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

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.

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 fast shutter speed should be combined with high-quality (slow) film and a high f-stop setting. However, this combination is not always possible in practice. SFAP, thus, represents a trade-off involving these factors.

Digital cameras generally operate in the same way with shutter speed and f-stop as the primary means to adjust the amount of light reaching the electronic detector array. Most SFAP is conducted under sunny conditions, so a fast shutter speed may be employed to produce sharp pictures from a moving platform. Many digital cameras have a sports or action mode that uses a fast shutter. Likewise some cameras allow selection of shutter priority in the camera setup. Some digital cameras also have built-in shake or vibration stabilization, again to acquire sharp pictures when the camera is moving.

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.

Given these and other factors, it should be clear that a photographic image of a scene would 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, which would not be suitable for SFAP.

Practical digital color-infrared cameras have been developed and marketed by Tetracam. The size, weight and functionality of these cameras make them useful for small-format aerial photography from manned or unmanned platforms (Aber et al. 2009). The primary purpose is to collect vegetation indices for agriculture, foresty, and related applications. With prices starting around $4000, however, flying one on a kite might be rather risky!

Tetracam multispectral cameras.

Related sites

References


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ES 555 © J.S. Aber (2015).