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By: Danette Mckay
Exposure is defined as how much light hits the camera’s sensor. It depends on the camera settings mainly aperture and shutter speed. Exposure value (known as EV) is a number that represents the exposure setting of the camera. We will explain what the number means and how it is used for exposure compensation.

The exposure depends on two things: how much light gets through the lenses to the camera’s sensor and for how long the sensor is exposed. The former is a function of the aperture value while the latter is a function of the shutter speed. Exposure value is a number that represents this potential amount of light that could hit the sensor. It is important to understand that exposure value is a measure of how exposed the sensor is to light and not a measure of how much light actually hits the sensor. The exposure value is independent of how lit the scene is. For example a pair of aperture value and shutter speed represents the same exposure value both if the camera is used during a very bright day or during a dark night.

Each exposure value number represents all the possible shutter and aperture settings that result in the same exposure. Although the exposure value is the same for different combinations of aperture values and shutter speeds the resulting photo can be very different (the aperture controls the depth of field while shutter speed controls how much motion is captured)

EV 0.0 is defined as the exposure when setting the aperture to f-number 1.0 and the shutter speed to 1 second. All other exposure values are relative to that number. Exposure values are on a base two logarithmic scale. This means that every single step of EV – plus or minus 1 – represents the exposure (actual light that hits the sensor) being halved or doubled.

It is not likely that you will find yourself setting absolute exposure values. Most cameras do not support entering exposure values at all. You will however use exposure values when setting exposure compensation. Exposure compensation is a way to set the camera to an exposure that is lower or higher than what was automatically detected. The camera auto exposure setting is not perfect and in some scenes results in over or under exposed photos. In such cases exposure compensation can solve the problem Exposure compensation is also useful when you like to get photos that are a bit over or under exposed.

When setting exposure compensation you select an exposure value that is added to the current exposure setting. Usually exposure compensation values are no more than +/-5 EV. For example a compensation of -2EV will make a photo darker (or less exposed) while a compensation of +2EV will make it brighter (or more exposed). A good example of using exposure compensation is on a bright day. Compensate to lower exposure using a value of -0.5EV or -1EV to get richer colors and more details.

Exposure values can also be used in conjunction with auto bracketing. When using exposure auto bracketing each time you take a photo the camera takes a series of extra photos for you each with a slightly different exposure. You can set the EV step between each photo.
This article and more are from Danette Mckay who is an expert in his field. photo printing provides more in depth information.
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