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The Exposure Triangle

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Posted 10-09-2010 at 08:49 AM by sevenfoldphotography

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When taking a picture the exposure is affected by three components: Aperture, Shutters-Speed and ISO Rating. The combination of these three elements is what determines the exposure of an image. After reading this article you will have a better understanding of the three key factors that are crucial for taking professional quality photographs.

Aperture
Alternately referred to as f-Stop, is the size of the lens opening. The size of the lens opening plays a vital role in deciding the volume of light that will reach the camera sensor. For more light to enter the camera, a larger aperture opening is needed: for less light, a smaller aperture opening is needed. Larger openings have smaller numbers and vice versa; for example, a small aperture would read f/22 and a larger aperture would read f/4.

Shutter Speed
This refers to the amount of time the aperture stays open. The longer the shutter-speed (the longer the aperture stays open) the more light reaches the camera sensor. When the shutter speed or aperture is shorter, less light reaches the camera sensor.

ISO Rating
On a digital camera the ISO is the sensorís measurement of light sensitivity, but for a film camera it is the measurement of the filmís measurement of light sensitivity. A lower ISO number (less sensitive) requires more light to obtain the same result as a higher ISO number (more sensitive) would give with less light. This could be done by using a longer shutter-speed, a larger aperture opening or both. The same overall exposure can be had by combining ISO, Shutter Speed and Aperture (f-stop).

A more advanced camera has an auto-exposure mode (AE), where the choice of aperture and shutter-speed is left to the camera and manual mode (M), where you get to choose the aperture and shutter-speed settings. There are also semi-automatic modes called aperture-priority and shutter-priority. In aperture-priority mode, you choose the aperture setting and the camera will choose the correctly corresponding shutter-speed setting. In shutter-priority mode, you choose the shutter-speed setting and the camera will choose the correctly corresponding aperture setting.

Analogy
A good analogy to help you understand these concepts better is to compare them to getting a suntan. The ISO, or sensitivity to light, can be compared to your tendency to burn when in the sun. You would be considered a high ISO person if you easily burn (film that has a high ISO number is more light sensitive). If you have a tendency to tan more easily than burning you would be considered a low ISO person.

You can compare shutter-speed to the length of time you are exposed to the sun. When you stay out to long you will more-than-likely be overexposed (sunburned). When you are not out for very long then you may not even get a tan (underexposed) depending on your level of sun (light) sensitivity.

The strength of sunscreen you use is comparable to aperture. Using a sunscreen with a high rating, such as SPF-50 can be likened to a large f-stop number indicating a smaller opening of the aperture which restricts the volume of light that enters the camera. When you use a sunscreen with a low rating, such as SPF-15, it is like choosing a small f-stop number indicating a larger opening of the aperture which lets more light enter the camera. But no matter how you look at it, aperture, shutter-speed and ISO are all equally affected by the brightness or intensity of the sun. Photographing someone in dim light would be the same as taking their picture under a cloudy sky.

Why you need to know these things
When using the automatic exposure setting on your camera you donít have to worry about aperture, shutter-speed and ISO because the camera does it for you. But the more you take pictures the more you will want to have control over them. Control over these features allows you to have more creative control over the outcome of your pictures.


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