Terms: stop - an increment of light. The difference between a shutter speed of 15 ( one 15th of a second) and 30 (one 30th of a second) is one stop. The difference between two adjacent apertures would also be called a stop. Cameras in the 1800s had a piece of metal called a Waterhouse stop that determined the size of the aperture, and therefore how much light came into the camera. You chose which stop to put into the camera based on how much light you wanted to come in.
A shutter speed of 15 lets in twice as much light as a shutter speed of 30. A shutter speed of 60 lets in half of the light as a shutter speed of 30.
equivalent exposure- different combinations of apertures and shutter speeds will let in the same amount of light, but there are aesthetic reasons for choosing one or the other. For example, if I start with an aperture of 8 and a shutter speed of 125, but I want to use a larger aperture for a shallower depth of field, I can change my aperture to 4. That 's two stops more light. Therefore, I have to change my shutter speed two stops to 500 to keep my exposure (the amount of light) the same. If I start with f/8 and 125 again, and want to blur motion, I can change my shutter speed to 30. Since I have changed my shutter speed to allow two stops more light, I must change my aperture to f/16 to keep the exposure the same.
Remember that aperture determines how large the opening is that allows light to come in. It also controls depth of field. Shutter speed determines how long the light strikes your image sensor, but it also determines how motion is recorded.
white balance - one of the properties of light is color. We experience this every day, but perhaps have not really thought about it. Morning light is cooler than evening sunset light. Tungsten lightbulbs give off a warm, yellowish light. Florescent bulbs give off a cool, greenish-blue light indoors. Digital cameras have settings to compensate for these different colors of light.
flash - an artificial light source. Most cameras come with a built-in flash. Your built-in flash is often not the best solution. It produces a harsh light close to the camera lens that makes your subject look flat and without texture. Often the subject is too bright and the background is too dark. Accessory flash units that connect to a hot shoe on the top of your camera give much greater flexibility in intensity and direction of light, and can even be used off-camera. A wonderful web site for reading about off-camera flash is strobist.com.
Seeing: This week, look for motion and light as you take your practice photos. Try to take both frozen action and blurred motion. Give panning a shot. Take both high key and low key photos, as well as a high contrast one. Try a window lit, back lit, and reflection picture. Have fun!
