HAROLD E. LOCKWOOD LINCOLN PERRY Technicolor Graphic Services, Inc. Houston, TX 77058

Shutter/Aperture Settings for Aerial Photography

Aerial camera shutter and aperture settings are determined based on sunlight, aircraft altitude, subject and season, film speed, and optical system.

INTRODUCTION rect exposure settings must receive high CIENTISTS, aerial photographers and priority. PTD strives to either control or com­ S processing laboratory personnel within pensate for the many variables which affect the Photographic Technology Division in-camera film exposures. Discussed here are (PTD) at NASA's Johnson Space Center(JSC) the variables which affect film exposure and in Houston, Texas have accumulated exten­ the steps taken by PTD to control those vari­ sive experience in the determination ofcam­ ables. era exposure settings for aerial photography. VARIABLES AFFECTING FILM EXPOSURE JSC is responsible for photography on major projects including the Earth Resources Air­ The variables which affect film exposures craft Program (ERAP) and manned space in aerial photography can create considerable

ABSTRACT: Determination of aerial camera shutter and aperture settings to produce consistently high quality aerial photographs is a task complicated by numerous variables. Presented in this article are brief discussions of each variable and specific data which may be used to systematically control each. The variables discussed include sunlight, aircraft altitude, subject and season,film speed, and optical system. Data which may be used as a base reference are included and encompass two sets of sensitometric specifications for two film­ chemistry processes along with camera-aircraft parameters which have been established and used to produce good exposures. Informa­ tion contained here may be used to design andimplement an exposure determination system for aerial photography. flight missions. The ERAP uses aircraft flown confusion and poor results if each is not con­ as low as 1000 feet and as high as 60,000 feet sidered and controlled where possible. over thousands of subject areas both on and These variables include: off the continental United States. Space flights, ofcourse, have enabled photography • Sunlight: The amount of illumination pro­ of sites around the world, with the largest vided the ground subject, which varies with collection of photographs taken during the time-of-day and time-of-year. • Altitude: The height ofthe aircraft or space three manned Skylab missions. craft above the ground. Exposure determinations for these projects • Subject: The reflectance or effective are made within PTD where the ERAP as brightness of the ground object or objects well as the space flight original imagery is being imaged on the film. processed, duplicated, and distributed. Cor- • Season: The time-of-year relative to the

PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING, 239 Vol. 42, No.2, February 1976, pp. 239-249. 240 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

type ofground cover to be expected. Season the colorbalance for colorfilms be acceptable may be considered as partofthe subject, but over the entire subject reflectance/image den­ is a frequently overlooked exposure vari­ sity range. Figure 1 shows the optimum able. placement of imagery for a 10: 1 effective • Film Speed: The effective sensitivity ofthe brightness range scene on a typical black­ film type and emul;ion batch relative to the subject being photographed. and-white film, Kodak Plus-X Aerographic • Optical system: The transmission of the Film 2402. Figure 2 shows the optimum im­ camera lenses, aircraft windows, and filters agery placement on the characteristic curve relative to the subject and film being used. of a typical color film, Kodak Aerial Color Film 80-242. It is obvious from these curves / Most ofthe above variables can result in a that only a narrow subject brightness range minimum of 1/2 [-stop changes in exposure (10 or 20: 1) may be accommodated with a and some may require as much as 3 f-stops or linear density relationship by these film more compensation in camera exposure to types. In most aerial photography above maintain correct exposure. 10,000 feet, however, the contrast range is typically only 4 or 5: 1. A goal for correct expo­ CORRECT EXPOSURE sure, then, may be specified as placement of A brief discussion of correct aerial expo­ imagery in the optimum recording range of sure is necessary if the best results are to be the film to be used. This optimum range usu­ achieved from aerial photographs. It is desir­ ally means placement of maximum subject able to image the subject on the photographic reflectances (in a positive image) at the low film such that (1) the subject reflectances density end of the straight line of the film­ have a linear relationship in the image and (2) process characteristic curve. When this is ac-

Kodak Plus-X aorographic Film 2402 Versamat 11tH Processor; 2 tanks. ~~6:I""~h~~;~iry

RELATIVE LOG EXPOSURE FIG. 1. Imagery placement for typical black-and-white film. SHUTTER/APERTURE SETTINGS FOR AERIAL PHOTOGRAPHY 241

O

3.• Kodak Aerial Color film 50-242 4'3.6 Versamat 1811 Processor -Jl-..-.---- E:;:""-'-'--'---"-EA-5 Chemi .try .-- 3.4

2.2

2.0

u

1.6

1.4

.3 .6 .9 1.2

RELATl VE LOG EXPOSURE

FIG. 2. Imagery placement for typical color film.

complished it will be found that mid-range documented. Sunlight relative to aerial densities in aerial color positive photographs photography includes not only the direct taken at lower altitudes (below 10,000 feet) light from the sun but also scattered light will frequently fall in the density range of0.9 from clouds and haze. Non-image forming to 1.3 for medium contrast films and in the 1.3 light at the aerial camera refle~ted by the to 1.6 range for higher contrast films. At clouds and haze in the atmosphere is a defi­ higher altitudes it will be found that the den­ nite factor in exposure determination. The sity range of imagery is lower because of effects ofsun elevation angle (the angle ofthe contrast attenuation due to the atmosphere. A sun, measured at the subject, between hori­ typical subject frequently results in image zontal and a line to the sun, Figure 3) best densities at higheror lower densities than the describe the gross sunlight available for aer­ optimum density range but exposures should ial photography. Sun elevation angle is also be centered about the optimum area. Most referenced as solar altitude in some publica­ subjects will be found to image in the op­ tions. timum range with specular highlights and The falloff of illumination on a horizontal dense shadows imaged outside that range. subject is caused both by attenuation of sun­ light by the Earth's atmosphere and by the VARIABLE SUNLIGHT angular relationship ofthe subject plane and The intensity of the illumination at the the sun.7 The net effect can be a large loss in ground subject is a major factor affecting aer­ illumination at the subject. When the sun ial film exposures. The effects of the sun elevation angle is 35° or lower, exposure elevation angle or solar altitudel as well as compensations are necessary because the a­ atmospheric effects 2.3 have been well vailable illumination at the subject is less by 242 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

TABLE 1. SUN ELEVATION ANGLE vs. EXPOSURE COMPENSATION.

Sun Elevation Angle Exposure

Subject A. 90°-35° Normal 2,~ S~n Elevation Angle 35°-25° Add V2 f-stop (1 V2X) -----...!-'--<...-1.------lll~I,orizon 25°-15° Add 1 f-stop (2X) FIG. 3. Sun elevation angle (solar altitude). 15°-10° Add 1% f-stop (3X) *10°- 5° Add 2 f-stop (4X) * 5°_ 0° Add 2%-4 f-stop (6-16X)

1/2 f-stop or more. A compensation system • Photography at these sun elevation angles should be avoided, used successfully at JSC is shown in Table l. The attenuation, of course, varies continu­ TABLE 2. TIME-OF-YEAR VS. EXPOSURE ously with Sun Elevation Angle (SEA) and COMPENSATION. compensations may be made at finer camera adjustments than 1/2 f-stop if desired. Refer­ Time-of-Year ences 1 and 7 can provide data for those ad­ (Noon, Exposure justments. Mid-Month) SEA* Compensation During the April to September months, five to six hours of aerial photography time Dec, Jan 29° pius % stop are available in the forty-eight contiguous Nov, Feb 36° plus V2 stop Oct, Mar states with no exposure compensation re­ 47° Normal Sept, Apr 58° Normal quired. The SEA during these periods re­ Aug, May 69° Normal mains within the 90° to 35° range. This is not July, June 72° Normal the case during the remainder of the year. Table 2 shows a comparison ofexposure fac­ • Latitude = 40·, tors necessary at 1200 hours at mid-month as time-of-year varies. Table II reflects data at TABLE 3. SEA vs. TIME-OF-DAY (400 LATITUDE) latitude 40° or the approximate locations of such U. S. cities as Philadelphia, Pittsburgh, Summer Spring Indianapolis, Kansas City, Denver, Salt Lake Time-of-Day SEA SEA City, and Reno. The SEA also varies from morning to night and typical summer and 0600,1800 15° 7° spring days at these same cities would have 0700,1700 26° 19° 0800,1600 38° 30° the variance shown in Table 3. Fortunately, 0900,1500 49° 42° SEA data tables for aerial photographers are 1000,1400 60° 51° available in the literature and are sum­ lloo,13oo 69° 59° marized in an Aerial Exposure Computer a­ 1200 73° 62° vailable from the Eastman Kodak Company to aid in determining the camera setting com- . pensation. wavelength regions than in the longervisible VARIABLE ALTITUDE wavelength regions. The major natural in­ Atmospheric effects, principally haze, im­ fluencing factor on color balance in aerial pact the camera exposures used in aerial photographs is scattering of light by the at­ photography. Color aerial photographs gen­ mosphere. Figure 5 shows the scattering ef­ erally will have a blue color balance, increas­ fect by the atmosphere as a function of ing with increasing altitude if no corrective wavelength for a minimum haze condition filtration is proVided. Aerial photographs also (Rayleigh Atmosphere). The scattering in the will receive increased exposure as altitude 450nm wavelength region (blue) is signifi­ increases as a result of sunlight scattered to­ cantly greater than in the 700nm wavelength ward the camera lens by aerosols and region (red). As the wavelength decreases the particulate matter in the atmosphere. effect worsens. (Rayleigh scattering is pro­ Figure 4 is a typical curve of atmospheric portional to the inverse of the wavelength to transmittance by wavelength. Transmittance the fourth power). A comparison of Figures 4 varies with the composition of the atmos­ and 5 clearly indicates that atmospheric scat­ phere and with the number of atmospheric tering has more influence on color balance thicknesses which the radiation must pene­ than atmospheric transmittance. . trate. In general, however, the atmosphere, as A typical remedy for the blue color balance shown, transmits less in the shorter is the addition ofa filter which absorbs short SHUTTER/APERTURE SETTINGS FOR AERIAL PHOTOGRAPHY 243

100 VARIABLE SUBJECT AND SEASON .. The reflectance properties of the ground subject or the effective brightness ofthe sub­ ! 50 ject in the photographic spectrum are impor­ tant considerations relative to correct expo­ 1.... sure. Table 5 lists nine categories of subject matter and the exposure compensation necessary. These data have been used suc­ cessfully for calculating exposures for high FIG. 4. Atmospheric transmittance versus altitude (60,000 feet) and space craft altitude wavelength3 . photography. The table may also be used as a starting reference for medium and low al­ titude photography, but a greater range of wavelength light and reduces the problem. A brightnesses will be apparent at these al­ filter equivalent to a Wratten 2A, Figure 6, titudes. will perform the task for many aerial photog­ Seasonal variations must be expected and raphyapplications. are of concern particularly where aircraft or Increasedexposure with increasedaltitude space craft have global coverage as a matter of is another effect resulting from scattering of routine. The presence of snow is an obvious light by the atmosphere. Using 5000 feet as a result of season but less obvious is the effect base altitude, an additional 1/2 stop exposure of plant growth. Agricultural or vegetated will be experienced at about 25,000 feet areas have varying brightnesses depending under normal haze conditions and 1/2 stop on plant growth. The effect is also depend~nt less exposure will be available at a 1000 foot on the spectral sensitivity of the film bemg altitude. Altitudes above 25,000 feet, includ­ used. For example, during a typical growing ing space craft altitudes, will result in 1/4 stop season the ground will be covered with vege­ more exposure. Table 4 lists factors based on tation and trees will be covered with leaves, a typical color film. The altitude effect is great­ which have relatively high reflectivity in the er for shorter wavelengths compared to near infrared and an average reflectivity in longer wavelengths; therefore, filtration will the visible spectrum. During the dormant be an additional factor with black-and-white season, when there is absence of ground films used in multispectral applications. cover the same area will have low reflectivity in the'near infrared and may have high reflec­ tivity in the visible region. The effect from

1000

?J TABLE 4. LIGHT vs. ALTITUDE. ~ c 600 ~ Altitude Light Factor '"> ... 1000 ft plus Y2 stop ~'" 200 5000 ft Normal 25000 ft minus Y2 stop 400 500 600 700 Space minus * stop Wavelength (nanometers) FIG. 5. Atmospheric Scattering versus TABLE 5. SUBJECT vs. EXPOSURE COMPENSATION. wavelength5 . Exposure Subject Category Compensation .1% A. Clouds Minus 2 stops B. Snow Minus Ilf2 stops '"u ...c C. Bright desert; e.g. Sahara Minus 1 stop ...'" 10l D. City (street & building) Minus 1 stop ~ E. Sandy soil; e.g. Mojave '=100l Desert Minus Y2 stop 3 0 400 500 600 700 ROO F. Vegetation and mountains Normal Wavelength (Nanometers) G. Coastal and shallow water Normal H. Deep Ocean Water Plus 1 stop FIG. 6. Wratten 2A filter for short I. Heavy vegetation (jungle) Plus 1 stop wavelength correction6 • 244 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

season-to-season may mean as much as a 1/2 ity of 0.4 is apparent. Figure 8 shows the stop variance in effective exposure depend­ effect of process gamma on effective speed ing on the amount of ground cover. measured at a density of0.4. A difference of1 1/4 f-stops between the slower and faster proc­ VARIABLE FILM SPEED ess rate for 2402, Kodak Plus-X Aerographic Film is apparent. A vital part ofany exposure determination Changes of 1/2 stop in effective film speed is the effective sensitivity or speed of the have been encountered with emulsion batch photographic film on which the subject is to changes making this an additional speed var­ be imaged. Wide variability will be encoun­ iable which must be controlled. Aerial tered not only between different film types photographs of consistently high quality are but also sometimes between emulsion possible, therefore, only ifthe film speed var­ batches of the same film type. Film­ iable is controlled. This may be ac­ processing systems also add a significant var­ complished by establishing two procedures: iable for consideration. It is imperative that a close working rela­ • Evaluate all emulsion batches ofevery film tionship be established between the aerial type for effective sensitivity and make ad­ photographer or person responsible for ex­ justments, if required, in exposure index to posing the film and the processing laboratory compensate for changes in emulsion sen­ sitivity. which is responsible for processing the ex­ • Establish process control curves for the posed films. Figure 7 illustrates potential var­ laboratory to use in maintaining process iability between processes with the same consistency so that there is minimal var­ SO-022, Kodak Panatomic-X Aerial Film. A iance in effective film speed due to process­ variability equivalent to 1 3/4 f-stops at a den- ing.

50-022 Film Kodak Panatomic-X Aerial Film

2 .•

2.0~III;~~~~~~ 2.21.8 ~~

.6 '2jiii'.,..._- .6 .9 1.2

RELATIVE LOG EXPOSURE

FIG. 7. Effective speed versus process. SHUTTER/APERTURE SETTINGS FOR AERIAL PHOTOGRAPHY 245

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_ ._ ••• , __••n_ U •••• ~--- Kodak Plus-X Aerographic Fil~ -2402 Fultron Processor; 14X-641

I I

.3 .. .9 u 1.B 2.1 2.4 2.7 3.0

Relative Log Exposure

FIG. 8. Effective speed change with gamma.

TEST EMULSION BATCH stop faster than standard" or "for high al­ Each emulsion batch should be tested by titude use only". the laboratory prior to use. To do this, sen­ sitometric exposures can be made both on the PROCESS CONTROL STANDARDS emulsion under test and on one in current Process control standards are established use; then the two sample films are processed for each film type with aerial application together. A density analysis ofthe results will compatibility being a major criterion. These show differences in speed, color balance, standards are maintained by a laboratory proc­ gamma, maximum density, and minimum ess control group, and prior to processing density. In most instances corrective actions any exposed aerial imagery the process is are possible ifdifferences do exist. It is also a sampled and evaluated to meet the standards good idea to test films with commonly used established. PTD maintains standards for camera filters to determine any gross prob­ each color film process and three standards lems with spectral sensitivity. PTD certifies for each black-and-white film process (one every emulsion batch received using these each for red, green, and blue filters) with the methods and in the case of 2443, Kodak exception of 2424, Kodak Infrared Aero­ Aerochrome Infrared Film, every cutofevery graphic film, which has a single standard for emulsion batch is. certified. Records are each process (with infrared transmitting fil­ maintained and the films are recertified ifthe ter). Once these standards are established, emulsion remains in use for six months or if the speed ofthe films are determined, and the the film is subjected to any questionable en­ film storage is adequate, a high confidence vironment. Stored with each emulsion are will be established that film-process vari­ pertinent notes regarding use; e.g., "one-half ables have been minimized. 246 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

VARIABLE OPTICAL SYSTEM vignetting filters may be labelled as percent The camera-aircraft optical system is transmission ofcenter versus edge, e.g., 40%, another important factor which impacts expo­ or as the exposure factor required, e.g. 2.2, to sure determinations. Camera lenses, camera correct for the center density of the filter. filters, and aircraft windows are the system Filters alone can add 3 f-stops or more to the components routinely encountered in aerial exposure required. In the case of unknown photography. spectral filters a sensitometric comparison with a known filter ofsimilar wavelength may Camera lens data either may be available be accomplished to determine the exposure from the manufacturer or may be determined factor. in the optical laboratory. Transmission should be measured at various wavelengths Aircraft windows are the remaining optical which cover those commonly recorded system variable commonly encountered. should special multispectral projects be un­ Windows are usually of high quality, high dertaken. It is not common, for example, for transmission glass and will present little lens transmission at shorter wavelengths problem. Perhaps the most severe window (400nm) to be one-half of the lens transmis­ problems are either cleanliness or condensa­ sion at longer wavelengths (700nm). tion caused by change in air temperature and altitude. Camera filters for spectral correction or spectral isolation along with anti-vignetting filters should be calibrated or checked so that MISCELLANEOUS the exposure or filter factor is known. Anti- There are added variables or considera-

RELATIVE lOG EXPOSURE

FIG. 9. Kodak Aerochrome infrared film Type 2443, visual densities. SHUTTER/APERTURE SETTINGS FOR AERIAL PHOTOGRAPHY 247

. 3.6

3.2.3.0 _ 2.'

2.6 2.2 2'''11_2.0 1.8

1.6 :::1.::°1111 ."

RELATIVE LOG EXPOSURE

FIG. 10. Kodak Aerochrome infrared film Type 2443, tri-color den­ sities.

tions which may affect exposure and are only sure determination; therefore, basic data for mentioned here so that they are not ignored two films are included here. when a photographic mission is planned. For example: EXAMPLE A Film: Kodak Ae.rochrome Infrared Film 2443 • Non-vertical photography from airborne Camera: Wild RC-8 with 150mm lens platforms requires special consideration Filter: Wratten 12 equivalent plus 2.2 Anti- because of the increased atmospheric path vignetting filter length influence. In general, the available light will increase and contrast will be re­ Altitude: 10,000 feet duced, appearing much the same as an in­ Exposure: 1/225 second at £5.6 crease in aircraft altitude. Absolute Log Exposure for visual density of 1.2: 8.9-10 m.c.s. • Aircraft speed versus altitude, VIH, will im­ Curves: Visual and tricolor density versus log pact exposure because image motion is af­ E data in Figures' 9 and 10. fected by VIH and the camera shutter speed 0 0 versus film speed ratio may be limited by Sun angle: 40 _60 aircraft operating parameters.

EXAMPLE B A STARTING PLACE Film: Kodak Plus-X Aerographic Film 2402 What is a basic exposure for a normal Camera: Hasselblad with 80mm lens scene? A starting place is essential for expo- Filter: Wratten 25 equivalent 248 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976

4.0

3.1

3.6

3.4

3.0 Versamat 11 C-M Processor 2.1 Kodak MX-641 Chemistry·

2.6

2.4

2.2

2.0

RELATIVE LOG EXPOSURE FIG. 11. Kodak Plus-X Aerographic film Type 2402.

Altitude: 8,000 feet nificant contributors to the background ofex­ Exposure: 1/250 second at £8.0 posure calculations, are gratefully acknowl­ Absolute log exposure fot visual density of edged. This work was performed at the 1.2; 9.06-10 m.c.s. NASA Johnson Space Center under Curve: Density versus log exposure curve as Technicolor Graphic Services, Inc., NASA Figure 11. Contract NAS-9-11500. Sun angle: 40°_60°

REFERENCES SUMMARY Determination of camera exposure set­ 1. M. R. Specht, N. L. Fritz, and A. L. Soren, tings, i.e., shutter speeds and lens apertures "The Change ofAerial Camera Exposure with for aerial photographs, is influenced by the Solar Altitude", Photographic Science and numerous variables described above. Con­ Engineering, Vol. 10, No.3, May-June 1966. trol ofthe variables may be accomplished ifit 2. John T. Smith, Manual ofColorAerial Photog­ raphy, first edition, American Society of is approached systematically.-Table 6 lists a Photogrammetry, Falls Church, Virginia, summary of steps which should be taken. 1968. 3. RCA, Electro-Optics Handbook, 1968, TCA, ACKNOWLEDGMENTS Commercial Engineering, Harrison, New Jer­ sey. The work and experience of Pete Stanley 4. Niels Jensen, Optical and Photographic Re­ (NASA) and the Aerial Photographers from connaissance Systems, John Wiley and Sons, ERAP along with Noel Lamar (NASA), sig- New York, 1968. SHUTTER/APERTURE SETTINGS FOR AERIAL PHOTOGRAPHY 249

TABLE 6. STEPS IN THE DETERMINATION OF EXPOSURE SETTINGS.

° Basic Exposure (a known starting place) -Reference* film/process (film calibration, certification) -Reference camera/lens -Reference shutter/aperture -Reference aircraft altitude -Reference sum elevation angle * Reference = a starting point parameter; e.g. those listed above

o New Parameters -Film/process effective recording speed. -Camera/lens transmission

° Sunlight Factor SEA Factor 90°-35° Normal 35°-25° Plus % stop 25°-15° Plus I stop 15°_10° Plus I % stops 10°_ 5° Plus 2 stops

° Altitude Factor 1000 ft. Plus V2 stop 5000 ft. Normal 25000 ft. Minus V2 stop Space Minus % stop

° Subject Factor Subject Factor Vegetation, mts Normal Sandy soil; e.g. Mojave Minus V2 stop Bright desert; e.g. Sahara Minus I stop Snow Minus I % stop Clouds Minus 2 stops Deep ocean Plus I stop Coastal ocean, shallow water Normal Heavy vegetation Plus 1 stop City (streets, buildings) Minus 1 stop

Result ° New Camera Settings

5. Jenkins and White, Fundamentals of Optics, 7. L. Jones, and H. Condit, "Sunlight and McGraw-Hill, New York, 1957. Skylight as Determinants of Photographic Ex­ 6. Kodak Filters for SCientifiC and' Technical posure", Journal of Optical Society of Uses, Eastman Kodak Book B-3. America, Vol. 23, No.2, February 1948.

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