a textbook by Philip Greenspun; revised January 2007
Exposure: (after last publication)
If you ask a professional for some exposure advice, the typical answer is "f/8 and be there." This is a bit of an in joke. The "f/8" part of it sounds vaguely technical and useful, since f/8 is an actual aperture that you can set on most lenses. But it doesn't mean anything without an accompanying shutter speed or film ISO. The "be there" reminds you that ultimately exposure is pretty easy. The most important thing to have is patience and dedication so that you're around when a great photograph is happening.
There is no correct exposure
As I noted in the chapter on film, the real world generally contains a wider range of tones than you can represent on paper, film, or even with the best digital sensors. You have to make an artistic decision about where you place those tones. Some detail will inevitably be lost as tones that are distinguishable in the real world are mapped to the same number out of a digital sensor or density on film.
This chapter will teach you how to control and predict which details are lost.
The Controls
Single-lens reflex cameras have an intimidating array of buttons. It will please you to know that there are only three controls that affect the imaged: focus, aperture, and shutter speed. The two controls that affect exposure are aperture and shutter speed.
Aperture
If neither the subject nor the camera are moving, the shutter speed is not very important. Aperture, however, affects the depth of field and therefore which portions of the image will be in focus.
What is aperture and why is it useful to change it? Aperture is the degree to which the iris or diaphragm inside the lens is opened. Lenses are designed for maximum light-gathering capability. The diaphragm is just like the iris in your eye; it can be closed or stopped down to block off a portion of the light coming through the lens. A lot of expense and weight went into making your lens fast or good at gathering light. Why would you want to throw away some of that capability away?
The first reason to stop down a lens is that the world might simply be too bright. If you're using high-speed (sensitive) film and have a slow shutter that must expose the film for at least 1/500th of a second, using a smaller aperture is the only way to prevent too much light from striking the film and overexposing it.
A more interesting reason is for aesthetic control of sharpness. Suppose the lens has a maximum aperture of f/2. The f-number is the lens length divided by the diameter of the aperture opening. So for a 100mm lens, this would be a 50mm opening. The depth of field will be shallow. Only the object on which you focussed will be sharp. Things closer or farther from the camera will be out of focus. The range of distances for which objects are acceptably sharp is called the "depth of field". Notice the word "acceptably" in the definition. What is acceptable in an 8x10 print viewed from across the room may not be acceptable in the same print viewed at arm's length. What is acceptable in an 8x10 print viewed at arm's length may not be acceptable in a 30x40 print viewed at arm's length.
If you want more objects in the scene to be acceptably focussed, you have to stop down the lens to a smaller aperture, e.g., f/16 or f/22. This nomenclature is a bit confusing at first for beginners because a smaller aperture means that the lens length divided by the aperture diameter gets larger, yielding a larger f-number. Even more confusing is the fact that lenses are calibrated with a strange succession of apertures: 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32, 45, 64. Each step represents a halving of the amount of light that comes through the lens. Why? The area of the aperture is proportional to half the diameter squared. So multiplying the f-number by the square root of 2 halves the amount of light coming through the lens.
With a long lens and a wide aperture, the depth of field is very narrow. Only those objects exactly at the focussed distance will be sharp. For example, here are a couple of images taken with a 600mm lens at f/4 or f/5.6:
Notice that only the birds are sharp and the backgrounds are soft. The effect may seem rather extreme given that f/4 and f/5.6 are not ordinarily considered super wide apertures. Depth of field is related to the absolute size of the aperture not the f-number (lens length divided by aperture diameter). A 600mm lens is a big honker and an f-number of 4 implies an aperture 150mm across. I.e., the depth of field at f/4 on a 600mm lens will be shallower than at f/1.0 on a 50mm lens.
One way to achieve overall image sharpness is to choose a composition where everything is roughly the same distance from the lens (50mm).
Another approach is to stop the lens down to a small aperture. Note here the leaves in the upper right corner of the frame and the trees at infinity. Both are sharp thanks to the f/16 aperture used on this 50mm lens.
If you're using a single-lens reflex camera, where what you see through the viewfinder is what the film will see after the mirror flips up, you might be confused at this point. You turn the aperture ring on the lens and the image remains just as bright in the viewfinder. Moreover, out of focus objects don't get any sharper as you stop down. You're using a lens with an automatic diaphragm, introduced in the 1960s. The lens will be stopped down by the camera an instant before exposure, just as the mirror is flipping up. If you're just viewing and composing pictures, the lens is kept wide open for maximum brightness. To see what the film will see, you press the depth of field preview button. This lets you visualize in the viewfinder the focus effects of stopping down the aperture but it takes some practice to adjust to the extreme dimming that occurs by f/11 or f/16.
Shutter Speed
For a given amount of exposure on the film, the shutter speed can be determined by the aperture that you set for aesthetic purposes. If you are taking a portrait and want to throw the background out of focus, choose a wide-open f/2.8 aperture. Suppose that implies a shutter speed of 1/125th of a second. If you change your mind and want to ensure that the background is sharp, stop down to f/22, 6 f-stops less light. The film will need to be exposed for 2^6 times as long. Two raised to the 6th power is 64 so you'll need a shutter speed of 1/2 second to achieve the same density of exposure on film.
A camera with built-in meter can do this calculation for you. Professional photographers most typically use an exposure mode called "aperture-priority autoexposure". The photographer picks the aperture and the camera picks the shutter speed. Does it matter what shutter speed the camera picks? Not as long as neither the camera nor subject is moving. If they are standing up, most subjects won't be able to hold acceptably still for the 1/2 second exposure mentioned above. The photographer will be advised to open the aperture until the shutter speed is 1/15th second or faster. If the photographer is handholding the camera, i.e., not using a tripod, the 1/15th of a second exposure will very likely result in an unacceptable amount of camera shake being recorded on film. When using a normal lens, the general rule is to use shutter speeds of 1/60th or faster. Longer lenses magnify the subject but they also magnify camera shake. The traditional rule for handheld photography is to use shutter speeds of at least 1/focal-length. So if you've got a 250mm lens you'd use shutter speeds of 1/250th or faster. You'll be well advised to use faster speeds if you intend to make big enlargements from your originals. You can get away with slower shutter speeds if you either (1) brace yourself against a solid object, (2) rest the camera/lens on a solid object, or (3) use a lens with electronic image stabilization.
There are sometimes aesthetic reasons to use different shutter speeds. If you are taking a picture of something moving and want to show the motion, you'll need a slowish shutter speed. If you're taking a picture of something moving and want to freeze the motion, you'll need a fastish shutter speed, the exact speed depending on the velocity with which your subject is moving and whether the direction of moving is towards the camera or sideways across the frame (note: the best way to freeze motion is with an electronic flash, which is actually a kind of strobe light; a cheap on-camera flash may have a duration as short as 1/30,000th of a second).
Help in Setting Aperture and Shutter Speed
Given the information and examples above you ought to have some idea of the aesthetic results you're trying to achieve. If you're interested in the blurring or stopping of motion on film, set the shutter speed first. If you're interested in what will be in focus, set the aperture first. If you can't get a combination that suits you, look for a different speed of film or put a neutral density filter over the lens to reduce the amount of light coming through without changing what is in focus.
How do you know that you're send the right number of photons through to the film so that your result won't be completely black (underexposed slide) or completely white (overexposed slide)? Old-timers using negative film would simply estimate the exposure from their experience, then fix up any minor errors in the darkroom. A somewhat more accurate technique is to RTFM. Here are the instructions included with Kodak Tri-X, a name shared by two confusingly different films (Tri-X Pan is ISO 400 and has good midtone separation; Tri-X Pan Professional is ISO 320 and has more highlight separation):
"Use the exposures in the table below for frontlighted subjects from 2 hours after sunrise to 2 hours before sunset."
| Lighting Conditions | Shutter Speed (Second) | |
| Tri-X Pan | Tri-X Pan | |
| Bright or Hazy Sun on Light Sand or Snow | 1/500 | 1/500 |
| Bright or Hazy Sun | 1/500 | 1/500 |
| Weak, Hazy Sun | 1/500 | 1/500 |
| Cloudy Bright | 1/500 | 1/500 |
| Heavy Overcast or | 1/500 | 1/500 |
| * Use f/5.6 at 1/500 for backlighted close-up subjects. | ||
to be continued………………
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