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, let’s assume that for a given ISO that 1/125th second and f8 would give the preferred 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 the aperture that gives 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.
Use the Chart. As an example, we find ourselves in a situation with our ISO already selected. 1/30 at F8 is giving us good exposure. 1/30 second is too slow to create a good exposure handheld. So let’s find a faster shutter speed. Follow the F8 column to find 1/30 to find our chosen exposure. (also follow the EV column on the left down to EV 11) Now from 1/30 move left and notice the indicated shutter speeds and getting “faster”. 1/60, then 1/125 and then 1/250. 1/250th will work for the situation.
So, we have our new shutter speed. We need the best aperture to give an equal exposure value to our original exposure. Follow the column that has our 1/250 to the top of the column, F2.8. It is the best aperture to give the same exposure value as we started with.
Recapping, 1/30 at F8 equals 1/250 at F2.8.