Light and Exposure Values (LV & EV)
Light Value (LV) and Exposure Value (EV) are scales for measuring the light value and exposure value, respectively. These are often confused, not least by the markings on some cameras. LV is absolute light intensity, while EV expresses the combination of shutter speed and aperture. The two are related by film speed; at ISO100 (DIN21) they are numerically equal. Each step up or down the scale doubles or halves the light respectively.
Typical values of LV in daylight vary between about 16 (bright sun), down to around 10 (dark clouds) and of course, lower in very bad weather, at night or indoors. A range of 2 to 18, as offered by many light meters, covers most common photographing conditions. Many light meters are not sensitive enough to reach even this range, although for some special purposes (such as astronomy) values beyond this range may be required.
The range LV 0 (semi darkness) to LV 16 (bright sunlight) corresponds to a light increase of 65,500 fold (216), easily handled by our eyes but impossible for any film or camera sensor to encompass.
Exposure meters often display EV which can be directly transferred to the exposure setting ring of some cameras, otherwise the value must be converted to corresponding aperture and shutter speed settings. These are usually shown on a calculator dial or scale on the meter itself.
The Light Value System is an arrangement of exposure controls used on some cameras (mostly ones with in-lens shutters), whereby the shutter speed and aperture controls are mechanically coupled. Once the exposure is set (using an EV scale), the exposure can easily be switched between different shutter/aperture combinations giving the same EV. For example, in the Retinette shown here, the EV is set (on the red scale) by depressing the lower (aperture) scale-ring to disengage it from the shutter scale, and turning it. Once re-engaged, the shutter speed and aperture scales turn together, maintaining a constant EV. Some cameras only have an EV scale (e.g. the Kodak Pony II or the Kodak Auto Colorsnap 35 ) and do not have separate speed or aperture markings.
The EV/LV system became popular during the 1960s, when the growth of colour photography made accurate exposure more important, but it died out again into the 1970s as cheaper, more compact, electronics made in-camera light meters possible and coupling to exposure controls made manually transferring values redundant.
EV can be calculated from the aperture and shutter speed thus:
EV = log2 (N2/t) (or, since log10(2) = ~3.322, EV = 3.322 log10(N2/t) )
- N is the relative aperture (f-stop)
- and t is the exposure time (shutter speed).
so, for example, an exposure of 1/250s at f/8 is an EV of log2(8*8*250) = 14 (to 2 figures, actually nearer 13.96578...)