Understanding Photography Exposure
Exposure is the amount of light a digital camera's sensor captures when a photo is taken. Too much light results in a washed out photo (overexposed). Too little light and the photo will be too dark (underexposed). A camera's Aperture, Shutter Speed and ISO settings directly affect exposure, but more importantly, they allow you to control how each photo will look.
When taking pictures, just remember the following: ISO affects Noise, Aperture affects Depth-of-Field (DOF), Shutter affects Motion.
Remember that using a very high ISO may add some digital noise. So always start with a low ISO and adjust if necessary to achieve the effect you want.
Think of Aperture and Shutter Speed as balanced variables. If your settings are giving you a good exposure but you want to increase the size of your Aperture by one stop (or click) - you will also need to decrease your shutter speed by one stop to get the same balanced exposure.
When in Aperture Priority mode keep in mind that when you use a small Aperture, the Shutter Speed will adjust to stay open longer. Long shutter times will pick-up any hand movement so use a tripod.
Shutter Priority Mode (represented by Tv), allows you to focus on how motion is being captured, while automatically setting your Aperture and ISO. So if you’re shooting a track meet or a car race, you will probably want to use Shutter Priority.
Shooting a scene with low light is going to need a larger aperture and/or a longer shutter speed. Remember to steady the camera if you are using a longer shutter speed.
Understanding the three elements that affect exposure (ISO, Shutter Speed and Aperture) and learning how to control them, will help you achieve the results you want in your final image.
The first step is to determine the environment you’ll be shooting in. This will help you determine your ISO. The ISO controls the sensitivity of the image sensor in your camera. A high ISO setting means that the image sensor is operating at full capability and capturing more, so you don’t need as much light when you shoot at a high ISO. A high ISO will allow you to shoot in low light situations – like indoors at a dinner. With a low ISO – your sensor is not running at full capacity, which means that you have more flexibility in the amount of light you can let into your camera. A low ISO setting is perfect for a bright sunny day.
The next step is to determine if you will be shooting a subject in motion, and, if so, how you want it to look. Your answer will help you set your shutter speed, which determines the length of time the shutter remains open. Leaving your shutter open a long time will let in lots of light and will capture motion as it moves – meaning you will see it as a blur. A short shutter speed, like a quick blink, freezes a moment in time without blur.
Lastly, you will determine what depth of field you want: If you only want just your subject in focus you will set a low aperture, which gives you a shallow depth of field. Conversely, a high aperture will lengthen your depth of field. The aperture controls the size of the lens opening. Like the pupil of the eye, open it wide in low light and make it small in full sunlight.
The Exposure Meter is your final check before you snap a shot. At a glance it tells you how your exposure will turn out based on the Aperture, Shutter Speed and ISO settings. A well exposed shot lines up right down the centre at zero. An underexposed shot (too little light) falls left of centre and an overexposed shot (too much light) falls right of centre. Use the Exposure Meter as a guide only, exposure is a matter of personal preference so don't be affraid to wander off of zero.
When the exposure indicator (the bar under the meter) aligns with the center point of the meter, as shown in the middle example, the current settings will produce a proper exposure. If the indicator moves toward the minus side of the scale, as in the left example in the figure, the image will be underexposed. If the indicator moves to the right of center, as in the right example, the image will be overexposed. The farther the indicator moves toward the plus or minus sign, the greater the potential exposure problem.
The shutter speed, f-stop, ISO speed and exposure meter appear in the viewfinder on Canon EOS.
You also can view the settings in the Shooting Settings display and LCD panel.
Also be aware that in the viewfinder, LCD panel, and on the monitor in Live View mode, shutter speeds are presented as whole numbers, even if the shutter speed is set to a fraction of a second. For example, for a shutter speed of 1/500 second, you see just the number 500. When the shutter speed slows to 1 second or more, you see quote marks after the number: 1" indicates a shutter speed of 1 second, 4" means 4 seconds, and so on.
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