Exposure: Getting it Right Every Time, Part II © Wendy Folse Dec 18, 2001
Shutter Speed With shutter speed you are essentially setting the time the shutter remains open. The longer it remains open, the more light it lets in. The less time it stays open the less light that reaches the film plane. Pretty simple concept. Shutter speed and aperture determine total exposure. This relationship is described as, "this amount of light for this length of time." Keep in mind the f‐stop principle of halving and doubling values represented by the f‐stops. The difference between one shutter speed setting and the next shutter speed setting is one f‐stop. The difference between one aperture setting and the next aperture setting is one f‐stop. If you open one f‐stop on the aperture, then set one shutter speed faster on the camera to keep the ratio the same. Why faster? Opening the aperture lets in more light, therefore the time needs to be less in order to keep the same exposure. What if you want to close down the aperture by two f‐stops in order to get a greater depth of field? Then you will need to set the shutter speed two stops slower to allow the light a longer time to reach the film. Both the aperture setting and the shutter speed setting control the total amount of light or exposure. (Shutter speed + Aperture = Total Exposure) Which one you use will depend on what effect you are trying to achieve. Movement of the subject equals blur, so that is your first clue. If the subject is moving in any way, you want a faster shutter speed. Camera shake is another movement that can effect sharpness. The general rule to compensating for camera shake is to set the shutter speed at an equal or greater setting than the focal length of the lens. This sounds confusing but it really isn't. Say you have a 50mm lens on the camera, then the shutter speed should be at least 1/60 sec. If you use a tripod then you can set a slower shutter speed because you have decreased the amount of movement. Here is an easy analogy. Suppose you want to cook a turkey in the oven. The directions say 375F for 4hrs. Now you don't have four hours to wait you only have three. What do you do? Or, here's another example. If you put a pot of water on the stove to boil you could turn the temperature to high and the reduce the amount of time or you could put it on medium and wait longer, or you could set the stove on low and wait forever. Which do you choose? The object is the same; the water must reach 312F before it will boil. How long it takes to boil depends on the ratio of time to heat. You can increase or decrease the ratio in several ways. If the burner is larger than the pot, heat is escaping on the sides and is wasted. If the burner is much smaller than the pot, it takes longer, right. You could also put a cover on the pot to increase the heat, thereby shortening the time it takes to reach 312F. The copyright of the article Exposure: Getting it Right Every Time, Part II in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Every Time, Part II in print or online must be granted by Page 1 the author in writing.
These same principles can be transferred to the idea of shutter speed. If you need more light, but also need a fast shutter speed to capture and freeze motion, then you could add light by using a flash or reflectors to keep in the light, just as you added a cover to keep in heat. On shutter priority modes, you set the required shutter speed based on what you are trying to achieve then the camera sets the necessary aperture to complete the ratio based on the meter's recommended exposure reading. The next article, part III of this series, will deal with controlling exposure with aperture settings
The copyright of the article Exposure: Getting it Right Every Time, Part II in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Every Time, Part II in print or online must be granted by Page 2 the author in writing.