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Basic Photography: Exposure

Properly Exposed Photo
Properly Exposed
Note that there are details throughout the photo.

Just about anyone can pick up a modern automatic camera and take reasonably good, well exposed photos under optimal lighting conditions. The electronics in modern cameras, especially those of expensive, auto-focus, single-lens-reflex (SLR) cameras, can cope with a variety of lighting conditions and users need merely set the camera to the full auto mode and snap away. A good photographer, however, will understand that some lighting conditions can fool the camera and result in poor exposure when using the camera's automatic modes. Understanding the basics of exposure and the effects of shutter speed, aperture setting and ISO rating on the final photograph will allow the photographer to over-ride the camera's recommendations to achieve good exposures under adverse conditions that would most likely fool the camera's automatic settings. This knowledge also gives the photographer the creative edge necessary to produce specific photographic effects in the resulting picture. It has been said that knowledge is power. This is definitely true when applied to the concepts of photography.

The Camera

A modern camera is a very sophisticated instrument with a myriad of controls that can seem baffling to a neophyte photographer. In its simplest form, however, a camera is merely a light-tight box that contains a light sensitive media with an opening that will allow light to enter the box and strike (or expose) the material, and a shutter to block the light from entering the camera until the photographer is ready to expose the media. The opening (or aperture), the shutter and the sensitivity of the light sensitive media are the elements the photographer must consider for proper exposure. Each of these elements can be manipulated to create specific photographic effects that will be discussed later in this article.

Keep in mind, there is no "correct" or perfect exposure. Exposure is a creative decision about what portions of the photograph will contain the most detail, but in general terms photographers refer to photos as being properly exposed, "underexposed" and "overexposed." Underexposure results from allowing too little light to hit the exposure media (film or a digital sensor), and overexposure results from allowing too much light to hit the media. The photographer can bias the exposure either one way or the other, thus allowing for darker shadows (underexposure) or brighter highlights (overexposure), but in doing so he or she risks losing the details in the photograph. This results from the lack of dynamic range in film or digital sensors.

Much of what is written about exposure in this article pertains equally to both film and digital cameras since optics and exposure principles are unchanged from one format to another. The term light sensitive material or exposure media refers to film or to the digital sensor. Film has a recommended light sensitivity rating determined by the manufacturer, but a digital sensor has an adjustable sensitivity than can be set by the photographer. There are limits to this sensitivity and you should always use the lowest sensitivity (ISO setting) for a the situation to avoid excessive noise (more on this later), but this is a great boon for photographers.

"Dynamic Range" and other terminology:

Photography seems to have its own language at times. You'll often hear photographers talking about "stopping down a lens", "two stops less exposure", "fast film", "a slow lens", and "two stops of exposure latitude" and "five stops of dynamic range." It's almost like you need a dictionary to understand what they are talking about. One key term is "stop" or "f-stop." A stop represents either doubling or halving the amount of light hitting the exposure media. It can also represent doubling or halving the exposure media's sensitivity to light. "Stopping down a lens" means reducing the size of the len's aperture to reduce the amount of light through the lens. "One stop more exposure" when referring to shutter speed means the shutter will be open twice as long allowing twice as much light to enter the camera.

Exceeding Dynamic Range
Exceeding dynamic range
Note the lack of detail in the highlights and deep shadows!

"Dynamic Range" refers to amount of difference in light between the darkest and brightest parts of the scene in which the exposure media can produce good detail. More simply, it is the range of subject brightness values over which a change in subject brightness will create a change in image brightness. For example, slide film and digital sensors are generally considered to have about 5 stops of dynamic range. Print films usually have about 7 stops of dynamic range. When the light falling on the scene exceeds the dynamic range of the exposure media, the shadows may become completely black with no details at all, or the highlights may become completely white (blown highlights), or both. This is called a "high contrast" scene or lighting condition.

Dealing with lighting in a scene is a topic for a future article and usually involves adding fill light to soften the shadows or diffusing the light source to reduce the contrast. For now, it is enough to simply know that very strong sunlight or bright lights in a dark environment create high contrast scenes. This can be used with some creative effect by photographers with a strong mastery of light (or who are just plain lucky), but the results are typically harsh and not always conducive to glamour or portrait photography.

 

 

Exposure and 18% Gray

Underexposed Photo
Underexposed
Note the dull, muddy colors.

Most cameras contains a light meter which measures the amount of light reflected from the scene. This light meter is set to assume that the average of all the dark and light areas of the scene is "18 % gray." For many scenes this will be appropriate. If, however, the scene contains many dark areas you want to keep dark, or many bright areas you want to keep bright, you probably won't get what you want if you rely completely on your camera's automatic exposure. For example, if the scene is dark the light meter is going to tell your camera to let in more light which will make your shadows too light and will "blow out" your highlights (overexposure). Conversely, if you have a brightly lit scene, such as sunlit snow, the automatic camera settings will try to reduce the amount of light hitting the scene producing a photo of gray snow and completely black shadows (underexposure).

Modern cameras have exposure algorithms programmed into their electronics that help them deal with these exposure problems, but a good photographer should anticipate these exposure problems and adjust the camera settings accordingly. This may mean bracketing exposures to ensure that one of several exposures will produced the desired effects. For example, for a dark scene the photographer may override the camera settings to shoot an additional photo at one stop below the recommended exposure setting and another at 2 stops below the recommended exposure. For a high contrast scene the photographer may shoot the scene at the recommended exposure plus one stop below the recommended exposure and one stop above the recommended exposure. By bracketing the photographer can improve the chances that at least one shot will have acceptible exposure. Additionally, improved software has made combining individually exposed images into a single High Dynamic Range (HDR) image a relatively painless experience, especially when the photographer takes care to use a tripod to keep the images aligned.

Overexposed Photo
Overexposed
Note the blown out highlights (no details)

Lens Aperture and Depth of Field

Photographic exposure is determined by three variables: the quantity of light hitting the media (determined by the lens aperture), the length of time the light is allowed to hit the media (shutter speed), and the sensitivity of the media to light (ISO rating). The lens aperture, used in conjunction with the other two variables, not only helps produce the proper exposure, but gives the photographer control over the photographic Depth of Field (dof).

The term depth of field refers to the range of acceptable focus in front of and behind the point of focus. Shallow depth of field refers to a very narrow range of acceptable focus and wide or deep depth of field means that most, if not all, of the photo will be very sharp. So, the question arises, why would anyone want a shallow depth of field? Wouldn't an expert photographer want the entire photograph to be sharply in focus? The answer is subject isolation. When the photographer wants to isolate a sharply focused subject from the background (or less often from the foreground), a shallow depth of field is the answer. By opening the lens aperture, the photographer can decrease depth of field to narrow the range of focus. Of course, shutter speed and ISO would have to be appropriately adjusted to compensate for exposure, and that will affect other creative aspects of the shot.

Shallow Depth of Field - Canon 135 f/2 L @ f/2
Shallow Depth of Field

 

In the example Shallow Depth of Field photograph, the busy background is out of focus minimizing the distraction from the main subject (who is distracting enough all on her own). In the Medium Depth of Field example, a smaller aperture was used to ensure the entire motorcyle was rendered in sharp focus.

Depth of field is also affected by the focal length of the lens used and the subject to lens distance as well as a few other more esoteric factors, but in general the longer the focal length of the lens and the shorter the lens to subject distance, the shallower the depth of field. For most amateur photographers, the aperture will be the most important variable in determining the the depth of field, and this variable strongly impacts exposure whereas the focal length and lens to subject distance have no discernible impact on exposure.

Lens aperture is measured is f-stops. An f-stop (or f-number) is the ratio of the focal length of the lens to the diameter of the aperture (or opening) in the lens and is expressed as a decimal number. So a 50mm lens with an effective aperture diameter of 25mm would have an f-stop of 2.0. For most lenses the range of variable f-stops goes from a maxium (largest opening) of f1.4 to a minumum (smallest opening) of about f22.0. Some lenses have a smaller or larger range of f-stops. The f-stop values represent a doubling or halving of light between each stop follow the following sequence:

Aperture (f-stop)Value Sequence
f/1.4
f/2.0
f/2.8
f/4.0
f/5.6
f8.0
f/11
f/16
f/22
f/32

 

Medium Depth of Field
Medium DOF
A small aperture creates more DOF

Starting at f/1.4, each subsequent f-stop value produces a smaller effective aperture (remember this is a ratio) and less light (1/2 the amount of the previous stop) is allowed to enter the camera. Also remember, the larger the f-stop number, the smaller the effective aperture of the lens and less light enters the camera (which requires a slower shutter speed or more sensitive media to compensate). Finally, keep in mind, the larger the aperture (smaller f-stop number), the shallower the depth of field, all other variables being equal. A fast lens has a large maximum aperture (f2.8 and below are considered fast) and stopping down a lens means setting a higher f-stop (smaller aperture).

Shutter Speed and Motion Blur

The second exposure variable is shutter speed. Shutter speed is a much more simple concept to understand. The shutter speed setting simply determines how long the exposure media is exposed to the light entering the camera. Shutter speed is typically expressed as a fraction of a second, and like aperture, the standard increments refer to doubling or halving the amount of light allowed to hit the exposure media. There is a much larger range of shutter speeds available than there are for apertures, especially on modern cameras, but typical settings are 1/60th, 1/125th, 1/250th, 1/500th and 1/1000th of a second. Some cameras can go has high as 1/8000th of a second and most cameras can easily do 30 second exposures. Some cameras also have a Bulb or B setting that will hold the shutter open for as long as the photographer presses the shutter button. This is useful for very long exposures in very low light.

The longer the shutter is held open, the greater the chance is that something (either the subject or the camera) will move during the exposure. If the camera moves significantly during the shot, the resulting photograph will look blurry and almost as if it is out of focus. Since no photographer can hold a camera completely still, long exposures are more likely to cause this phenomena called "camera shake." So for long exposures a photographer should use a tripod to hold the camera still. Longer focal length lenses (which magnify the subject image) are more prone to this problem than wide angle lenses (which reduce the subject image). A good general rule to use for hand-held photography is to set your shutter speed to no slower than the reciprocal of the lens focal length. So, for example, if your lens has a focal length of 50mm, you would use a shutter speed of 1/60th of a second (the closest setting to 1/50). With good technique an experienced photographer may be able to shoot 1 stop slower, say 1/30th, but the above rule is a good place to start for beginners.

As with aperture, shutter speed can be used with creative effect. Shooting a moving subject, such as flowing water, with a slow shutter speed can create a silky look to the water or create a sense of movement for an otherwise static subject. A fast shutter speed can freeze action and stop a bird in mid-flight. In glamour photography, unless striving for an unusual effect, the shutter speed will be determined more by consistency with the aperture setting or by the focal length rule than by anything else.

Motion Blur of Water
Slow Shutter Speed
The silky look of the water resulted from a slow shutter speed on a tripod mounted camera.

Creating a Sense of Motion with Shutter Speed
Fast Shutter Speed
The shutter speed for this shot was fast enough to capture the individual drops of water, but slow enough to create an elongation of the droplet.

ISO Ratings and Exposure

High ISO Photo
High ISO Photo

ISO stands for International Organization for Standards and an ISO rating refers to the standard sensitivity rating of film. Digital cameras also use ISO ratings, but in the digital realm this really refers to the electronic gain or amplification of the digital sensor and is a setting on the camera itself.

The ISO values a photographer will most likely encounter are 100 ISO, 200 ISO, 400 ISO, and 800 ISO. Some films and digital SLR cameras can go as high as 3200 ISO and as low as 50 ISO. As with aperture and shutter speed, these standard ISO values represent a doubling or halving of light sensitivity. For example, 200 ISO film is twice as light sensitive as 100 ISO film and half as light sensitive as 400 ISO film.

As with the other exposure variables, there are advantages and disadvantages inherent in the ISO setting. Very slow ISO films as settings allow for virtually noiseless (digital) or grainless (film) images, but require correspondingly slower shutter speeds and/or larger aperture combinations. Slow ISO ratings are best for finely detailed images or brightly lit conditions. Fast films (high ISO) can allow a photogapher to shoot hand-held in lower light or allow the use of smaller apertures to get greater depth of field, but at a loss of fine image detail and an increase in grain or noise. So the choice of ISO settings and film is as much a creative decision for the photographer as is the choice of shutter speed and aperture combination.

The example photograph was shot at 3200 ISO using a professional digital SLR camera with the only light coming from a set of candles below the model. You can easily see the noise inherent in the photo at this high ISO level. Sometimes noise, (but more often. film grain is used for special creative effects. This photo could not have been taken at a lower ISO rating, but there was certainly a trade off in detail and in noise.

Putting It All Together

Contolling exposure and the creative elements associated with each element of exposure are what help to differentiate the photographer as a artist rather than merely the user of a recording device. Composition and lighting are also very important and will be discussed in future articles, but understanding exposure is the starting point for any photographer seeking to do more than simply take pictures. A photographer who understands the difference of making an exposure with a 50mm lens at 100 ISO, f/2.0, and 1/60th of a second or making the same exposure with a 100mm lens at 800 ISO, f/4.0, and 1/125th of second has much more creative control of the final result than snapshooter who does not.

This article presents only a fraction of the information avalible about photographic exposure. Other recommended sources are:

Recommended Readings
On the Web
Books
Understanding Exposure - Bryan Peterson
The Photographer's Guide to Exposure - Neubart
Photography - London and Upton

 

 

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