[This is a compendium of information on photographic exposure control assembled from Austin Community College lectures by Lynn Jones and Mark Daughn and from several photography books, including Bruce Warren's Photography (West, 1993).]
Light is a form of electromagnetic energy made up of waves made up of particles called photons.
When light reacts with photographic film, photons strike and partially disrupt molecules of silver bromide or silver iodide in the film emulsion. The disrupted molecules constitute a latent image that can then be chemically developed into a visible image on the film and/or on photographic paper. The silver salts reduce to metallic salts, and the density of the film image is the accumulated silver. The more silver in an area of an image, the denser that part of the image is and the darker it appears.
A point source emits light in all directions from a single point (or approximation, like a light bulb) and produces hard edged shadows when it falls on a subject. According to the inverse square law, light intensity from a point source varies inversely as the square of the distance from the light source.
As a light source is moved nearer or further from an object, its intensity changes geometrically: the same amount of light is spread over a smaller or greater area as the distance decreases or increases. Cutting the distance by half quadruples the illumination, and doubling the distance quarters it.
Different colors of light have different wavelengths, measured in nanometers (millionths of a meter): Violet=400, Blue=500, Yellow=600, Red=700.
In inches, the size range for the spectrum of visible light is .000016 to .000028 inch. Ultraviolet rays and Infrared rays have wavelengths just below and just above the wavelengths of colors visible to the human eye.
The color of light can be correlated to temperature and compared to the color of a heated body (like iron) at various temperatures. Color temperature is the temperature at which a given color is emitted by an object that absorbs it perfectly, measured in degrees Kelvin. Just as blue has a shorter wavelength than red, so it also has a higher temperature.
Film is material sensitive to light. The lens of a camera gathers light from the subject and forms an image on the film. Film in a camera provides the means to record an image; the camera lens provides the means to resolve the image (bring it into sharp focus).
Daylight balanced color film is set for 5500 degrees Kelvin, the temperature of direct sunlight at midday and roughly that of incandescent filament bulb lights. Tungsten balanced color film is set for 3200 degrees Kelvin, roughly the temperature of fluorescent lights.
Since the higher the temperature, the bluer the light, using daylight film with fluorescent lighting gives colors a reddish cast, and using tungsten film with daylight gives colors a bluish cast. These effects can be adjusted for with filters.
Contrast is sharpness you can see: images with strong distinctions between highlight and shadow areas look sharp even when they aren't quite in sharp focus.
Print (negative) film captures about 50% of the contrast on the subject in real life. Slide (positive or transparency) film captures 135%. Put another way, the contrast ratio of slide to print film is 2.75:1.
A film's latitude is the degree to which it can produce a usable image even if given less or more exposure than its film speed calls for.
Slide film is tolerant of underexposure; it's best never to be more than half a stop overexposed. Print film is tolerant of overexposure; it's best never to be more than half a stop underexposed.
The reactions of a particular film to a sequence of increments of exposure can be graphed as its characteristic curve. Where the curve rises diagonal to the axis, every increase in light results in a proportionate increase in density. The flattening of the curve at the low end of the exposure range indicates underexposure--the film doesn't see differences between light and dark. The flattening of the curve at the upper end indicates overexposure--the film doesn't see details in highlight areas.
A film's exposure depends on subject illuminance and reflection and camera settings
(fstop and shutter speed).
Exposure = illuminance of the image multiplied by time.
Illuminance of the image = amount of light falling on and reflected from a subject
Time = how long the image falls on the film.
Fstop (f stands for "factor") is the camera mechanism to control illuminance. A camera lens collects the illuminance of the image and projects it on the film through a variable sized opening called the aperture. The size of the aperture determines the volume of light that strikes the film; fstop is the exposure index for aperture sizes.
Fstop settings are readable as fractions. The larger the aperture number, the smaller the physical opening through which light strikes the film. For example, when a lens is set at f16 the opening is 1/16th the total area of the lens surface, or when a lens is set at f4, the opening is 1/4 of that area--and 1/4 is bigger than 1/16.
The "fnumber" of a lens can be calculated by dividing its focal length by the effective diameter of its lens aperture (if aperture is 100mm in diameter and focal length is 50mm, result is 1/2 and number is f2). Though different lenses set at a given fstop have different sized openings, the amount of exposure produced is the same.
Changing one fstop changes exposure by a factor of two. Opening up one fstop doubles the exposure; stopping down one fstop cuts the exposure in half.
Some lenses include "fractional"; fstops (like 1.2, 1.8, 3.5, and 4.5). Changing from one of these to the next higher or lower one is not a full stop difference in exposure. Many lenses have click settings halfway between stops, and because aperture size change is continuous, it's OK to position the aperture speed dial anywhere between stops.
The more closed down the lens aperture, the greater the depth of field (the span between nearest and farthest distance acceptably sharp in the image). The more opened up the aperture, the smaller the depth of field.
When an image is in focus, a point on the subject is a point on the film. But when it is not, a point on the subject is a circle of light (called a circle of confusion). The smaller the aperture, the narrower the cone of light that falls on the film--and thus the smaller the circles of confusion and the sharper the image.
Film is classified according to its measured sensitivity to light, and the scale is an exposure index of film speed numbers. The most widely used index is that adopted by the American Standards Association and the International Standards Organization (ISO).
Each number in the ISO scale represents a 1/3 stop change, and the ISO doubles every third number. Each speed in bold above varies from the one above or below it by 1 fstop.
The ISO for a film indicates the amount of exposure required to produce an image of "normal" density. The higher the ISO number, the faster the film. The faster the film, the more sensitive to light it is, and the less light is required to expose an image on it.
The lower the ISO number, the slower the film. The slower the film, less sensitive to light it is, and the more light is required to expose an image on it.
Both contrast and latitude correlate to film speed. Films are low contrast if large changes in exposure result in small differences of density, and high contrast if small changes in exposure result in large differences of density. Films with "fast"speeds (ISO 400+) are less contrasty and have more exposure latitude than "slow" films. Since medium and fast speed films exhibit less tonal contrast than slow films, they may be more suitable for subjects that have both bright highlights and deep shadows.
Shutter speed is the camera mechanism to control time. It measures duration of time light falls on the film. The shutter speed exposure index is based on length of time the shutter is open, expressed in seconds and readable as fractions.
For practical purposes, each shorter shutter speed is half the preceding one: each change in shutter speed alters film exposure by a factor of two.
The longer the time, the more any movement (of either subject or camera) is blurred. The shorter the time, the more movement is frozen.
Though technically the term stop applies only to the aperture, it is commonly used to describe exposure changes made with shutter speed and manipulations to film speed as well.
According to the law of reciprocity, the effect of exposure on the film [the density of the image] is the same regardless of the rate at which the exposure is achieved.
(Reciprocity fails at long or very short exposure times, giving less density than expected. The correction is to increase the exposure time or aperture size.)
Since both shutter speed and aperture settings are based on a factor of two, the same film exposure can be achieved with a number of different shutter speed and fstop pairs. Thus 1/500 @ f8 and 1/125 @ f16 and 1/30 @ f32 all produce the same amount of exposure.
Changing the aperture fstop by one setting doubles or cuts in half the amount of light reaching the film. So does changing the shutter speed by one setting. And so does changing the ISO by one fstop (e.g. 400 to 200).
Some cameras have an override mechanism to adjust the fstop determined by the in- camera meter. Here are the common override settings and their effects:
|+1||2 times the exposure|
|+2||4 times the exposure|
|+3||8 times the exposure|
|+4||16 times the exposure|
|-1||1/2 the exposure|
|-2||1/4 the exposure|
|-3||1/8 the exposure|
|-4||1/16 the exposure|
Underexposure results in loss of detail in the dark subject tone areas, while overexposure results in loss of detail in the highlights.
For negative/print film, the best exposure is the least exposure that retains detail in the areas representing the dark tones of the subject. For positive/slide film, since transparencies get lighter with increasing exposure, the best exposure is the greatest exposure that retains detail in the light subject tone areas.
Bracketing is making multiple exposures with different camera exposure settings, intentionally over- and underexposing film relative to the metered light reading. Bracketing is especially necessary when working with slide film, which doesnt tolerate error or lend itself to darkroom manipulation the way print film does.
Slide film can be overexposed by adjusting its ISO down one or two settings (e.g. 100 to 64 or 50) and then underdeveloped (by reducing the amount of time it's left in the developer); this is called pull processing, The purpose/effect is to reduce contrast.
Print film can be underexposed by adjusting its ISO up one or two settings and then overdeveloped (by increasing the amount of time it's left in the developer) to resolve as much of the image as possible; this is called push processing. The purpose/effect is to deal with low light situations or increase depth of field.
To compensate for incorrect settings, pull processing may save overexposed film and push processing may save underexposed film. But such processing will affect color rendition, grain structure, and image contrast.
The main reason to add light sources to existing light is to achieve printable images on film! Sunlight or any sort of ambient light may not "work" as is. In a bright sunlit scene, the human eye can distinguish 200 levels of brightness, but film compacts that range greatly, and then reproduction compacts it even more.
Though many scenes are 6-8 levels or stops in brightness range, with strong contrasts between lights and darks, and though some film can capture up to 10, only 4 will reproduce in print (via the 4-color process) after the photo goes through scanning and color separation.
The exact reproducible range is 4 1/3 stops--18% gray plus or minus 3 1/3 stops, to white with detail one way and to black with detail the other.
The 18% in 18% gray refers to the percent of light reflected off a subject. The gray is tonality, not color--every color has it. The Zone System of exposure control makes use of the gray scale (with the aim of reproducing specific tones of a subject as tones in a print):
|2||2.25% gray||black with detail|
|6||36% gray||"white" skin|
|8||144% gray||light source|
The purpose of manipulating lighting and camera settings for photographic exposure is to "Get the range in the range that'll fit." The aim is to get enough detail to print, to push the scene into reproducible range.
Light can be added to with reflectors or reduced with diffusers, and various kinds of flash devices, and spot and strobe lights can be directed at the subject to change its ambient illuminance. About 80% of location shooting, like shots of people outdoors, requires such control—it's not possible to fight the sun.
With flash photography, exposure is controlled by changing lens aperture, power of flash, or subject to flash distance--the shutter speed is constant (most cameras synchronize flash at 1/60 second). The duration of the electronic flash that lights the subject is much shorter than the time the shutter is open.
An incident light meter measures the light falling on a subject and typically will give the correct tonality of the subject. That is, if the camera settings match an incident reading, a black subject will look black in the resulting image and so on. An incident reading tells nothing about the subject per se but is a good place to start for setting the camera.
A reflective light meter (also called a spot meter because it typically has a very narrow angle of view) measures the light bouncing off/being reflected from a subject and tells how to record that subject at 18% gray. Reflective metering is a way of gauging the relationships of tonality in an image—specular highlights, shadows, and diffused areas are all relative to 18% gray. A reflective meter reading takes into account both the light falling on the subject (incident light) and the light being reflected off the subject (reflected light), so it's accurate. But the reading must be adjusted to fit the subject and desired image relationships. Otherwise, light subjects will be overexposed and dark ones underexposed.
Contrast control is adding or subtracting light from the subject to control the background and effect, mixing ambient and flash light sources.
Since past 3 1/3 stops up or down from reflective meter reading is black or white, with enough flash power (4 stops), you can make night of day or vice versa.
Additive contrast control means adding light to the subject to control the tonality of the background, to darken it.
Subtractive contrast control means taking light away from the subject (say by using a scrim) to control the tonality of the background, to lighten it.
|0 background||same reading for background and subject|
|+1 background||one stop more light than on subject|
|+2 background||two stops more ("airy")|
|-2 background||two stops less|