Exposure Value

2006-12-05_02

Exposure Value, or EV, is the mechanism on a camera that allows you to shift the exposure away from the value that the camera’s light sensor gets. When a camera manufacturer develops the light meter for a camera, they use several stock colors and images to fine tune the meter. Usually, there is a snow scene, a night scene, a neutral scene, and several others. Once the light meter in the camera is programmed with each scene, it can approximate what it’s looking at in the field by referencing the built in scenes and the values that the manufacturer has associated with them. The camera “recognizes” the scene by choosing the closest match in its memory. It then distills that scene into aperture and shutter speed values that it presents to the user.

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Left: An even exposure Right: Underexposed

The problem with this method is that some scenes can fool the built in meter. If your picture has some elements of a pre programmed scene, the camera may choose those settings, even if the picture you’re taking is completely different. A bald eagle is a good example. Eagles have dark body feathers and white heads. If the meter places more weight on the head, the body will become underexposed. If the meter weights the dark body feathers, the head will be overexposed. EV is the manufacturer’s way of giving the photographer a method to compensate for these meter errors.

Positive EV will add exposure value to the settings that the camera calculates. In the bald eagle example, if you meter for the bird’s head and set the EV value to +1, the camera will add one stop worth of exposure to the internal value. The longer exposure will account for the darker feathers in the eagle’s body. It takes some experimentation to get an exposure that’s long enough to expose the dark feathers, but short enough to not overexpose the eagle’s head. A 1 stop compensation is a good place to start. If the camera’s internal meter reads a shutter speed of 1/500th of a second, +1 EV will force the camera to the next stop, or 1/360th of a second.

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Left: the camera’s meter was fooled by the light waterfall and underexposed the rest of the picture.  Right:  I used matrix/intelligent metering for this scene, which metered the entire scene.

The way that a camera sensor captures light is analogous to pressing a felt tipped pen onto a piece of paper. The longer you hold the pen on the paper, the more ink spreads over the surface. The longer the shutter stays open, the more light the sensor captures, and the more exposed that part of the image becomes. Light colored areas of a picture expose more quickly than dark areas, so pushing the exposure value too far will overexpose the light areas of the picture.

The exposure value that the camera calculates will also change based on which meter you use. A spot meter only reads a small area of the scene and chooses exposure values based solely on that. A center weighted meter adds a small area around the spot to the calculation. Usually, a camera will weight the spot at 75% and the small surrounding area at 25%. A matrix/intelligent meter will choose a value that properly exposes as much of the scene as possible. Each of these meters will give a different exposure value for the same scene. When you start experimenting with EV compensation, try the spot meter first. This is the simplest meter and will give you a good idea of how changes in the EV value will translate into changes in the finished picture. I usually like to start with the lightest part of the subject and add +EV until I get an exposure that covers the dark parts of the subject. Metering the dark areas of the scene and using -EV will work just as well.

Grace 2007-07-08_070-crop

Both of these pictures used center weighted metering.  The difference is in the position of the flash.  In the left picture, I held the flash close to the kitten because I wanted to overexpose parts of the kitten’s fur to emphasize the light falling on it.  In the right picture, I held the flash further away from the kitten to balance the light hitting it.

Flash EV works by regulating the amount of light that the flash puts out. Instead of changing the shutter speed, the flash pushes more or less light onto the sensor during the same exposure. There are subtle differences between shutter EV and flash EV. Too much +Flash EV can give a harsher light to the picture. -FlashEV is usually used in sunlight to provide fill light that illuminates a person’s face.  In bright environments, cameras will often be fooled into underexposing faces because of the large amount of ambient light in the scene.  I don’t use flash EV most of the time. I prefer to manually change the flash value by using a remote flash and changing the angle that I hold the flash at. This combines EV compensation with an artistic element. By changing the angle of the flash, I can achieve more light contrast in the picture while also changing the amount of light that’s hitting the subject.

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Left: Even though this looks like a dynamic scene, to a camera meter it’s relatively bland and easy to meter because the EV values of the different colors aren’t that far apart and don’t change the exposure very much.  Right:  I metered for the balloon and let the rest of the scene overexpose because I wanted to emphasize that part of the picture.

EV compensation is a useful tool for a photographer because it provides a quick, dynamic way to push exposure values while still retaining the electronic metering assistance that the camera meter provides. The alternative is to meter the scene, then change the manual settings on the camera to the exposure you want. Modern cameras have very sophisticated meters in them. Even though the particular scene you’re photographing may not be properly metered in certain circumstances, the internal camera meter will usually put you within a stop or two of the correct value. Using EV lets you keep the advantages of the camera meter while tailoring the resulting picture to your tastes.

One Response to “Exposure Value”

  1. Manuel Says:

    Believe it or not, I know NOTHING about EV. Literally, that is the most I have ever read about it.


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