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Exposure: Getting it Right Everytime, Part I

© Wendy Folse

Ways of Controlling Exposure

Many people consider photography too difficult to learn simply because of the constant barrage of numbers photographers toss around, but once the secret language of the numbers is understood it really is very easy. The magic code is what photographers call the f-stops. It is a system of numbers that represents the halving and doubling of light values. The beautiful thing about the system is that everything adheres to the same principle. The f-stops merely represent the quantity of light exposing the film. The math has already been figured out for us and in its place is the f-stop system. If you understand the f-stops and how they work, then it isn't necessary to do the math.

In this series of articles we will be covering exposure and how to learn to get it right, everytime. Once you have a grasp on the basic concepts you will quickly learn how to judge for yourself and make the proper adjustments for each different type of exposure.

There are several ways of controlling exposure with an slr. One of the easiest ways is with film choice and it is often the most misunderstood. People know that film speed relates to the amount of light needed for exposure but beyond that, a lot of people get confused as to what film speed actually represents. The difference between ISO 100-speed film and ISO 200-speed film is one full f-stop. The difference between ISO 200 and ISO400 is one full f-stop. So, the difference between ISO 100 and ISO 400 is two full f-stops. With most automatic slrs, the film speed is set by the camera according to the DX coding on the film canister. However, for manual cameras that allow the user to set the film speed, this is a great way to adjust exposure when it is required for the entire roll.

For example, how do you know when to use which speed of film? Under a given situation, you take a meter reading and it shows that with the current film loaded in the camera, you do not have enough exposure range for the lens you are using. What do you do? Switch film? Which speed will be appropriate? That depends on how many f-stops more you need. If you need two more f-stops, then go up two f-stops on your film choice, from ISO 100 to ISO 400.

Another way to use film speed to control exposure is by tricking the camera and meter into thinking it has a different speed film. The meter will then give you readings based on the film speed that you set on the camera. There is one full f-stop difference between each film speed. Therefore, if you are taking snow pictures and you wish to open up or overexpose by one f-stop, you can halve the film speed number setting on the camera. Example: ISO 200 film loaded in the camera, film speed dial set to 100. This will overexpose the entire roll by one f-stop. This is not the same as pushing or pulling film, which involves the altering of development process as well. This technique is only used to purposely overexpose or underexpose an entire roll of film in order to achieve a desired result.