LV is a numerical scale used to judge the correct exposure for the level of light in a given scene. Although LV is often confused with EV, the two are in fact very different. EV is simply a shorthand way of combining shutter speed and aperture settings and therefore only describes how much of the light from a scene is allowed into the camera. EV does not, by itself, give a correct exposure for a given level of light, as it does not take into account the speed of the film being used. LV, by contrast, factors in all three of the essential parameters for exposure: film speed, shutter speed, and aperture.
The EV and LV scales are the same at ISO 100. However, because EV does not take film speed into account, if you change the speed of the film, the exposure changes and the EV numbers change accordingly. At ISO 100 the EV for scenes in full daylight is EV15, but at ISO 400 it’s EV17: two stops down to compensate for the increased sensitivity of the film. This can get confusing as you have to remember a different EV for each film speed. LV numbers are constant and always represent the same lighting conditions. The Sunny 16 Rule (for scenes lit by full daylight) is always LV15, regardless of the film speed. It is therefore easier to deal in LV when mentally calculating camera settings for specific lighting conditions.
