Camera Basics I: Exposure
One of the most important concepts to understand, while operating a camera, is what determines the overall exposure of a photograph. On a single-lens reflex (slr) camera, three controls determine the exposure. The controls are the shutter speeds, the aperture, and the film speed. Those settings have a direct relationship to one another which, after grasped will make exposure and the choices surrounding it easy.
First, lets get some terminology out of the way. The aperture in your camera is a mechanical device that determines how much light travels from your lens to the film plane. The aperture is often referred to as an F-stop (Ex. F2.8) and for simplicity, can be considered a circle in-between the lens and film. The control will either be found on the lens itself or in modern slrs on the lcd screen.
Shutter speeds also determine how much light reaches the film. For simplicity, consider the shutter a mechanical curtain that opens and closes on your command. The settings on the camera refer to a time such as 1/125 of a second. However, the camera setting will appear as a whole number. That means the 125 on the camera means 1/125 of a second, 60 equals 1/60 of a second and so on. From here, I will refer to the shutter speeds as the whole numbers for ease.
Film speed (ISO) is the third variable that you control with the camera to determine exposure. The range of film speeds vary from 25-6400 ISO. With lower ISOs, more light is required to affect the exposure. Therefore a film with an ISO of 50 would be considered a “slow film.” Film with an ISO of 800 would be considered a “fast” film, meaning a shorter amount of time is necessary for exposure .Consider each control equal to the other. Moving from an ISO of 100 to an ISO of 200 (film speed) is on “stop” of light. Increasing the size of your aperture (F-stop) from f4 to f2.8 is also one “stop” of light. Each of these actions are allowing the same amount of light increase. Each measure doubles your previous light, just as slowing down the shutter speed from 125 to 60 allows the same light increase, one “stop.” Here is a chart to further your understanding of how “stops” of light are controlled by the reciprocal valued mechanisms of the camera.
Hypothetical Camera Settings:
ISO 100 F2.8 1/60 sec
To double your light: (1 stop)
ISO 100 F2.8 1/30 sec
Or ISO 100 F1.8 1/60 sec
Or ISO 200 F2.8 1/60 sec
Each control individually can double the amount of light!
Another Example:
ISO 400 F11 1/250 sec
To halve your light: (1 stop)
ISO 200 F11 1/250 sec
Or ISO 400 F16 1/250 sec
Or ISO 400 F11 1/500 sec
Another Example: In this case you are shooting a landscape, so the same aperture is required to maintain depth of field (focus). You’re photographing a sunset so it is great light but it is decreasing. You’re near the end of the roll of film with many other choices in the bag. The settings now are:
ISO 50 F11 1/8 sec
15 minutes later, the light has decreased by 2 stop of light. You can adjust 2 ways to attain the proper exposure. You’re film is ready to change, so it’s either film speed, shutter speed, or a combination of both. Here are some of your choices:
ISO 100 F11 ¼ sec
ISO 200 F11 1/8 sec
ISO 50 F11 ½ sec
All of these would work to adjust for the conditions. You would need to make the decisions so that exposure is correct and you are able to express what you want to with the photograph. If you decided in this scenario that the depth of field is not as important then you could decide to alter the aperture to also reach the desired exposure.
Hopefully this will give you a better understanding of exposure and how you can manipulate your camera to assure you get the image you want. If you have any questions on another situation, please contract me through email on the contacts page.
Thanks,
Jonathan Jackson
