Exposure: Getting it Right Every Time, Part IV © Wendy Folse Dec 28, 2001
Three ways of controlling exposure The camera companies have come up with all sorts of on‐board computer functions to take some of the confusion out of the calculations. Now days, the camera knows what lens is connected and will give you a reading for that particular lens. However, for the ease of explaining how to achieve proper exposure it helps to forget that the camera knows everything, because it doesn't. It doesn't know what you want to do, only what it was programmed to do. So far, we have discussed three ways to control the total exposure necessary to produce an image, using shutter speed, aperture setting, or film speed. Where do we get the correct ratio? From the meter, either a hand held meter or the camera's built in meter. Once you have a correct exposure reading for the scene, then you can alter the ratio to suit your needs by adjusting either the shutter speed, aperture setting, or film speed. As long as the ratio stays the same, the exposure will stay the same. That is the beauty of the f‐stop system. It is like a bartering system. If you trade one f‐stop for this effect, you give an f‐stop somewhere else. The object is to keep the ratio in perfect balance for the correct exposure. If you change it here, you must also change it there. It all starts with an accurate or at least an educated guess at the correct exposure. From there, it is a trading game. Do you need more opening in the aperture for a creative effect? If so, then you will have to compensate for the extra f‐stop by changing the shutter speed to match. If you need a faster shutter speed to stop the movement of the subject, then you will have to give it up in the aperture setting. Say for instance you go out one morning and want to capture a beautiful garden scene, but your meter is giving you a setting of say f2.8 at 1/60 and your lens is not capable of handling f2.8. What do you do? Say your smallest aperture setting is f5.6. That means that your lens opens two f‐stops less than the setting for f2.8. Therefore in order to preserve the meter suggested ratio, you will need to increase the amount of time that the shutter remains open in order to let in more light. That means setting the shutter speed at two stops slower or to 1/15sec. That is one way, now lets consider the other two. Take the same scenario as above. The meter is telling you that you do not have enough light for the current setting of f2.8 at 1/60 sec with 100speed film. What do you do, wait and come back later when the light is better? No not really. Maybe you can't use a slower shutter speed because you don't want to The copyright of the article Exposure: Getting it Right Everytime, Part IV in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Everytime, Part IV in print or online must be granted by Page 1 the author in writing.
risk a blurry picture or you don't have a tripod or you may be using a long lens. There are still at least two other quick ways to get the right exposure. The last of the big three is the film speed. We talked about the difference in f‐stops between each speed of film. There is one f‐stop difference between each film speed. In our above scenario, we need to make up two whole f‐stops of exposure. If we are using 100 ISO speed film then we could switch to a 400 ISO speed film therefore gaining the needed two f‐stops to maintain the correct ratio. That is how we use the three major ways of controlling exposure with an slr camera. But what if you have struck out all three possibilities and still need more light? That is when your flash comes into play and it too is based on the f‐stop system of halving and doubling. (More about how to use your flash in another article.) For now, just mount the flash and set the shutter speed to 1/60 second, this is the speed at which most cameras will synchronize with the flash. Set your flash for the right exposure. When all else fails and you get confused, with 100speed film just set the camera to f/8 with the shutter speed at 1/60. If you leave your flash on it will synchronize at 1/60 if you need more light. What if you have 400‐speed film in the camera? Remember the rule about film speed, each film speed is one f‐stop difference from the lower one. So 400‐speed film is two stops more sensitive than 100‐speed film so you have gained two f‐stops. Now what? This gain of two f‐stops means that you can use two f‐stops smaller, because the film is more sensitive you need less light or less time to get the correct exposure. How much less? Two f‐stops less. Therefore, if you have the old rule of 100‐speed film with f/8 at 1/60sec, then you adjust one part of the equation. You have already adjusted the film speed by two stops so you will either adjust the aperture by two f‐stops or the shutter speed by two settings: one or the other but not both. If you want you can split the two gained f‐stops and give one on the aperture and one on the shutter speed. But not two stops on both, it is two stops combined. •
100 ISO ‐‐‐ f/8 at 1/60sec
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200 ISO ‐‐‐ f/11 at 1/60 or f/8 with 1/125sec or f/5.6 with 1/250
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400 ISO ‐‐‐ f/16 at 1/60 or f/11 with 1/125 or f/8 with 1/250 or f/5.6 at 1/500
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800 ISO ‐‐‐ f/22 at 1/60 or f/16 with 1/125 or f/11 with 1/250 or f/8 with 1/500 or f/5.6 at 1/1000
There really isn't a need for the above chart because the camera will be set for the correct film speed when it is loaded, either you set it manually or the camera reads the DX coding on the film canister and automatically sets it for you. The meter then gives you a correct range based on the film speed since you will use the same film speed for the entire roll of film. The reason that it is good to be familiar with the above calculations is that it will help you understand the relationship between the three variables. It also helps to understand when you need to increase your film speed for certain jobs and by how much. Given the previous scenario of the early morning exposure with a reading of f/2.8 and 1/60 let's see how this principle could help you out. The reading was based on 100‐speed film and your lens only went to The copyright of the article Exposure: Getting it Right Everytime, Part IV in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Everytime, Part IV in print or online must be granted by Page 2 the author in writing.
f/5.6. Basically, there was not enough light for you to make the exposure without risking serious blur. What do you do? What type of film would you need in order to get into an acceptable range? •
100 ISO ‐‐ f/2.8 at 1/60, or f/4 at 1/30, or f/5.6 at 1/15
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200 ISO ‐‐ f/5.6 at 1/30
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400 ISO ‐‐ f5.6 at 1/60
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800 ISO ‐‐ f5.6 at 1/125 or f/8 at 1/60
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1000 ISO ‐‐f/5.6 at 1/250 or f/8 at 1/125 or f/11 at 1/60
See how easy that was. The camera doesn't know what to do so it doesn't know what to tell you. It can only tell you a reading at a given setting, by knowing how to alter the setting you can control what you want to do. During the past four weeks we have covered how to use and understand the various techniques for controlling exposure in order to get it right every time. Take control of your equipment and you'll be on your way to taking the great pictures you want to take, not just the ones your camera lets you take.
The copyright of the article Exposure: Getting it Right Everytime, Part IV in Photography is owned by Wendy Folse. Permission to republish Exposure: Getting it Right Everytime, Part IV in print or online must be granted by Page 3 the author in writing.