Learning Centre: Understanding exposure
by edit@cnet.com.au (Alexandra Savvides)
Once you know the bits and pieces of your digital camera, you're ready to move into a bold new territory. Automatic mode? Out the window. Let the camera do the work for you? Forget it. We're going fully manual - welcome to the world of exposure.
What is exposure?
You've probably come across the terminology "obtaining a correct exposure" in your photographic travels. Exposure is the measure of light that reaches the image sensor in order to capture an image. There are three main elements that determine how either a camera (in automatic modes) or a photographer (in manual modes) determines exposure.
Exposure is made up of three elements, working together. (Credit: CBSi)
Back in the old days of film, each roll would have an ASA or ISO rating, which meant how fast or how sensitive to light it was. Digital cameras work on the same principle, but this time, replace film with the image sensor. ISO is the sensitivity level, so an ISO of 100 is relatively slow, and an ISO of 400 and above is considered fast.
You would use a "slow" ISO when there is a lot of light so the image sensor is less sensitive, and a "fast" ISO when there is less light and it is difficult to get a decent exposure without introducing camera shake into your images.
Think of aperture like the pupil in your eye. Aperture is a measure of how much light is let into the camera through the lens. Like your pupil, the lens can open up (widen its aperture) to let more light in, or close down (narrow its aperture) to let less light in. Aperture is measured in f/ stops and affects depth of field.
Using a wide aperture (small f/ stop) will produce an image with a blurred background and sharp foreground, or area of focus, and a small aperture (large f/ stop) will produce an image with sharpness across more of the image. This will be explained further when we discuss depth of field.
The lens on the left is stopped down to f/22 (letting in the least amount of light), in the middle is f/8, and on the right, f/2.8. (Credit: CBSi)
The length of time the shutter is open, measured in fractions of a second. A fast shutter speed will let in less light than a slower shutter speed. For example, a shutter speed of 1/60 will let in more light than a shutter speed of 1/125.
In the following sequence, each shutter speed lets in half the amount of light than the one that came before.
1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500
The same principle applies for aperture, but when you change the measure it's called "stopping up" or "stopping down", or just a stop.
f/2.8, f/4, f/5.6, f/8, f/11/, f/16, f/22
The same exposure can be reached through a combination of apertures and shutter speeds. For example, an exposure of f/8 at 1/125 is equivalent to f/5.6 at f/250.
Depth of field
Exposure modes
Once you've understood the basics of what makes up an exposure, you can start to familiarise yourself with the manual modes on your camera. Most digital SLRs will have the following options available on the mode dial.
| Program mode (P) | The camera will choose the optimum combination of aperture and shutter speed for you, but you can control variables such as white balance and ISO. On some cameras you can also toggle between different exposure combinations for a little more control. |
|---|---|
| Aperture priority mode (A) | You select the aperture you want in order to achieve a desired effect (for example, opening up the aperture to blur the background) and the camera will select the optimum shutter speed required for obtaining a correct exposure. You also have control over ISO and white balance. | Shutter priority (or Tv) mode (S) | Depending on what brand of camera you have, this mode will either be S or Tv (Shutter priority or Time value). Like in Aperture priority mode, you get to choose the shutter speed you want to achieve a desired effect, and the camera will automatically determine the aperture required to obtain a correct exposure. Again, you have control over ISO and white balance. | Manual mode (M) | You have complete control over all aspects of the exposure and can adjust aperture and shutter speed freely. You also have control over ISO and white balance as before. |
| Bulb mode (B) | All digital SLRs have this mode but only a few will have it on the mode dial. B stands for bulb mode, used for long exposures where the shutter needs to stay open for a significant length of time. |
Light meter
So now you're probably wondering how your camera determines a correct exposure. It all comes from the light meter. A light meter measures the reflected light off a subject and uses that measure to determine the optimum exposure.
Light meters come in two flavours, internal and stand-alone. A stand-alone light meter looks like the image pictured below.
(Credit: Sekonic)
All digital cameras have a light meter built into them so photographers no longer have to carry around a dedicated meter, or refer to techniques like the zone system and the sunny 16 rule.
If you have a digital SLR, look inside your viewfinder while you are in P, A, S or M mode. You should see a line or a bar in the middle. Depending on your brand it will look something like the following:
| Nikon light meter | Canon light meter |
|---|---|
|
|
Alternatively, on some digital cameras you can look at the light meter and exposure measures on the LCD screen.
(Credit: Nikon)
When the pointer is in the centre sitting on 0 it means that you have a correct exposure according to the camera's light meter. The light meter will continue to measure the incoming light as you adjust the combination of aperture and shutter speed (and ISO) while in manual mode. You can choose to underexpose, or overexpose, despite what the light meter tells you, to achieve different effects.
Adjusting the aperture and shutter speed is a different process depending on what brand of camera you have. Usually, there will be a rotating dial located just underneath the shutter button that you can turn to adjust shutter speed. Often, the same dial will be used to adjust aperture as well, and a specific button needs to be held while you turn the dial to toggle between shutter and aperture selection.
Nikon cameras, like the D90, have the aperture/shutter speed selector dial around the front of the camera. Canon cameras, like the 50D, have the dial on top of the camera body. (Credit: Nikon, Canon, CBSi)
Medium grey
As we've discovered, all cameras take a light reading using the internal light meter in order to determine a correct exposure. This reading comes from what's called reflected light - so the light bouncing off an object or subject. Light meters are calibrated to assume that all light is reflected off "medium grey".
As you've probably guessed, not everything you point your camera at will have the same light-reflecting properties of medium grey, therefore giving a less than optimal exposure. This is why you might see some photographers carrying around a specially designed medium grey card from which to take an ambient light reading and set their exposure accordingly.
In the next instalment we'll look at the role of exposure bracketing, as well as exposure compensation and the histogram.
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