Light and Exposure Values (LV & EV)
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.
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 EV is then converted into a combination of aperture and shutter settings. 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).
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| EV scale and locked aperture/shutter rings on a Kodak Retinette |
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, which locks the aperture and speed rings together; the speed/aperture can then be adjusted together.
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| Exposure calculator on a Kodak Bantam Colorsnap |
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.
In photography the EV or Exposure Value is a scale used for light measurements. Light meters are often scaled in EVs, and many cameras feature an EV scale for setting the exposure.
The EV can be calculated from the aperture and shutter speed thus:
EV = log2 (N2/t)
where
- 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...)
