Exposure Value

Exposure Value (EV) is a number that represents a combination of shutter speed and aperture that equals a certain exposure at a given ISO.  For a given scene and level of luminance lets assume that for a given ISO that 1/125th second and f8 would give a correct exposure.  But in our situation we’d like to limit our depth or field so we’ll move our f-stop to 2.8.  That’s a move of three stops, from f8 to f5.6 to f4 to f2.8. So we can shoot at f2.8 but the shutter speed must be adjusted to compensate for the over-exposure. So we adjust the shutter speed from 1/125 to 1/250 to 1/500 to 1/1000.  So our resulting exposure is 1/1000th second at f2.8 and is said to have the same exposure value as 1/125th at f8.   So the exposure variables across the scale of optional shutter speeds with aperture that give your “correct” density at a given ISO are all the same exposure value.  This chart is from Wikipedia and it shows the exposure combinations that are equal to each other.  If you follow across the top to the f-stop you’d like to use, then follow down to the shutter speed needed. Once you arrive there move to the left and right to adjust your exposure for the specific situation.  All these on the same row are equal in exposure value.  In the left-most column is the exposure value for your f-stop/shutter speed combination at a given ISO.  Any Questions? Drop me a note.

Exposure-Value-Chart-2

 

 

 

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©Gary Culley 2014

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