In 1950s, Friedrich Deckel, a German shutter manufacturer, first developed the concept of exposure value (EV). He attempted to simplify choosing among combinations of equivalent camera settings.
Exposure value is defined as the exact amount of light hitting a photographic film or image sensor determined by lens aperture and shutter speed, to product a picture which is correctly exposed – neither too light nor too dark.
WHAT IS IT?
In photography, EV is a numerical scale that represents a combination of a camera’s shutter speeds (determine the amount of motion blur) and f-numbers (determine the depth of field), such that all combinations that yield the same exposure have the same EV value.
Even though all camera settings giving the same EV, it does not mean that they give the same picture. For instance, if you fasten up the shutter speed, the aperture will become larger automatically for compensation, in turns your image maintain the same EV but with shallower depth of field.
EV also indicates an interval on the photographic exposure scale, 1 EV equals to a standard power of 2 exposure steps, that is, an increment of one step on the EV scale indicates a one step (also called as a stop) increase in exposure, and vice versa.
If you think your image is quite dark, you can increase the EV. Conversely, if the image is too bright, you can simply decrease the EV.
In fact, many instant cameras provide the exposure controls, like Polaroid cameras and MiNT InstantFlex TL70. For example, the EV numbers on Polaroid Land camera Model 180 range from EV 5 to EV 22 while MiNT InstantFlex TL70’s offers EV +/-1.