Light value

From Academic Kids

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 17White object in full sunlight
LV 16Light object in full sunlight
LV 15Typical object in full sunlight
LV 13Object in shadow, sunny conditions
LV 10Object in dark, overcast day
LV 7Object indoors, well lit
LV 2Object in typical night street
LV -5Object 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, <math>S<math>, to a speed value, <math>S_V<math>:

<math>

S_V = lg2 \frac {S} {100}

<math>

The exposure value is:

<math>

E_V = L_V + S_V

<math>

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.

See also: exposure value

External links

  • Camera FAQs (http://jcchen.acc.tku.edu.tw/camfaq.htm)
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