Photographic Terms

 

Digital Photography is a science as well as an art form, and like any scientific subject, it has its fair share of technical terms. Without getting too immersed in jargon, the definitions below will explain some of the more common terms.


Exposure

Exposure in digital photography simply means the amount of light that is allowed to reach the sensor. More light produces a brighter image, less light a darker one. Too much light will lead to the photo being overexposed with bright areas where no details can be seen, whereas with too little light, dark areas will be pure black. The ideal exposure gives the sensor enough light to detect details in the shadowed areas of the image without being dazzled by too much light in the highlights.

The amount of light reaching the sensor depends on the length of time the shutter is kept open and the size of the aperture through which the light passes. The sensitivity (ISO value) controls how fast the sensor reacts to the light it receives. The ideal exposure comes from a proper balance between shutter speed, aperture and sensitivity and thus any changes made to one require corresponding changes to one or both of the others.

In practice, exposure is usually controlled by changing shutter and aperture settings. If a smaller aperture is chosen, the shutter must stay open for longer to let in the same amount of light and when a faster shutter speed is required, the aperture has to be widened accordingly. In circumstances where the chosen combination of shutter speed and aperture would make the image too dark or too bright, the ISO settings can be altered in compensation.

Although any of the three factors (shutter speed, aperture and ISO settings) can be specified individually and the correct exposure obtained by altering the others to suit, each has its own implications. As well as controlling how much light is available to the sensor,

  • SHUTTER SPEED controls whether moving objects are captured sharply and whether the camera can be hand-held or needs to be steadied on a tripod
  • APERTURE SETTINGS determine how much of the image will be sharply in focus (See also Depth of Field)
  • INCREASED ISO VALUES can also increase the amount of digital noise in the image, possibly to unacceptable levels.

Digital cameras measure the levels of light entering through the lens and calculate settings that will achieve a standard, balanced exposure. Fully automatic "point and shoot" cameras and mid-range or DSLR cameras set to the basic capture mode are designed to choose an average, "one size fits all" combination of settings, but the result can be disappointing if your subject falls outside the average range for any reason. Fortunately, except for the simplest models, most modern digital cameras offer features such as a range of capture modes that allow you to change at least some of the settings and choose how you want to capture a particular image.

Shutter

The shutter is opened by pressing the shutter release button, usually located conveniently on top of the camera body at the right hand side. While the shutter is open, light can enter the camera and cause the sensor to react, and the term "shutter speed" refers to the length of time the shutter is open. Under normal circumstances, a single press on the release button causes the shutter to open, remain open for a predetermined period and then close again automatically (for exceptions, see Bulb Mode).

Shutter speeds are measured in fractions of a second - a setting of 1000 opens the shutter for 1/1000 of a second, 500 for 1/500, 250 for 1/250 and so on, and a setting of 1 (i.e. 1/1) opens the shutter for 1 second. Blurred photos are often attributed to bad focus when the real cause of the problem is movement, either of the camera or the subject. With a steady hand, it should be possible to hold the camera still for 1/250 of a second or so, but it is better to use a tripod than risk blurring the shot by accidentally moving the camera while the shutter is open.

Even if the camera is held perfectly still, the subject itself may move during the shot (leaves on a windy day; birds in flight; people, animals or vehicles moving past, etc). Movement can often be "frozen in time" by using a faster shutter speed, but you can also choose to accentuate the appearance of motion by deliberately using a relatively slower setting. See Shutter Speed Demonstration for the results of experimenting with different speeds.

Aperture

The aperture is the "hole" through which light enters the camera from the outside world, via the lens. The size of the aperture is controlled by a diaphragm consisting of overlapping metal plates that can widen and contract in much the same way as the iris of the eye, to allow more or less light to enter the camera. The actual amount of light that reaches the camera depends on the ratio between the size of the aperture and the focal length of the lens, but to avoid having to do complicated calculations when adjusting the aperture, aperture settings are simply expressed in terms of "f" numbers.

Here is a typical sequence of settings:

f/1.0f/1.2f/1.4f/1.7f/2 f/2.4f/2.8f/3.3f/4f/4.8 f/5.6f/6.7f/8f/9.5f/11 f/13f/16f/19f/22...etc.

Note that the lower numbers refer to wider apertures, higher numbers to narrower apertures. The widest available aperture (in this example, f/1) will be limited by the diameter of the particular lens in use.

Dividing both shutter and aperture settings into steps known as "f stops" establishes a relationship between them and makes calculations easier when aiming for a balanced exposure. Each "f stop" in the sequence below decreases the size of the aperture by a factor of 2 from the previous one.

f/1.0 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 ...etc.

To see how this works in practice, suppose the correct exposure can be obtained by using a shutter speed of 1/125 of a second and an aperture of f/16. If we slow down the shutter speed by one stop to 1/60 and change the aperture setting by 1 stop to f/22, we have doubled the length of time the shutter will remain open and halved the size of the aperture, allowing the same amount of light to reach the sensor as at the original setting. Alternatively, we could use a shutter speed of 1/250 and an aperture of f/11 to get the same result.

Depth of Field

Both shutter and aperture settings do more than just control the amount of light reaching the sensor. The shutter speed is used to manage the blurring effect caused by movement, while setting the aperture wider or narrower will control how much of the image appears to be sharply in focus.

When the lens is focused on an object at a certain distance from the camera, either manually or using autofocus, anything at the same distance (on the same plane) will also be in focus. Only one plane in an image can actually be in perfect focus at any one time, but there is a limited area, extending approximately one third in front and two thirds behind this plane, that still appears sharp to the human eye. This area, where the image is still acceptably sharp, is called the depth of field. Narrowing the aperture by setting the f stop to a higher number will increase the depth of field, giving a greater area that still appears sharp, while setting the f stop to a lower number will decrease it.

There are various ways you can use this feature to improve your photos. When shooting a landscape scene, you will probably want to have as much of the image as possible appear in sharp focus, so you can increase the depth of field by “stopping down” (narrowing) the aperture by choosing a higher f stop number.

You may, on the other hand, want to feature a subject close to the camera and distract attention from the rest of the scene. In this case you can set a wider aperture to narrow the depth of field to the area immediately around the subject so that it stands out sharply against a blurred background.

Depending on the current capture mode, the camera may automatically alter the shutter speed to suit when you change the aperture setting, but if not you may have to make the necessary adjustment manually. See Depth of Field Demonstration for examples.

Sensitivity (ISO value)

ISO settings (50, 100, 200, 400, 800, 1600, etc, depending on the individual camera), determine how quickly the electrical cells on the sensor will react as light reaches them from the outside world. Switching the ISO setting is the digital equivalent of changing a film to one with a different ASA speed, with the advantage that it can be done on an image by image basis.

Digital images are formed when light hitting the sensor produces an electrical charge in each cell. An increase in the ISO value boosts the charge so that the sensor will react faster to the available light. Although this can be useful by making it possible to use a faster shutter speed or smaller aperture than would otherwise be required, it unfortunately often results in loss of quality. The charge generated to increase sensitivity can cause a phenomenon known as digital noise, where cells start to register random specks of colour that interfere with the image. In black and white film photography, a certain graininess could add character to a photograph, but digital noise is actually a degeneration of the image. While small amounts may be hardly noticeable in a small print, an image badly affected by noise can look rather like a multicoloured snowstorm and be completely ruined for all practical purposes.

Setting the sensitivity to 'auto' allows the camera to raise the ISO as well as balancing the exposure by means of shutter and aperture settings, but take care when using this setting. Noise often starts to appear at settings above ISO 200, so the best way to avoid it is simply to set your ISO manually to the lowest allowed by the camera (ISO 200 or less) and only raise it as a last resort when the light is very low and you need a fast shutter speed.

Sensor

The physical size of the sensor is an important factor affecting the amount of noise in an image. Sensors with a smaller surface area need more of a boost and are more likely to be affected by noise, so for the best quality images, choose a camera with as large a sensor as you can afford. Note that it is the surface area of the sensor that is in question, not the number of megapixels. It makes sense that the tiny sensor in a mobile phone will not give the same quality as the one in a full frame DSLR.

Viewfinder

The purpose of the viewfinder is to give the photographer a preview of the final picture in order to compose and in some cases to focus the image.

Simple viewfinder

On a simple or compact camera, the viewfinder takes the form of a reverse telescope, separate from the main lens. The photographer may look directly through the viewfinder, or view the image projected on to a small LCD monitor on the back of the camera.

Depending on the camera’s design, the viewfinder may be offset slightly from the main lens. This can cause problems when composing the picture, because the image in the viewfinder is not seen from exactly the same angle as the one seen by the lens, and the difference can be more noticeable when the subject is quite close to the camera. The composition may look perfect through the viewfinder, but appear to have moved up or to one side in the final picture. The image on the monitor type of viewfinder is likely to be more accurate, but lighting conditions can sometimes make it difficult to see clearly.

Single lens reflex (SLR, DSLR)

An SLR camera uses a mirror to transfer the image as seen by the main lens to the viewfinder, by-passing the aperture. When the shutter is released, the mirror flips out of the way to allow the light through the aperture and into the camera. Some cameras also allow the mirror to be moved manually to give a preview through the aperture of the actual image that will be captured, which is useful for checking the depth of field.

Lens

The area of the subject that is captured in the image depends on the focal length of the lens. A longer focal length magnifies the image so that a smaller proportion of the scene fills the frame, giving the impression of being closer to the subject. A camera may have a fixed-length lens or be fitted with a zoom lens where it is possible to vary the focal length. It may also be possible to remove the lens from the camera and exchange it for another with a different range.

Wide angle

Lenses classed as ‘wide angle’ have a short focal length (on a digital camera, a lens with a focal length of 18mm would be classed as ‘wide angle’) and so give a large picture angle, resulting in a wide field of view. In general, wide angle lenses tend to have a greater depth of field and closer minimum focusing distances.

Telephoto

Telephoto lenses have a long focal length (on a digital camera, a lens with a focal length of 300mm would be classed as ‘telephoto’). The long focal length gives a small picture angle, with a narrower field of view as a result.

Telephoto lenses can be used to draw attention to a specific subject, where wide angle lenses can be used to show the vastness of a scene.

Zoom

A zoom lens is one where the focal length can be altered (by manually turning a ring on the lens or by operating a lever on the camera) within certain limits. Examples of zoom lenses for digital cameras would be those where the focal length can be altered between 18mm and 55mm or between 75mm and 300mm.

Digital zoom

Digital zoom is a method of enlarging an area of the scene by increasing the size of the pixels in the image. This method often gives poor results and can cause a significant reduction in the quality of images when printed.