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# Science Fair Project Encyclopedia

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# Light value

In photography, the term light value, usually abbreviated LV, is a value given to the amount of light coming from a scene in absolute terms, as measured by a light meter. It is a base 2 logarithmic scale, and an increase of one corresponds to half the amount of light admitted. The traditional definition is:

LV 10 = 10 candles / foot²

To obtain correct exposure for a specific film speed, the light value will have to converted to an exposure value, EV. For a film of ISO rating 100, the relationship is one to one. For an ISO 200 film, the EV is obtained by adding one to the LV. For ISO 400, add two, etc. For slower film, like ISO 50, subtract one and so on. Further compensation may be desired to compensate for the brightness of the object, and to achieve artistic effects.

Some typical values are:

 LV 17 White object in full sunlight LV 16 Light object in full sunlight LV 15 Typical object in full sunlight LV 13 Object in shadow, sunny conditions LV 10 Object in dark, overcast day LV 7 Object indoors, well lit LV 2 Object in typical night street LV -5 Object in typical moonlight

To obtain correct exposure for a specific film speed, the light value will have to converted to an exposure value, EV. This is done by converting the film speed ISO rating, S, to a speed value, SV:

$S_V = lg2 \frac {S} {100}$

The exposure value is:

EV = LV + SV

For ISO 100 film, the relationship is one to one. Further compensation may be desired to compensate for the brightness of the object, and to achieve artistic effects