic brightness, may be isolated for experimentalstudy.

Some previous work has been done on this subject.Moon and Spencer' have examined the usefulness ofthe brightness range concept for outdoor scenes, andcompared the results of a number of investigationsincluding that of Jones and Condit.2 Carman andCarruthers3 have made a statistical study of brightnessand brightness ratios occurring on fine detail in airphotography.

PROCEDURE

Measurements have now been made of the photo-graphic brightnesses and brightness ratios occurring onfine detail in exterior ground scenes on cloudless daysby a procedure which provides reasonably random sta-tistical sampling. In view of the photographic applica-tion a logarithmic brightness scale has been used. Ob-

' P. Moon and D. E. Spencer, J. Soc. Motion Picture TelevisionEngrs. 63, 237-239 (1954).

2 L. A. Jones and H. R. Condit, J. Opt. Soc. Am. 31, 11, 651-678(941).

3 P. D. Carman and R. A. F. Carruthers, J. Opt. Soc. Am. 41,305-310 (1951).

servations have been analyzed to provide two kinds ofinformation, the log brightness of each piece of finedetail measured, and the difference of log brightness(or log brightness ratio) for every two adjacent piecesof fine detail observed. In the image of each sceneseveral lines have been selected for examination and allbrightnesses along each line have been measured. Thelines were chosen to be representative. The only delib-erate bias involved in their selection was the avoidanceof large pieces of absolutely featureless areas such ascloudless sky. Some other accidental bias may be pres-ent, but it will be much less than would have arisen inany procedure in which particular small pieces of detailwere chosen for measurement.

Both photographic and photoelectric measurementprocedures were used to provide a means of detectingany possible systematic errors and incidentally to eval-uate the reliability of a photoelectric procedure suchas had been used exclusively in Carman and Carruthers'earlier work.

Both photometers were arranged to observe thescene as it would be seen by a representative camera

of 4-in. focal length, operating at f/11, and usingSuper XX film with no filter. Both differed from thishypothetical camera by having better resolution andless veiling glare than would normally be present. In

both, the angular diameter of the circular field on whichthe photometric measurement was made was 0.0005rad. This corresponds to 0.05 mm in the image plane ofthe hypothetical average camera. Both photometerswere focused for an object distance of 30 feet, corre-

sponding to the hyperfocal distance of the representativecamera for a circle of confusion with diameter f/1000.

629June 1959

P. D. CARMAN AND H. BROWN

APPARATUS

The photoelectric photometer consisted of a telescopewith photocell, an oscillograph, and a recording camera.The telescope objective, of 8-in. focal length wasstopped down to 0.36 in. A circular aperture of 0.004-in.diameter was placed in its image plane. A lens hoodextended 12 in. in front of the objective and containeddiaphragms to minimize stray light. Behind the 0.004-in. field stop were a filter and a 1P21 multiplier photo-tube selected from a number for which the spectralresponse had been determined. The filter consisted oftwo 1-cm cells, one containing Orange II, 0.0375 mg/Lin methanol, the other Methyl Violet 2.4 mg/L inmethanol. The degree of correction to Super XX sen-sitivity is shown in Fig. 1.

The photomultiplier was operated from B batteries.Its output was connected to the horizontal plates on aDuMont type 304H oscilloscope. Spot deflections wererecorded by a Fairchild oscillo-record camera on movingfilm, while the telescope was swung across the scene.The telescope was swung on a vertical axis at 45 sec/radso that each 0.0005 rad was seen for 0.022 sec. This wasabout the slowest speed which safely eliminated fluctu-ations due to motions of trees and grass in the wind. Itwas chosen to permit phototube noise to be reduced toan insignificant level by reducing the band widthcovered.

The photographic photometer was a 4-in.X5-in.Crown View camera fitted with a 14-in. Kodak Ektarlens stopped down to 0.36 in. A baffled lens hood about14 in. long was added to reduce stray light. Photo-graphs were taken on Super XX film with exposuresranging from 1/10 to 1/50 sec. Development was inD76 to a gamma of 1.0. To reduce errors arising fromreduction in image illumination, only a central area of

2.0

PHOTOCELLX ~~~AND FILTERS

, t

0 SUPER XX

W ~~~~~FILM

- WAVELENGTH (my)

FIG. 1. Spectral sensitivity of photocell with filters.

the negative 2 in.X3' in. was considered for latermeasurements.

Calibrations of both photometers were based onreference objects placed in the scene. Two pieces offlat white cardboard about 24-in. square were heldupright on easels. Their brightnesses were establishedin foot-lamberts with a Macbeth photometer. Two grayscales, each containing 5 steps about 2 in. square,previously calibrated for reflectivity were also included.

The photoelectric telephotometer was separatelychecked for linearity and the accuracy of a 1oX elec-trical range change was established.

In the photographic procedure a sensitometric ex-posure was processed along with each group of threenegatives from the camera.

Comparison of photographic and photoelectric re-sults showed only minor differences which were of nopractical importance. The photoelectric measurementswere inherently more precise since they avoided therandom errors arising in the photographic process fromsuch factors as grain and slightly uneven development.This type of "noise" could significantly increase errorson small difference of log brightness. Consequently thephotoelectric data were established as the more reliable,and only they are reported here.

ANALYSIS

It is difficult to establish an unassailable procedurefor analyzing the data. The frequency of occurrence ofthe various brightnesses can be obtained by any one ofa number of methods and is unlikely to be seriously de-pendent on the procedure chosen. Changes of brightnessbetween adjacent pieces of fine detail are much moredifficult to describe numerically. An attractive labor-saving procedure would be to feed the photomultiplieroutput into a wave analyzer or a differentiating andsorting device. Information obtained by such a pro-cedure might be useful to applications such as televisionwhere electrical transmission of pictures was involved.However, as a general form of analysis it has two short-comings. Real scenes contain a large proportion ofabrupt discontinuous transitions from one brightnessto another mixed with a few transitions which are con-tinuous. The general usefulness of approximating thesestep functions by combinations of sine waves seemsdoubtful. In the measurement procedures these realdiscontinuities are masked by instrumental limitationsand the recorded apparent rate of change of brightnessremains finite. Such a limitation will occur in anycamera or any photometer but the actual limit on therate of change, or the "frequency response," will varywidely between different instruments. This practicallimitation further reduces the general usefulness ofwave analysis or similar procedures, since the resultsso obtained would depend considerably on the char-acteristics of the particular measuring instrument used.

The method of analysis which has been used was

630 Vol. 49

FINE DETAIL IN GROUND PHOTOGRAPHY

(a) (b)

(c) (d)

(e ) (f )

(g) (h)

FIG. 2. Representative portions of scenes, with scan lines shown.Scan lines were selected for best sampling of the whole scene somay not appear optimum for the portion shown.

(i)

June 1959 631

632 P. D. CARMAN

chosen primarily to provide information on the sizesof the steps occurring in the brightness discontinuitiesin the scene. The formal procedure adopted also tooksome account of gradual brightness changes but thesewere so few in number that values obtained for theformal procedure differed little from informal readingsof step sizes. In the formal procedure, the brightnessat some starting point on the line to be scanned wasnoted, then a region corresponding to angular distancesbetween 0.0008 and 0.0016 rad from the starting pointwas scanned. The point in this region giving the greatestdifference in brightness from the starting point wasfound, the brightness noted and this point then used asa new starting point. One step of this procedure is

0

0

0

0.5 0.9 1.3

0.0 0.2 0.4DIFFERENCE

G 30X

0 25

0

< 20

0. 15

ID

1.7 2.1 2.5 2.9LOG BRIGHTNESS

0.6 0.8 1.0 1.2

OF LOG BRIGHTNESS

AND H. BROWN Vol.49

illustrated in Fig. 4 where three vertical lines represent,from left to right, a starting point, the beginning of thescanned region, and its end. Data read in this mannerwere used for examining both frequency of occurrenceof log brightnesses, and frequency of occurrence ofdifferences of log brightness.

EXPERIMENTAL DATA

Photographs of representative portions of the scenesstudied are shown in Fig. 2. The data obtained fromscans of the complete scenes are presented graphicallyin Fig. 3. For each of nine types of scene and for allthese scenes combined, the percentage frequency ofoccurrence of log brightness in each 0.20 range is plotted

25 I-

0

0

I-

3.3 3.7 4.1

C

0.5 0.9 1.3 1.7 2.1 2.5 2.9 3.3 3.7 4.1

LOG BRIGHTNESS

0.0 0.2 0.4 0.6 0.8 1.0 1.2

DIFFERENCE OF LOG BRIGHTNESS

45

40

35

30

0

0

.u

a:

25

20

I!

IC

C

'.5 0.9 1.3 1.7 2.1 2.5 2.9 3.3 3.7 4.1LOG BRIGHTNESS

0.0 0.2 0.4 0.6 0.8 1.0 1.2


(d)NEARBY BEACH SCENE

FRONT TO SIDE LIGHTINGTOTAL OCCURRENCESBRIGHTNESSES 3898DIFFERENCES 3430

I~~~ ~ _ _ _ _ I0.5

0.0

0.9 1.3 1.7 2.1 2.5 2.9

LOG BRIGHTNESS

0.2 0.4 0.6 0.8 1.0 1.2DIFFERENCE OF LOG BRIGHTNESS

(b)NEARBY LANDSCAPE

BACK LIGHTINGTOTAL OCCURRENCESBRIGHTNESSES 4116DIFFERENCES 3561

I C I I I I II I I I I I

(Legend on opposite page)

I

3.3 3.7 4.1


30

25z

20

0,; is

i1 XU0

0 U.,0

0.5 0.9 1.3 1.7LOG

0.0 0.2 0.4 0;'6DIFFERENCE OF LOG

25

0

D° 20

0

25-

0

U..

0o

2.1 2.5 2.9BRIGHTNESS

0.8 1.0 1.2BRIGHTNESS

0.5 0.9 1.3 1.7 2.1 2.5 2.9 3.3LOG BRIGHTNESS

0.0 0.2 0.4 0.6 0.8 1.0 1.2DIFFERENCE OF LOG BRIGHTNESS

I I I I I I 10.5 0.9 1.3 1.7 2.1 2.5 2.9

LOG BRIGHTNESS0.0 0.2 0.4 0.6 0.8 1.0 1.2


3.3 3.7 4.1

O II I I

0.5 0.9 1.3 1.7 2.1 2.5 2.9 3.3 3.7 4.1LOG BRIGHTNESS


30

W 25

z20

O

o 15IL

CD 10

z

C 5.

0

3.7 4.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9LOG BRIGHTNESS


WU,

z

a:W

0

U-0W0

'C

3.3 3.7 4.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9LOG BRIGHTNESS


3.3 3.7 4.1

3.3 3.7 4.1

FIG. 3. Frequency diagrams of log brightnesses and of differences of log brightness for adjacent points.Brightnesses are in foot lamberts for gray objects.

June 1959 633

(h)ARCHITECTUREMOSTLY SHADED

TOTAL OCCURRENCESBRIGHTNESSES 3412DIFFERENCES 2261

1 _

1 I I I I I I

(n

z

'C00

0a-W

a.

l l l l l l l l

I

P. D. CARMON AND H. BROWN

TABLE I.

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (I1) (12)

Log brightnesses (ft-L on gray) Sum DifferencesRange Scene (8) of logExtreme range 99% range with index and brightness

Scene Min Max Range Min Max Range sky Mean 10 (9) Mean Max

(a) Nearby landscapeFront lighting 0.70 3.84 3.14 1.22 3.30 2.08 2.05 2.42 0.3 2.7 0.17 1.13

(b) Nearby landscapeBack lighting 0.84 3.70 2.86 1.00 3.36 2.36 2.36 1.87 0.61 2.5a 0.19 1.12

(c) People in nearby landscapeFront lighting 1.13 2.93 1.80 1.40 2.90 1.50 1.49 2.41 0.3 2.7 0.14 0.96

(d) Nearby beach sceneFront to side lighting 1.79 3.59 1.80 2.20 3.30 1.10 1.10 2.82 0.1 2.9 0.12 0.96

(e) Distant landscapeFront lighting 1.30 3.83 2.53 1.54 3.40 1.86 1.70 2.80 0 2.8 0.11 0.96

(f) Marine, beach and distantFront, side, and back lighting 1.76 3.83 2.07 2.10 3.80 1.70 1.60 3.11 -0.2 2.9 0.10 1.13

(g) ArchitectureFront lighting 0.78 3.82 3.04 1.02 3.78 2.76 2.62 2.65 0.3 3.0 0.19 1.15

(h) ArchitectureMostly shaded 0.69 3.63 2.94 1.42 3.24 1.82 1.80 2.29 0.6 2.9 0.15 0.88

(i) SnowFront lighting 1.54 3.76 2.22 1.74 3.70 1.96 1.91 3.03 0.1 3.1 0.13 1.14

Mean (a) to (i) 1.17 3.66 2.49 1.52 3.42 1.90 1.84 2.60 0.15 1.05(j) All scenes 0.69 3.84 3.15 1.22 3.65 2.43 2.38 2.61 0.15 1.15

- The scene is partly "unshaded, backlighted" with a scene index of 6 as used here and partly "heavy shade" with a scene index of 9 for a sum of 2.8

against the midpoint of the range. Similarly percentagefrequency of occurrence of difference of log brightnessin each 0.02 range is plotted against the midpoint of therange. Differences below 0.025 have not been shownsince the 0.005 to 0.025 group would contain a consid-erable proportion of occurrences which were not realbut were due to instrument "noise" superimposed on areal zero difference.

The major shape characteristic of the log brightnesscurves are readily explicable from practical photo-graphic experience. The curves are generally unsym-metrical. Front-lighted scenes show negative skewnessdue to a large proportion of relatively high brightnesson directly sunlit objects and a small proportion ofshaded objects. Back-lighted scenes show positiveskewness for the reverse reason. Distant landscape(e) compared with nearby landscape (a) shows fewerlow brightnesses. This is caused by atmospheric effectsand perhaps also by the reduced angular size of naturalshaded areas making them less important. Lowerbrightnesses are also scarce in the marine scene (f),where there is very little shade. The combined data forall scenes give a distribution curve witl negative skew-ness due to the predominance of front-l-hted scenes inthe sampling. This sampling is typical of average pho-tography so a curve with moderate negative skewnesscan be considered typical. However too much impor-tance should not be attached to this shape since widevariations occur.

The brightness range of a scene is a quantity whichis difficult to define in a useful manner. It is simple totake the difference in log brightness between thebrightest and darkest objects measured. Such extremevalues are indicated in Fig. 3 by short vertical lines and

are recorded in columns 1 and 2 of Table I. Differencesbetween them are given in column 3. However thesevalues are not of much significance since further ex-amination of the scene would have disclosed bothbrighter and darker objects. In fact examination ofmost sunlit scenes with a photometer of sufficient reso-lution would disclose specular reflections of the sunwith brightnesses in the range 7.0 to 8.5 log ft-L* andopen windows or other holes sufficiently dark that theirapparent brightness was caused almost entirely bylight scattered toward the camera lens by the air be-tween the lens and the hole, giving log brightnesses inthe range from 1.3 to 0.6 for a 30-ft distance and clearconditions.t 4' 5 Thus in most scenes ranges of logbrightness of 6.4 (brightness ratios of 2 500 000: 1) ormore could be found with sufficient searching. In viewof this, any pretense that some smaller value actuallyrepresents the maximum brightness range of an outdoorscene seems questionable. As an improvement, a rangemight be stated which included some definite largefraction of the brightnesses measured in a randomsampling of the scene. A range containing the central99% of the observed brightnesses is suggested. Limitsof such 99% ranges are indicated by the medium-lengthvertical lines in Fig. 3 and recorded in columns 4 and 5of Table I. The 99% ranges are given in column 6.These 99% ranges are not independent of the measuringphotometer, since its small but finite field of view will

* Sun brightness 4.7X 108 ft-L. Fresnel reflection normal towater 0.020. Chromium plate reflectivity 0.66.

t Visibility 200 000 yd, horizon brightness 1000 ft-L to visibility20 000 yd, horizon brightness 2000 ft-L.

4Coleman, Morris, and Rosenberger, J. Opt. Soc. Am. 39,515-521 (1949).

,1 R. G. Hopkinson, J. Opt. Soc. Am. 44, 455-459 (1954).

634 Vol. 49


FIG. 4. Portion of back-lighted scene with photoelectric trace reproduced to same horizontal scale.

let it respond only partially to very small areas of ex-tremely low or extremely high brightnesses. That is, theranges depend on the choice of a 4-in. f/1l1 camera lensfocused for 30 ft as typical. The sampling of the scenes,which was not random in its omission of large uniformareas such as sky, will also have affected the range valuesto some extent. However a correction for this can be cal-culated since the sky brightnesses will not lie in the %areas at either end of the frequency distribution curves,and the proportion of sky area in each scene used isknown. These corrections, which are included in thedata given in column "7" of Table I, are not largeenough to be of much practical importance.

The averages of the log brightnesses measured ineach scene are indicated by long vertical lines in Fig. 3and are given in column 8 of Table I. These averagelog brightnesses are systematically lower than the log-arithms of the average brightnesses which would beread by some exposure meters since sky is neglectedand the averaging of brightnesses is effectively geo-metric rather than arithmetic. However the averagelog brightnesses are in general accord with photographicexposure experience. The degree of accord may be ex-amined by adding to the average log brightness one-tenth of the scene index given in the American StandardPhotographic Exposure Computer and examining thesums for constancy. This is done in columns 9 and 10of Table I. The sums are moderately constant. Anycombinations of one-tenth the scene index with min-imum or maximum log brightnesses show much largervariations.

Frequency distributions of differences of log bright-ness are generally similar in shape from scene to scene.Average differences are indicated by dotted verticallines in Fig. 3 and are given in column 11 of Table I.

Back lighting (b) gives a higher average difference thanfront lighting (a). Beach scenes (d), distant landscape(e), and marine scenes (f) give progressively loweraverage differences. The highest average difference foran entire scene is found for sunlit architecture. Thebuilding examined was of cut stone with gray mortar.It is apparent that more high differences would occuron some types of brick construction. Some extremeback-lighted areas also give higher values. In the marinescene (f) one scan of the horizon sky below the suncrossed a number of clearly spaced trees at a distanceof about 1200 ft. This scan gave an average differenceof 0.30. A portion of this scene and the correspondingphotoelectric trace are reproduced in Fig. 4. An in-formal reading of the steps in this portion of the trace,ignoring steps below 0.03 gives a mean difference of0.27. For this rather distant sample, the effect of "airlight" must be considered. It happens that the value ofair light calculated from the visibility, the backgroundsky brightness and the distance agrees within aboutten percent with the value obtained from the apparentbrightness difference on distant and nearby tree trunks.Subtraction of the mean air light, 250 ft-L, from themeasured brightnesses in this small sample increasesthe average difference of log brightness from 0.30 to0.36. The lowest average difference for an entire scenewas 0.10 in the marine scene. However many areasgave lower averages. Side-lighted grass at 30 ft gave anaverage difference of 0.06. Small waves in part of themarine scene showed an average difference of onlyabout 0.03 yet contributed considerably to the appear-ance of the water in the photograph.

Of most importance is the general predominance oflow values of differences of log brightness on adjacentpieces of fine detail. For all scenes the average value of

June 1959 635

P. D. CARMAN AND H. BROWN

10,000

1,000

100

10-

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4


FIG. 5. For differences of log brightness, plotting log frequencyof occurrence gives an approximation to a straight line.

this difference of log brightness is 0.15. Two-thirds ofthe differences measured are below this mean value.Only 10% are above 0.33 and only 1% above 0.73. Themean difference of log brightness of 0.15 represents theaverage small object which a camera is required toreproduce with good definition. Consequently it is ap-plicable in the establishment of a laboratory cameratest procedure which will be likely to correlate wellwith performance in outdoor use. If a single test objectcontrast must be selected for reasons of economy andconvenience, it should be near this 0.15 log value. Thishappens to be' reasonably close to the log brightnessdifference of 0.20 which has had quite general accept-

ance for low contrast targets used chiefly in testing aircamera lenses. Consequently it appears that 0.20 logbrightness difference would be a reasonable standardcontrast for testing cameras to be used on outdoorground scenes. Where more thorough tests involvingmore than one target contrast were practicable it wouldappear desirable to use differences of log brightnessboth below and above 0.20 and combine the resultsfrom different contrasts on the basis of their relativefrequency of occurrence.

Efforts were made to find simple mathematical rela-tionships applicable to the frequency distribution ofdifferences of log brightness. It was found that plottinglog frequency against difference of log brightnessyielded an approximately straight line relationship,with the accuracy of the fit varying greatly betweenscenes. This form of plot is shown in Fig. 5 for thecombined data from all scenes. The approximate rela-tionship is of the form

f=Ae-BX

where f is the frequency, X the difference of log bright-ness, and A and B constants. Integration shows that, ifthis relationship applies, the average difference of logbrightness should be given by

X= 1/B.

Best straight lines were estimated on log frequencyplots for each of the nine scenes and the resultingvalues of 1/B were compared with calculated values ofX which are given in column 11 of Table I. The alge-braic mean deviation was 0.002, the arithmetic meandeviation 0.02, and the largest single deviation 0.06. Itwas apparent that the larger deviations occurred fordata which fitted a straight line poorly. The straightline drawn on Fig. 5 has its slope appropriate to theaverage difference of log brightness. It appears that therelationship f=Ae-BX is far from exact but might besufficiently good for some practical use.

636 Vol. 49

brightness of fine detail in ground photography

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