BASIC EXPOSURE APERTURES, SHUTTER SPEEDS AND PHOTO TERMINOLOGY EXPOSURE Exposure is the response of a photographic emulsion to light. The emulsion is made of a material (silver) that is sensitive to light. When the silver halides (microscopic particles of silver bromide, iodide, fluoride or chloride) in the emulsion are struck by light a latent (invisible) image is formed. The developer changes this to a visible image, which is composed of black metallic silver. The amount of silver darkened depends on how much light struck the film and the degree to which the film is sensitive to light. Many types of film are available which differ greatly in their sensitivity to light, but each is sensitive over a fairly short range. The photographer must precisely control the amount of light that enters the cameras. If too much exposure (light) is given the image will be too dark, and will likewise be too light if the exposure is insufficient. If you consider what we have said so far about the film s response to light, you will understand why the film exposed in the camera becomes a negative image; that is, one in which the light and dark tones are reversed from the original scene. The image is made dark wherever light strikes it. The more light striking a given area, the darker it will be. Areas or parts of the subject which are light in tone (like the sky) will darken the film most. Dark areas will cause the least response (darkening) and tones in between will cause in between responses, depending on their relative brightness. On the negative we call the light, or thin, area (which received little or no light), SHADOWS. The dark, or dense, areas (which recieved t the most light) are called HIGHLIGHTS. The average negative should contain both highlights and shadows, as well as many tones or shades in between. The overall darkness of the negative is referred to as its DENSITY, and the degree of difference between the most dense (darkest) areas and the least dense (lightest) areas is its CONTRAST. EXPOSURE CONTROL The amount of exposure that a film receives is determined by two factors. One is the INTENSITY,or amount of the light that strikes the film and the other is the EXPOSURE DURATION,or the length of time the light is allowed to react upon the film The higher the intensity, the greater the exposure; and the longer the time, the greater the exposure. These two factors are controlled by the photographer. The camera contains an APERTURE to regulate the intensity (amount) of light passing through the lens, and there is also a SHUTTER to determine the duration (length of time) of the exposure. F/STOPS The aperture is an opening formed by blades or leaves in a circular shape that changes in diameter to determine the size of the hole through which the light will pass through. If
the subject is dark or poorly lit the aperture can be made larger to pass more light through. The term we us is OPENED UP. Conversely, if the scene is bright the aperture can be made smaller to prevent the film from being over exposed. The proper term is CLOSED DOWN. The aperture is controlled by a ring on the lens mount, for older cameras, andusually a wheel or toggle switch for newer cameras.the various positions of the opening are indicated by a series of numbers called F/STOPS and they tell the photographer what will be the intensity of the light he permits the lens to project onto the film. The series of f/stops used on most cameras are f/1, f/1.4, f/2, /2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, & f/32. These are the standard f/stops although some electronic cameras may show half stops that fall in between these standard stops. Most 35 mm cameras will not have the smallest and largest f/stops shown in this series. The largest size opening is formed at f/1 and it becomes progressively smaller as the number becomes larger, like fractions. (because they are fractions). It is important for you to remember that the larger the number, the smaller the opening, and viceversa. Most lenses do not have the entire range of f/stops. The largest opening (smallest number) on any lens is referred to as the SPEED of the lens. Lenses having large f/stops (f/1.4, for example, as compared to f/4) are called FAST lenses. This is because they are capable of permitting more light into the camera allowing you to use faster shutter speeds. You will notice the the f/stops indicated on most 35mm cameras are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, & f/22. Memorize these numbers and remember that f/1.4 lets in the most light and f/22 lets in the least light. You must also know the relationship between f/stops. The difference between one stop and another is a two times (2x) change in the amount of light transmitted. For example, if a given amount of light is passed at f/8, opening up to f/5.6 will give 2X (twice) as much light (because f/5.6 forms a larger opening). In the same way stopping down from f/8 to f/11, the next smaller opening, will decrease the amount of light by 2X (half) as much light. If we continue progression, f/16 transmits half as much light as f/11 and f/22 half as much as f/16. You can see that a change of one stop either cuts the light in half is we stop down, or transmits twice as much if we open up. A change of two stops would make a 4X change in the amount of light. For example, f/8 transmits 4X as much light as f/16 but only 1/4 as much as f/4. In the same way, a three stop change produces an 8X difference, and so on. SHUTTER SPEEDS The camera must also incorporate a timing device to control the length of time light will strike the film. this is the SHUTTER, and the most common type for 35 mm cameras is located at the back of the camera at the focal plane where the image is focused and is therefore called a focal plane shutter. It consists of a curtain with a slit or opening in it.
The curtain is either open or closed. During exposure it remains open for a pre-selected time, then closes. The shutter speeds (which are fractions of a second) will be marked on the exterior of the camera where you will be able to set them. On most 35 mm cameras you will find these markings: B, 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000. These are the standard shutter speeds although some electronic shutters may allow shutter speeds which fall inbetween the standard settings. Most newer cameras will have shutter speeds as short as 1/4000 and 1/8000 of a second and as long as 4, 8, or 16 seconds. The B allows you to manually control the length of exposure. As long as you hold the shutter release down in the B position the shutter will remain open. The other numbers are fractions representing segments of time, 1 being one second, 2 (really the fraction 1/2) being one half second and so on to the fastest speed, 1/1000 second. Memorize the series: 1 second, 1/2 second, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/500, 1/10000 second. The relationship that we saw between f/stops also exists between shutter speeds. That is, each change from one shutter speed to another results in an exposure change of 2X. 1/60 of a second is twice as long as 1/125, so that at a setting of 1/60 the film receives twice as much exposure as at 1/125; and 1/250 gives 1/2 as much exposure as 1/125. As a change of one setting to the next on the shutter produces the same change in exposure as a one stop change with the aperture, we refer to a change of shutter speeds as a change of so many stops, meaning that each stop change is an exposure change of two times. It is imortant to note that a change in either aperture or shutter speed by one unit is referred to as a one stop difference. This is a term you will hear often when describing camera settings, film sensitivity, and the amount of light. SELECTING EXPOSURE With such a range of f/stops and shutter speeds to choose from, how do you select the proper combination to obtain correct exposure? Under most circumstances you will use a light meter, either a hand held meter or the metering system built into most 35mm cameras. Correct exposure is based on these four variables: film sensitivity, light intensity, subject brightness, and direction of light. Film sensitivity and amount of light are the most important variables which you will need to reference constantly. Subjct brightness and direction of light are less important, but still need to be considered in certain situations.let s look more closely at each of these factors. FILM SENSITIVITY Films differ in several respects. The chief factor which differs from film to film is speed, or sensitivity to light. The greater the films speed, the more quickly it reacts to light. Films are assigned a numerical rating to indicate their sensitivity to light. This number is referred to as the film speed. this is designated as the ISO which represents a standardized method of rating film sensitivity. (In the recent past, the designation was ASA). A slow speed film might be rated from ISO 5 to ISO 30; a medium speed around ISO 100, and a fast film from ISO 500 up to about ISO 3200. These numbers are directly proportional, meaning that a film rated ISO 100 is twice as fast as an ISO 50 film and four times faster than one rated ISO 25.
Under identical conditions one film,being faster, might give the same density using 1/100 second at f/22 as a slower film using 1/100 at f/11. The latter exposure permits more light into the camera but the density produced will be identical with that of the first exposure with a film four times as fast. In computing exposure under daylight conditions we are going to correlate film speed with shutter speed. We use a rule commonly known as The Sunny 16 Rule. This rule states that to obtain correct exposure on a bright sunny day with a subject of average illumination you (1) Set the shutter speed to the number closest to the ISO of the film and (2) you set the aperture to f/16. If your ISO is 125 - set your shutter speed to 1/125. If your ISO is 400 - set your shutter speed to 1/500 etc. You do not always need to use a shutter speed that corresponds to the matching film speed, but we use it as a starting point that allows us to use the aperture to compensate for the other variables. Once you have determined the basic exposure you may use any other combination of shutter speed and f/stop which will give you an equivalent exposure. EQUIVALENT EXPOSURE This is any combination of F-stop and shutter speed that yields the same overall exposure. For example, f/16 @ 125 is the same exposure (total amount of light) as f/11 @250. In the second exposure, f/11 lets in twice as much light as f/16, but the shutter speed of 250 only lets the light in for half as long as 125. Twice the light for half the time will make the same or equivalent exposure. These two settings (assuming the amount of light is the same) will result in negatives that have the same density. F/22 @ 60 and f/8 @ 500 work in the same way and will also rusult in degatives that have the same density. They are all equivalent exposures. Remember, any exposure change on one side must be balanced by an equal change on the other. ILLUMINATION Light is an essential element for all photographers. No light no pictures. The type of light you use (natural sunlight, flash, incandescent, tungsten, etc) and its intensity will vary greatly depending on where you are, what you are shooting and the time of day and weather conditions. For now we will focus on natural (sun) light and outside shooting conditions. The amount of sunlight changes dramatically throughout the day, from dark, overcast early morning light to bright, sunny, light during mid-day. As the light changes, you will need to change your exposure accordingly to mantain the correct exposure for each scene. This change is most easily done by keeping your shutter speed constant and changing only the aperture to compensate for a different amount of light. The light meter built into your camera will be your guide to how much you need to change your exposure as the light changes. You may find yourself shooting in the shadow of a building or large tree on a bright sunny day, but your exposure will be very different than if you were shooting in the bright sun. Just the intensity of the sun, without clouds, can change by 7 or 8 stops throughout the day.
Remember, increasing expousre for darker lighting means using a larger aperture to let more light in. This would require using an aperture with a smaller number like f/4 instead of f/8. Conversely, decreasing exposure for brighter situations means using a smaller aperture to let in less light. This would require using an aperture with a larger number like f/16 instead of f/8. LIGHT METERS Photographers images live and die by being able to accurately judge the amount of light present and adjusting exposure for subtle changes in light. They do this by using a light meter, either the one built into their camera, or and hand held one. Most of the time light meters are very accurate, but occasionally they give the wrong information which the photographer must recognize and compensate for. Most of the time we shoot pictures that have a nice balance of highlights, shadows, and middle values. If we mixed all these values of light, we would be pretty close to an average middle value of light we refer to as middle or 18% grey. Light meters are designed to assume all of our pictures have this perfect range of values, and they recommend exposures based on that assumption. The truth is that the light meter has no idea what we are shooting, and this is why they sometimes don t give us accurate information. If our subject is mostly dark values, and the light meter assumes it is a middle grey, it will give us an exposure that over exposes our dark scene and makes it a middle grey. Not good. Conversley, if we are shoting in a situation that is mostly bright values (say the beach or the snow) and the light meter assumes the scene is a middle grey, it will give an exposure that underexposes the scene and makes it a middle grey. In both of these situations it is essential for the photographer to recognize the problem and compensate the light meters recommendation to achieve the proper exposure. This issue leads to the two less important factors to deal with for choosing the correct exposure that were mentioned earlier. LIGHT DIRECTION Another factor of illumination is light direction. On a bright sunny day or under a hazy sun the light is strong and will cast dark shadows. If the subject is SIDELIT, ie, the light is coming at the subject at a right angle, the side of the subject opposite the sun will be in shadow, causing the values to be darker than we want, throwing off the light meter. If you expose as you normally would the shaded area will be underexposed. To compensate we allow one stop increase in exposure (+1 stop exposure) for side lit subjects. With the sun completely behind the subject, it becomes BACKLIT and will be entirely in shadow. To compensate for this we must give an exposure increase of two stops (+2 stops exposure). Remember, for side lit subjects open one stop and for back lit subjects open two stops. SUBJECT BRIGHTNESS You know that when any object is struck by light some of the light is absorbed and some is reflected from the object. The image on film is formed by the light reflected and as objects differ greatly in the amount of light they reflect, we must consider the brightness of the subject of the picture when computing exposure. Because the subject may not have an average value, the readings from your light merter may be incorrect. To make
the proper exposure, a bright subject would require that we give more exposure by opening up, and a dark subject would require that we give less exposure, or close down. The subject brightness categories are: AVERAGE SUBJECT To be technical, an average subject is one which reflects 18% of the light striking it. That is an average gray. Probably most subjects will fall into this classification; people of medium color, clothing, most buildings, landscapes with trees, etc. DARK SUBJECT - (-1 stop exposure) A dark subject reflects only about 9% of the light striking it. It is only half as bright as an average subject. The light meter reads this as average and will indicate an exposure that rill render the subject middle grey. In reality it requires less exposure to make it darker. Therefore, for a dark subject, close down one stop (an decrease in exposure of 1/2). People of dark color, dark buildings, dark animals, etc. would fall in this category. BRIGHT SUBJECT - (+1 stop exposure) A bright subject reflects twice as much light as an average subject. Again, the light meter, assuming the subject has average luminance, will indicate an exposure thet will make the subject middle grey. Since it is really twice as bright as an average subject it requires 2x the exposure, or an increase of one stop. Bright subjects might be fair skinned blondes, people in light clothes, light colored buildings, etc. BRILLIANT SUBJECT - (+2 stop exposure) A brilliant subject reflects four times as much light as an average subject and requires two stops more exposure. A brilliant subject might be a white sailboat against the sky, people in light clothing on the beach, snowscapes, etc. These are all basic guidlines designed to familiarize you with some of the concepts and terminology we will be dealing with in the mechanics of photography. These are universal concepts that work the same way whether you are shooting with film or shooting digitally. All of these concepts will be dealt with in depth individually. *This information was gathered by Ken Slosberg at Orange Coast College and modified by Jonathan Fletcher.