Exposure Basics

It is virtually impossible to buy a digital camera without automatic exposure capability, so it’s no surprise that most digital camera owners don’t learn the very basics. Good automatic exposure systems result in decent results most of the time, but as you have likely noticed, in others the results are disappointing. If you want to avoid photos that suck, you need to understand three basic exposure controls: Media sensitivity, shutter speed and lens aperture.

Media sensitivity refers to how sensitive the recording medium is to light. In the film world, you’ve probably heard this referred to as ‘film speed’ or the film’s ISO number. Most digital cameras express their sensitivity in ISO as well. For example, ISO 400 requires half as much light to record the same image as ISO 200. In general, both film and digital sensors create higher quality images at lower speeds. More specifically, in the digital world, the camera increases the gain on the sensor to achieve higher “speeds”, which amplifies both the image and digital noise. Unless you’re looking for a specific effect, use the lowest practical ISO setting your camera offers, usually 100. Some experimentation will allow you to understand your particular camera’s characteristics.

Shutter speed is the amount of time the shutter is open, allowing light to pass through to the sensor (or film, in the case of a film camera). Unless you’re making a long exposure, shutter speed is specified in fractions of a second, such as 1/60 to 1/1000. Higher shutter speeds mean a faster exposure, and, therefore, less opportunity for movement to result in blur. As a general rule, if you want to avoid a blurred image, use a faster shutter speed. Many photographers use the lens focal length as a rough guide. For example, if you’re shooting with the 35 mm equivalent of a 50 mm lens, you normally want to use a shutter speed of 1/50th or faster. With a 200 mm lens, you’d ordinarily use a shutter speed of 1/200 or faster.

The lens aperture setting is measured in ‘f-stops’. A setting of f2.8 lets in much more light than f22. The aperture is essentially a diaphragm that opens or closes like the pupil of your eye to control the amount of light passing through the lens. The main influence of aperture (other than as an exposure control) is on ‘depth of field’ or the range of distances from the camera that are in sharp focus. Large apertures (such as f2.8) have a very narrow depth of field, while smaller apertures (such as f22) have a wider depth of field. A large aperture is commonly used to focus on the subject and push the background out of focus. On the other hand, landscape photographers often favour small apertures in their quest to keep both foreground and background objects in focus.

These three factors – sensitivity, shutter speed and aperture – work together to control exposure. For example, if you increase the shutter speed by one full stop (for example 1/125 to 1/250), you would need to either double the sensitivity (for example from 200 to 400) or open the aperture one full stop (for example from f16 to f8) to achieve the same exposure.

One of the great benefits of digital cameras is that many allow you to change the ISO setting at any time. For example, if you’re shooting handheld at 1/60, f5.6, ISO 100, and you run across a landscape where you need more depth of field, you could increase your ISO set-ting to 200 and decrease your aperture to f8. You could also use a tripod and shoot at 1/30 or 1/15, assuming that the wind isn’t creating too much movement.

At this point, at least some of you will be wondering how to do this with your specific camera. If you have a digital SLR, you’ll probably find that you have an ‘aperture priority mode’, in which you can set the aperture you wish to use and the camera’s automatic exposure system will select the corresponding shutter speed. You’ll just need to keep an eye on it to ensure that the speed doesn’t get slow enough to create undesired blur.

However, if you don’t have an SLR, not all is lost!  Even the most basic cameras have different shooting modes. If you recall the relationship between shutter speed and aperture, you can use these modes to trick your camera into doing what you want. For example, a ‘landscape’ favours a small aperture (for better depth of field), while a ‘sport’ mode usually favours a fast shutter to avoid blur. Some experimentation will help you understand your camera’s characteristics and convince it to do what you want by selecting a mode even if it doesn’t correspond to what you are shooting. For example, if you want to blur a cyclist riding by, try your ‘landscape’ mode. If you want to separate your child from the background, try ‘sport’ mode.

It’s important to understand that automatic exposure isn’t magic. While different systems exist, they all make assumptions about the scene. For example, most cameras make the assumption that that average of the scene is a neutral grey, so if you’re shooting in snow or sand, you end up with an underexposed image. One of the advantages of a digital camera is that you can get immediate feedback. Some digital cameras also offer a histogram that can be used to quickly understand the current exposure situation, and I’ll go into the details of using the histogram in another post.

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