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Old Jul 25, 2007, 6:06 AM   #11
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DIGuy wrote:
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Very informative posts....Off topic question maybe but what exactly is Exposure Values and what are they used for?
EV stands for Exposure Value, which is a way of representing how much light you have. See the link to an Exposure Calculator in my last post to this thread.

With each one EV move higher or lower, you double or halve the light.

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Most digital cameras I have seen have them preset at 0 and you have the option to go positive or negative...Not sure what the benefit or use of this is....Any clarification?
What you're seeing is the camera's meter in the viewfinder, and it's designed to let you know if your settings are going to result in a darker or brighter exposure, compared to the way the camera's metering thinks it should be exposed.

When you are shooting in non-manual modes, this meter is used for a feature called Exposure Compensation. If you set the camera so that the pointer is higher than zero, it will take a brighter exposure than the camera would have used by default. If it's set to a -EV value, it will expose it darker than the metering thinks is needed.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both of these variables, although most cameras have a meter that shows you how your settings are impacting exposure).

Exposure Compensation lets you alter the way a camera's autoexposure/metering algorithms expose an image (brighten or darken it compared to the way the camera metered the scene). It's one of my most frequently used settings on most cameras.

A +EV value gives you a brighter exposure. The camera uses a slower shutter speed and/or larger aperture (smaller f/stop number) to get a brighter exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

A -EV value gives you a darker exposure. The camera uses a faster shutter speed and/or smaller aperture (higher f/stop number) to get a darker exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

If you're in Av Mode (Aperture Priority) and use Exposure Compensation, the camera will vary the shutter speed (since you're setting the aperture). If you're using Tv (Shutter Priority) mode and use Exposure Compensation, the camera will vary the Aperture (since you're controlling the shutter speed).

If you're in Auto (or other similar modes), the camera may vary aperture or shutter speed when you use Exposure Compensation. In low light, since your aperture is already wide open, it varies shutter speed if you use a -EV setting.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both).

Correct Exposure comes down to the amount of light, the ISO speed, the shutter speed, and the aperture. A variety of combinations will produce identical exposure.

You use Exposure Compensation if you want a brighter or darker image compared to what the camera's metering would normally give you in the same conditions.

An example of when you may want to use a +EV setting is for a backlit subject, where the subject would normally be much darker than the rest of the image. Since the camera has a limited dynamic range, it doesn't know that you want the dark subject exposed properly (at the expense of the rest of the image). So, you can make the darker subject brighter for correct exposure (which might cause the rest of the scene to be overexposed some).

If your subject is much brighter than the rest of the image (for example, direct sunlight hitting your subject, even though most of the photo is in shadows), you may want to use a -EV setting for Exposure Compensation so that your subject is not overexposed (making the rest of the image darker, too).

The camera has a limited range of bright to dark that it can capture. So, it makes choices so that most of the iimage is correctly exposed, depending on your metering mode. Sometimes that may not be what you want. That's where exposure compensation comes in.

If you reach the camera's ISO speed limits and your shutter speeds are still slower than desired, you can also use exposure compensation to get faster shutter speeds by deliberately underexposing using a -EV Setting. Then, brighten the images later using software so that they appear to be correctly exposed.

But, this will increase noise levels, just as if the camera had an even higher ISO speed available (especially after you brighten the underexposed images with software), and deliberately underexposing and brightening images later also results in some loss of dynamic range. So, don't use this technique unless you have to.

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Old Jul 25, 2007, 12:16 PM   #12
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Very informative JimC...Thanx for the help
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