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Exposure means the same in photography as it does to getting a suntan. Referring to the intensity of light and the length of time that light it is allowed to act on photographic material.
The light intensity is determined by the aperture or f-stop setting.
The length of time is controlled by the shutter speed.
To be technically correct, exposure is measured in a term called steps. However, photographers seldom use this term. The term in common usage is f-stop or f-stops.
Intensity of Light and Length of Time
Exposure is a combination of light intensity and the length of time that light is allowed to remain in contact with the film. Without considering both the length of time and the intensity of the light, the concept of exposure is meaningless.
Exposure is measured in steps of f-stops. An increase of one stop (+1) means the exposure has been doubled the exposure. A reduction of one stop (-1) means that the exposure has been reduced by half of what it was.
If exposure is increased one stop, it doubles, decrease one stop and it is cut in half.
This term is loosely used to include a change in the shutter speed as well as a change in the lens aperture. An important thing to remember is that a change of one shutter speed or one f-stop corresponds to a change in the exposure of one stop.
An Example From the Beach
Think of getting a sun tan. Bright light results in a tan in a short time. Less bright sunlight can give the same tan, but it takes longer.
Lets consider a day at the beach. Just above you is a sliding door, and above the door are Venetian blinds. You will only be exposed to the sun when the door is open and the Venetian blinds are open.
When the door is open the intensity of the light is controlled by the blinds.
If the blinds are fully open the full intensity of the sun will be present. If the blinds are partly closed there will be less light reaching you.
What we have here is a situation similar to what happens inside the camera. Here you are the film. The door is the camera's shutter and the
blinds are the camera's aperture, or f-stop.
Now take the sensitivity of human skin into account and our example includes different film speed as well. The higher a film's ISO number the more sensitive to light that film is.
So, if you burn easily, your skin is fast. Here the term 'fast' refers to the fact that your skin, or film, reacts quickly to the action of light. This introduces the terms fast, medium speed, and slow films. These terms refer to the sensitivity of a film. You could say that a fast, sensitive, film results in a faster shutter speed. That may be where the term came.
Photographic exposure functions just like the above explanation. The shutter speed will control the length of time that the film is exposed to light. Light intensity is controlled by the aperture.
Choose low intensity light (a small aperture), acting for a long time (a long shutter speed), or bright light (a large aperture), acting for a short time (a short shutter speed). time.
Let's talk about exposure in photographic terms. Assuming an exposure of 1/60 of a second at f8 (1/60 @ f8).
To increase the exposure (give the film more light or more time) by one stop. Either the shutter speed or the aperture to adjust the exposure.
First, consider changing the shutter speed . To increase the light keep the shutter open longer, so change the speed from 1/60 to 1/30. This lets in twice as much light (1/30 of a second is twice as long as 1/60 of a second).
To do the same thing using the aperture open the lens
one f-stop. So, set the lens at f5.6, this is one stop larger than f8.
Spot Meters, Averaging Meters, and Multizone Metering
In Chapter One we briefly mentioned these two forms of metering. The averaging meter does just what it implies: it gives one overall exposure reading for the entire scene that you see through the viewfinder. The spot meter will meter only a small portion of the entire frame, perhaps only two or three degrees in the very center of the viewfinder.
Both spot and averaging meters are used extensively. For example: A spot meter will easily give you the exposure on a persons face for a portrait shot. It can be used to determine the exposure on a road side sign when the sun is setting in the background and you want to expose for the sign and not the bright sun. Note, if you do this be very careful not to damage your eye by looking at the sun, especially if using a telephoto lens. You can do permanent eye damage so be careful. The spot meter is perfect for metering one or more specific elements on the photo. We have used a digital camera with a spot meter to photograph the tops of utility poles because an averaging meter would have read the sky and silhouetted the pole top and we wanted the pole top equipment not a silhouette.
The Averaging Meter
Your averaging meter will do a fine job for just about everything. In fact many cameras have only an averaging meter. This meter will take all the dark and light elements in the frame and give you a single exposure reading for the whole
thing. This is almost always acceptable. Watch out for bright spots, reflections, lights, the sun, anything that might fool the meter into thinking that the entire scene is that bright. In this situation some experience will go a long way. The next time you fact this situation take several shots of varying exposure and note the settings you used.
Wether you use the spot or averaging meter it will give you an exposure reading composes of a shutter speed and an aperture setting. Depending on the camera you may be able to choose an aperture and the camera will choose the shutter speed or you choose the shutter speed and the camera will choose the aperture. In manual mode or with a manual camera you will need to choose both the shutter speed and the shutter speed. This will be especially true if you are using a hand-held exposure meter. This and other types of metering that sense the light that has entered the camera are called through the lens, TTL, metering systems.
The type of metering will be built into your camera and will determine the exposure by reading the light from several locations of the picture frame. This allows the camera's microcomputer to analyze the light from three to sixteen different locations in the scene and set the cameras shutter and/or the aperture for proper exposure.
While multizone metering covers the entire viewfinder the number location and configuration of its various light sensitive locations appear only in the camera manual, not in the viewfinder. This is the type of metering most often used in cameras with automatic exposure control because of its excellent ability to determine the correct exposure for almost any scene.
Using an Exposure Meter Correctly
Your exposure meter will allow you to determine the exposure of the scene or subject and it will allow you to accurately determine the contrast range of that scene by measuring the lightest and darkest portions of the scene. With indoor lighting this allows you the opportunity to adjust the lighting before the shot is made. Outdoors you will need to use reflectors or recompose the shot. By doing this shadows and backgrounds that are too dark or light can be found and adjusted by fill lighting or by being toned down.
Using a Reflectance Meter
This type of light meter reads the light being reflected or coming from the subject and is likely the one you are most familiar with. Almost all cameras with light metering systems determine exposure from the light coming from the subject and this type of meter is no different in that respect. Using this meter is simplicity itself, just make sure that the correct film speed is entered, point the meter at the subject read the light and you are done.
These meters can be used in averaging mode or in spot mode. Whichever mode you choose be sure to pay attention to the most important element in the picture. If that element is overly dark or light, you may need to compensate for it by increasing the exposure for a dark element and decreasing exposure for a light one. This is a perfect situation for a spot meter by which you can determine the precise exposure for your light or dark element.
Using an Incidence Meter
This type of meter you are likely not familiar with. It determines exposure by metering the light falling on the subject. Most light meters have an incidence metering attachment built in. You will see a white plastic hemisphere that can be slid in front of the light-sensitive cell. That is the incidence metering attachment. To use the meter in incidence mode just slide the white hemisphere over the photo cell, point the meter toward the camera, and determine the exposure. What could be easier?
Using an 18% Photographic Grey Card
An 18% grey card is a cardboard card that has a reflectance of 18%. That means that the card is designed and manufactured so that it will reflect 18% of the light that falls on the card and is the medium tone to which all exposure meters are calibrated. A grey card will be used when you are taking a picture of a scene that cannot be directly metered because, let's say, you cannot get close enough. In that case use the grey card to read the light at your location and use that reading to make the exposure. This will work well as long as the light on the card is similar to the light illuminating the scene to be photographed. When using this, or the white card technique below, be prepared to bracket your exposure. That means to make several exposures at values surrounding the exposure you determine. Like this: Let's say you determine an exposure of email@example.com. to bracket this reading make one exposure at 125@f4 and 125@f8. These are one-stop less and one stop more than firstname.lastname@example.org. Or you could adjust the shutter to do the same thing by taking the shot at email@example.com and firstname.lastname@example.org. By varying either the shutter or the aperture you increase and decrease the exposure by one stop.
When used with your camera the grey card must totally fill the frame so that the camera's meter sees it and nothing else. When used with a hand held meter take care to be sure you are only metering the card. Grey cards can be found at advanced camera shops and in publications dealing with exposure.
Using a 90% White Card
This technique will alloy to determine a working exposure when in very dim conditions. As you will likely not have a 90% reflectance piece of paper with you a page from your notebook can be used. This will work wonders when in a castle, cave, mine, or anywhere else that is dimly lit. So, when the subject or area that you are working with is too dark to get a decent exposure reading here is a technique that may allow you to get the shot. Your white paper will be far more reflective than a grey card, about 5 times actually, so you will need to modify the exposure that your camera gives you by multiplying the exposure found using the white card by five. Like this: Let's assume that by reading a white card you determine the exposure to be 5seconds @ f2. By multiplying the time, 5 seconds by 5 we determine the exposure to be 25 seconds @f2. Well, most cameras do not have a 25 second setting so use the time or bulb setting to keep the shutter open.
While this sounds a little wild, to be sure, you would do well to remember it especially when traveling. There are many times that you will find yourself in old dark castles, buildings, or outside at twilight. When all else fails this is one more arrow in your bag of tricks that may allow you to get the shot when other people are packing up their gear.
With a digital or other camera that will not allow you to increase the shutter speed beyond 15 seconds, or so, there will be nothing you can do to capture an image that requires a longer exposure time. So when you go shopping be aware of the limitations of the camera you are buying.
Tying It All Together
When using ISO 200 film how much exposure difference is there between someone using ISO 25 film? How many stops difference are there between films?
Every time the ISO number is doubled the film speed that's one stop. So:
You have three stops difference in the films.
Your film needs three stops more light that your friend.
Suppose your friend is shooting at 1/125 second at f8 (1/125 @ f8).
If you wish to shoot at 1/125, your f-stop must be f2.8 (that's three stops more light than f8).
If you need to take the picture at f8 you must change your shutter speed to 1/15 second (this gives three stops more exposure).
Reciprocity FailureThis is an apparent loss of film speed when film is subjected to very long or very short exposures. Very long is more than a few minutes. Very short is less than one-ten-thousandth of a second or so with consumer type film.
This is rarely a problem except in astronomical photography where exposures are measured in hours. This type of photography may require film specifically made for this purpose or special processing for over the counter film. Kodak 400 ISO black and white film, Tri-X, performs fine. Look into push processing the film, for this contact local professional processing labs.
On the short exposure side electronic flash set to a very low power
setting can be problematic because low power bursts are extremely short, perhaps
one-ten thousandth of a second or so. The light may not interact with the film long
enough for adequate exposure. Check the technical information that that came with the
Give the film a great tan.
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