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Light Value (LV) and Exposure Value (EV) are numerical scales for measuring the amount of light, and the exposure, respectively. These are often confused - not least by the labeling on some cameras; the difference is that LV measures how much light there is, and EV measures how much of the light is allowed into the camera. The scale is logarithmic- that is, each step up on the LV scale is twice the amount of light, and a step up on the EV scale correspondingly halves the exposure. Typical values of LV in daylight vary between about 16 (extremely bright sun), down to around 10 (dark clouds) and of course, lower in very bad weather, at night or indoors. For most purposes, a range of (say) 2 to 18 covers most conditions where the photographer can still see their subject. 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.
LV is often used as a scale on a light meter; the LV must then have a factor added for the film speed to get the required EV. The film speed factor for 100ASA is zero, so EV and LV numbers are the same at this speed; slower speeds require a negative factor - so 25ASA film would mean that the EV is LV-2 (i.e. two stops slower than for 100ASA). The single-number EV is then converted into a set of combinations of aperture and shutter settings - for example, EV 14 may be 1/250s at f/8, 1/125s at f/11 or 1/60s at f/16 - and others, limited by the shutter speeds and apertures available.
The Light Value System is a method of using the LV/EV on a camera, where the exposure is set by dialling the LV or EV; the camera may then have a fixed aperture/shutter combination, or possibly give the user an adjustment which moves the aperture and shutter settings together to maintain the same EV.
In the photo (left) the EV is set on the red scale by turning the lower ring, which locks the aperture and speed rings together; the speed/aperture can then be adjusted in sync using the upper ring. Some cameras only have the EV scale (e.g. the Kodak Pony II or the Kodak Auto Colorsnap 35 ), and do not have separate speed or aperture markings.
The photo on the right shows an LV exposure calculator; the dial is rotated to set the weather on the left scale, and the EV read off on the right, against the film speed - which is shown in DIN and two example film types. The EV is then set on a scale on the lens.
The centre of the dial is the red window for frame counting.
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 measurements possible and coupling of light meters to exposure controls made manually transferring values redundant.
The EV can be calculated from the aperture and shutter speed thus:
EV = log2 (N2/t) (or, since log2(10) = ~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 f8 is an EV of log2(8*8*250) = 14 (to 2 figures, actually nearer 13.96578...)