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Exposure (Part 3)
Creative use of exposure
The shutter speed and aperture controls can be used to change the look of an image in more ways than just making the image lighter or darker. Whilst this will be covered more in later sections, here's a brief introduction, taking two equivalent exposures:
1/1000 at f/4 Let’s call this exposure X
1/30 at f/22 And this exposure Y
We have already discussed in the previous sections that both these exposures let the same amount of light onto the sensor (ie. have the same exposure value). Let's assume that your camera meter is telling you both of these exposures are correct. Why would you pick one over the other?
Fast shutter speeds freeze motion; slow shutter speeds show motion.
If you wish to show motion in your image, you select a slow shutter speed that will make a moving subject blur (exposure Y). If you wish to freeze a brief moment in time, you use a faster shutter speed (exposure X).
Wide apertures lead to a lower depth-of-field than narrow apertures.
The primary creative reason for selecting a specific aperture is to control the depth-of-field in the image.
Depth-of-field is the range of distances in front of the camera lens that is in focus. An image with large depth-of-field will have sharp focus of near and far objects in the image; an image with low or shallow depth-of-field will have only a narrow range of distances in focus. This is a creative choice the photographer makes by adjusting the aperture size. For example a wide aperture can be used to isolate your subject against a blurred background, an effect often used in portraits.
This will be covered in more detail in the section on depth-of-field, however for now the image below should illustrate the point.
Striking a balance
The choice of exposure therefore depends on your priorities for the image. If you need the image to be sharp from the nearest objects to the furthest ones (eg. in a landscape), you'd choose a narrow aperture such as in exposure Y and accept that a slower shutter speed will result. If you wish to freeze motion then you need a faster shutter speed such as in exposure X.
These two exposures are for illustration only though, bear in mind the actual combination of shutter speed, aperture and ISO you need will depend upon the amount of light available. A dark scene indoors is going to require a different shutter speed / aperture combination that a sunny day at the beach, however, there will still be a range of these combinations so you will still have creative control.
This is very important and deserves to be reiterated. You cannot just go to a sports event and say "a-ha! I want to freeze these guys running around so I'll use 1/1000 at f/4". The specific exposure will depend on how much light is available. If it is dark then you may not be able to get a shutter speed of 1/1000 without your picture being too dark as not enough light is hitting the sensor. You can compensate for a lack of available light by using a higher ISO so the sensor doesn't need so much light but you may reach a point where you are using your highest ISO with your widest aperture and your exposure meter is still telling you the picture will be underexposed at your selected shutter speed.
Aperture and image sharpness
Each lens has an optimum aperture for image sharpness which is usually around two stops smaller than its maximum aperture. For example, on a lens with a maximum aperture of f/2.8, the optimum is probably around f/5.6. Smaller apertures create a slightly softer image due to narrow slit diffraction (high school physics anyone?). More will be said about image sharpness in the section on lenses.
Does the camera ever get it wrong?
In the auto and semi-auto modes (covered in the next section entitled 'Camera Modes (Part 2)', the camera uses its built in light meter to try to work out what shutter speed and/or aperture should be used to get a correctly exposed image. Does it ever get this wrong and produce an image that it over or under exposed?
That depends on what you define as wrong. If you show 10 photographers the same picture taken with a variety of different exposures (some brighter or darker than others) and ask them to pick the ‘correct’ one, you will get different answers. What is correct to you might be over or underexposed to someone else.
The camera doesn’t have the human understanding of what it is taking a picture of. The built in light meter will look at the tonal range entering the camera and make a decision on the exposure such that the average luminance (brightness) corresponds to a mid-grey level. For most pictures this will give a reasonably exposed image but a classic example of where a camera often gets it wrong is when taking pictures at night. The average brightness in the scene will be much closer to black than normal, and if you want the picture to reflect reality, you need the picture to be fairly black too. The problem here is the camera will try and gather as much light as possible so that the average is around the mid-grey tone, leading to a flat image which will probably be marred by camera shake due to a very slow shutter speed.
Our advice here is twofold:
1. Learn how to effectively use the camera's metering modes (covered in the relevant section later on).
2. Use your eyes. Digital camera users have one huge advantage over film photographers - they have a colour LCD screen on the camera on which to review the picture just taken. Look at it and decide if you think the picture is under or over exposed. If you’re not sure, take another one using exposure compensation (see the next section). Just one word of warning here: the brightness of the LCD screen can make a difference so use other tools available to you on your camera such as the image histogram. Take some test shots with a new camera to see if the screen accurately represents the brightness of the image.
Trust your eyes! If the exposure looks right to you, then it is right!
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