ES 555 Lighting and exposure

Lighting and Exposure
ES 555 Small Format
Aerial Photography
James S. Aber

Table of Contents
Introduction Exposure control
Light meters Human eye
Color-infrared Related sites

**Table of Contents**
Introduction	Exposure control
Light meters	Human eye
Color-infrared	Related sites

Introduction

The most important variable for successful outdoor photography is correctly exposing the film or CCD to the available light. Natural light is tremendously diverse in its characteristics, which include direction, diffusion, harshness, and color (Zuckerman 1996). Thus, natural light varies with season, time of day, latitude, altitude, cloud cover, humidity, dust, and other ephemeral atmospheric conditions.

To achieve gentle illumination and warm colors, professional photographers prefer the lighting conditions just after sunrise or before sunset, when a diffuse golden glow fills the sky.
However, typical aerial photography is taken during mid-day hours when the sun is relatively high in the sky, in order to provide for full illumination or toplighting of objects as seen from above. For purposes of the following discussion, we will assume that SFAP takes place under sunny sky between mid-morning and mid-afternoon hours.

Controls of Exposure

The lens aperture and shutter speed are fundamental controls of how much light reaches the film. Film speed determines how much light is required for correct exposure of the photographic emulsion. Each of these factors will be examined in turn.

Lens aperture -- The diameter of the lens opening is given by a value called f-stop, which is defined as lens opening diameter divided by lens focal length. F-stop is thus a fraction. On most cameras, f-stop values are arranged in sequence, such that each interval represents a doubling (or halving) of light values. Smaller f-stop values mean more light entering the camera; larger values indicate less light. The typical sequence of f-stops is given below (Shaw 1994).

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

Shutter speed -- The length of time the shutter remains open is called shutter speed. On most cameras, shutter speed values are arranged in sequence such that each interval is twice as long (or short) as the next. In other words, changing shutter speed by one interval either doubles or reduces by half the time interval. Longer time intervals mean more light entering the camera; shorter intervals indicate less light. The typical sequence of shutter speeds is given below in seconds (Shaw 1994).

1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000

Film speed -- Different photographic emulsions on film vary greatly in their sensitivity to light--some require very little, others need much light to properly record an image. This factor is known as film speed, which is indicated by a standard ISO rating. Low ISO values indicate "slow" films that need much light. High ISO values, in contrast, are typical of "fast" films that require little light. Possible film speeds are displayed below. The main speeds are given in bold, and intermediate speeds are shown for one-third stop increments (Shaw 1994).

12, 16, 20, 25, 32, 40 50, 64, 80 100, 125, 160 200, 250, 320, 400, 500, 640, 800

Settings for film exposure are best understood using the concept of stops. A stop is defined as doubling or halving of any factor that effects the exposure (Shaw 1994). Notice that values for each variable--shutter speed, f-stop, film speed--are arranged by doubling (halving) intervals. Among these variables there is a relationship called reciprocity. In other words, changing one variable by one stop can be exactly matched by changing another variable in the opposite direction by one stop.

For photographs under bright sun, the sunny f/16 rule can be employed (Caulfield 1987). For a given film under full-sun conditions, the shutter speed should be the (approximate) inverse of film speed at f/16. For example, ISO 100 film should be exposed at f/16 and 1/125 shutter speed.

Comparison of equivalent exposure factors, based on the sunny f/16 rule.
Film ISO	Shutter speed*	F-stop
100	125	16
100	250	11
50	60	16
50	125	11
200	250	16
200	125	22

* Only the fraction denominator is given.

The sunny f/16 rule can be modified for subjects that are intrinsically bright or dark (Caulfield 1987). Snow, sand, or polar bears, for example are intrinsically bright. For photographing these subjects, the sunny f/16 rule becomes the sunny f/22 rule, in other words the correct exposure is the reciprocal of film speed at f/22. A corrollary is the sunny f/12.6 rule for dark subjects, such as black soil, burned wood, etc.

The exposure factors have other influences on the resulting photograph. In general faster shutter speeds are desirable for SFAP in order to "freeze" the motion between the airborne camera and the ground. This necessitates using fast film (high ISO ratings) and/or lower f-stops. Fast films (ISO > 100) tend to be poor in quality, due to grainy appearance, in comparison to slower film. Lower f-stops reduce the depth of field, which refers to the range of distance over which the image is in good focus. For ideal SFAP, a very fast shutter speed should be combined with high-quality (slow) film and a high f-stop setting. However, this combination simply does not work in practice. SFAP, thus, represents a trade-off involving these factors.

Light Meters and Automatic Cameras

Most modern cameras employed for SFAP utilize built-in light meters and automatic adjustment of shutter speed and f-stop. Such automated photography introduces certain artifacts in the picture taking process. Inexpensive point-and-shoot cameras normally have a light meter that is separate from the lens or view finder; whereas the light meter in a single-lens reflex (SLR) camera operates through the lens. The latter is clearly preferable, as the meter registers the light actually entering the camera through the lens. In either case, such light meters measure the amount of light reflected from the ground, not the amount of incident light that illuminates the ground. To measure incident light, a separate handheld light meter must be employed.

Cameras normally determine exposure settings based on a medium gray tone to represent the average value sensed by the light meter. This works well for a scene in which most objects are uniformly lighted--for example blue sky or green grass. However, the results may be unsatisfactory for a scene made up of bright highlights and dark shadows, such as sunlit mountain tops and valleys in shadows. In this case, the highlights will be overexposed and washed out, while the shadows remain dark and lacking in details. Special photographic techniques and experience are necessary to deal with such lighting conditions.

Photography versus the Human Eye

Photographs are often considered to be "accurate" portrayals of visual scenes. However, photographs do not record images in the same way that the human eye would respond to the identical scenes. For the following discussion, we will ignore special techniques, such as use of artificial lights (flash), filters, unusual films, etc. The discussion will focus on natural-light photography under sunlit conditions. There are several fundamental ways in which photographic images differ from what is seen by the human eye.

Field of View -- Camera lens focal length determines the field of view which is focused onto the film. A focal length of ~100 mm (for 35-mm format film) approximates human vision in the central zone of the visual field, in which human color discrimination is best. A wide-angle lens compresses more field of view onto the film; whereas a telephoto lens severely limits (crops) the field of view. The human eye provides for an extremely wide angle of peripheral vision--160° to 170° in most people. This peripheral vision is good for recognition of black-white patterns and detection of movement, but has almost no color capability. Photographs do not provide any peripheral field of view.
Latitude -- Latitude refers to the range of highlights and dark features that are properly exposed in a photograph. Color-slide film has a total latitude of about five stops (Shaw 1994). Assuming the mid-tone of the scene is correctly exposed, this means that brighter or darker features also will appear correct within ±2½ stops. Features more than 2½ stops brighter will be "burned out" and objects more than 2½ stops darker will be all black. Digital cameras generally have more restricted latitude. The human eye, in contrast, has a much greater latitude equivalent to 12-14 stops.
Color Saturation -- The color rendition of color film may be different from the human eye. People tend to prefer vibrant, rich, saturated colors in photographs. Film makers have responded to this preference in different ways. Fuji's approach is for color-saturated films; whereas Kodak films render color as close as possible to reality (Zuckerman 1996). Similar differences are apparent in digital cameras.

Given these and other factors, it should be clear that a photographic image of a scene will be different in several ways from the human visual impression of that same scene. The photograph represents a selection of certain elements--field of view, exposure range and color saturation, which are in general more limited than a human observer would sense. On the other hand, a photograph is a permanent record of the scene, while the human visual impression is stored as memory that cannot be reproduced fully for analysis or sharing with others.

Color-Infrared Film

Color-infrared film is sensitive to visible and near-infrared portions of the spectrum. In normal practice, a yellow filter is employed to eliminate blue (and ultraviolet) wavelengths. In some cases, orange or red filters may be used to further restrict visible light from reaching the film. Color-infrared film carries no ISO number; nor do conventional light meters provide correct indications of infrared radiation. Without an ISO bar code on the film case, most cameras cannot make automatic settings. Therefore, taking photographs with color-infrared film requires manual settings for exposure based on estimates of available light. When using a film without an ISO rating, most cameras will default to ISO 100 for setting adjustments. The following table gives manual corrections for color-infrared film.

Manual compensation for SFAP color-infrared
film for default value of ISO 100.
Lighting conditions	Exposure correction*
Very bright sun, mid-day--clean, dry atmosphere	ISO 200 (+1 f-stop)
Light, but not bright sun--hazy, humid, dusty air	ISO 160 (+½ f-stop)
Slightly overcast, indirect light--thin clouds	ISO 100 (no correction)
Pale, diffuse light--early morning or late afternoon	ISO 80 (-½ f-stop)
Overcast, indirect, rather dark--heavy clouds	ISO 50 (-1 f-stop)

* Based on Pentax SLR camera with 50 mm lens
and orange filter (Marzolff, pers. com. 1998).

Given that most SFAP will take place under bright, sunny conditions, color-infrared film can be treated as ISO 200 according to Marzolff's settings. Following the sunny f/16 rule of thumb, the camera setting should be equivalent to shutter speed 1/250 and f/16. However, other camera/lens combinations may produce quite different results. The author formerly used a Canon Rebel SLR camera with a zoom lens for color-infrared kite aerial photographs (Aber et al. 2001). Best settings are 1/250 shutter speed and f-11 for full sun and active vegetation. Aside from these empirical results, aerial photography with color-infrared film remains an uncertain proposition--results cannot be predicted well. A final, and nearly insurmountable problem is that most photo laboratories are no longer willing to develop color-infrared film.

Digital infrared imagery is also possible. The CCDs of most digital cameras are sensitive to near-infrared radiation; however, some cameras exclude infrared energy with a filter. For those digital cameras without infrared-blocking filters, it is possible to acquire infrared images by blocking visible light with a filter. The only drawback at present is the relatively low sensitivity of conventional CCDs to near-infrared radiation. Thus, relatively long exposure times (1/30th second) are required.

Recently a practical digital color-infrared camera has been developed and marketed by Tetracam. The size, weight and functionality of this camera make it suitable for small-format aerial photography from manned or unmanned platforms. Its primary purpose is to collect vegetation indices for agriculture, foresty, and related applications. With a price tag of $5000, however, flying one on a kite might be rather risky!

ADC camera by Tetracam.

Related sites

More information on color-infrared film from Kodak.
Digital infrared photography by Eric. H. Cheng.
Infrared photography FAQ from CoCom, United Kingdom.

References

Aber, J.S., Aber, S.W. and Leffler, B. 2001. Challenge of infrared kite aerial photography. Kansas Academy Science, Transactions 104:18-27. See full article.
Caulfield, P. 1987. Capturing the landscape with your camera. Amphoto, Watson-Guptill Publications, New York, 160 p.
Shaw, J. 1994. John Shaw's landscape photography. Amphoto, Watson-Guptill Publications, New York, 144 p.
Zuckerman, J. 1996. Techniques of natural light photography. Writer's Digest Books, Cincinnati, Ohio, 134 p.

	To achieve gentle illumination and warm colors, professional photographers prefer the lighting conditions just after sunrise or before sunset, when a diffuse golden glow fills the sky.
	However, typical aerial photography is taken during mid-day hours when the sun is relatively high in the sky, in order to provide for full illumination or toplighting of objects as seen from above. For purposes of the following discussion, we will assume that SFAP takes place under sunny sky between mid-morning and mid-afternoon hours.