Understanding the Exposure Triangle

The exposure triangle is a concept introduced by Bryan Peterson in his book 'Understanding Exposure'. The triangle represents the relationship between the three factors that determine a digital image's exposure. The three points are ISO, aperture and shutter speed. Each factor has equal weight in influencing exposure.

ISO

The ISO is the digital equivalent of film speed. It determines the photographic medium's sensitivity to light. The higher the ISO level, the less light needed for exposure. ISO settings that are high are not as sharp and clear in quality as their lower ISO counterparts. With a high ISO, you need less light for exposure, but you end up having an image that is very grainy. There is little you can do to improve the quality in a photo editing program. 

So, when adjusting your camera's settings to create an even exposure, you want to leave the ISO as low as possible. But sometimes, the only way to get an exposure is by raising the ISO, like shooting photos of a night out in a club where everything moves fast and the light is low.

Aperture

The aperture is a hole that controls how much light gets to pass through the lens to the photographic medium. A closed aperture (higher f/stop) allows less light to pass through as opposed to an aperture that is opened wide (lower f/stop). You use an open aperture to compensate for a lack of light. Ideally though, you want to have a higher f/stop. The quality of the overall image looks better. Higher f/stops also offer you a great range of depth of field.

Shutter Speed

The shutter speed controls how long the photographic medium is exposed for. When you do not wish to raise the ISO and can't open your aperture anymore, the only option left for a balanced exposure is to slow down the shutter speed. An image that is being exposed for half a second needs far less light than an image being exposed for a quarter of a second. 

If you're using a slow shutter speed to compensate for exposure, be aware that you should be using a tripod. The slightest movement during the exposure will cause your image to be blurred.

Add More Lights

The exposure triangle shows the relationships between ISO, aperture and shutter speed. You can change one to compensate for another. However, if you're in a situation with a lot of control, you should always consider adding another light before altering one of these settings. The best looking images have a low ISO and high f/stop. You also don't want to slow the shutter unless you have to, so you avoid image blurs.


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