Exposure: Getting it Right Every Time, Part III © Wendy Folse Dec 23, 2001
In the previous two articles on controlling exposure, we have covered various ways to use film choice and shutter speed in order to get the correct exposure settings. In this article, we will talk about the third way, and possibly the most popular way of controlling exposure by using the aperture setting to control the amount of light. The aperture setting controls the amount of light that reachs the film plane in order to expose the film. This corresponds to the circular opening in the diaphragm and is marked with the f‐stop numbers on the barrel of the lens. This is what most people associate as the f‐stops. When we talk about gaining an f‐stop or losing a stop, it is this principle that we are referring to. The shutter speed is marked in parts per second. The aperture settings are marked as f/5.6, f/8, etc. In between each f‐stop marking is considered a half‐stop, so that if you want 1/4 less light to enter you can set the f‐stop marking between two whole f‐stops. When you open one full f‐stop (go to a smaller number)you are doubling the amount of light that enters through the diaphragm. When you stop down or close one full f‐stop, (go to a larger number) you are decreasing the light by half. Remember, half down and double up.
Aperture In several previous articles, we have covered the use of aperture to control the exposure in very creative ways. If it helps, go back and re‐read those articles. Aperture as it pertains to getting the correct exposure refers to the size of the opening of the diaphragm. This is what controls how much light is allowed to enter and expose the film. The shutter speed controls how long the diaphragm remains open and the aperture refers to how much it opens. Remember, both the aperture setting and the shutter speed setting control the total amount of light or exposure. Shutter speed + Aperture = Total Exposure (Film speed refers to how much total exposure the film is capable of recording.) When the aperture is opened, the f‐stop number is lower. When the aperture is closed down, the f‐stop number gets larger. This confuses people perhaps more than anything thing else. A simple trick to help you remember the concept is to think of the diaphragm as a pizza and the f‐stop number as the number of slices. Don't over think it. If a pizza is cut into 11 slices then the slices are smaller. If it is cut into four slices, then the slices are larger. The f‐stop numbers are based on fractions.
The copyright of the article Exposure: Getting it Right Every Time, Part III in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Every Time, Part III in print or online must be granted by Page 1 the author in writing.
Here is the range of f-stop numbers that you will most often encounter from largest to smallest:
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f/1.4
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f/2
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f/2.8
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f/4
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f/5.6
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f/8
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f/11
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f/16
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f/22
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f/32
You may not encounter the smallest or largest f-stops but it helps to be aware of what they are just in case. For most simple slrs and point and shoots, the range is from about f/5.6 to f/16. The most common settings used on a camera is around f/5.6-f/8 at 1/60th sec. This is where we get the old adage "F/8 and be there." On a normal sunlight day with medium speed film, a setting of f/8 at 1/60 sec will give you the best overall general exposure. Exposure Compensation (Ev +/-) The exposure compensation feature on most slrs works by setting an exposure value at plus or minus the number of f-stops it is set to. For instance, take a snow scene. You know you want to over expose by one stop in order to render the snow white instead of gray. Set the exposure compensation to +1. Now you shoot the scene using the camera's built in meter readings just like you would normally. The camera will automatically adjust the exposure for one stop over. This saves you from having to think about each shot. You must remember to turn off the exposure compensation feature after or all of your other shots will also be over exposed. This works great when you plan to shoot an entire roll under the same conditions and don't want to have to constantly check and recheck exposure.
In the next an final part of the series we will be putting the parts back together and learning how to control exposure using all three components that we have learned in the previous articles
The copyright of the article Exposure: Getting it Right Every Time, Part III in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Every Time, Part III in print or online must be granted by Page 2 the author in writing.