The Fundamentals of Exposure

Light Value (LV)

In order to achieve a correct exposure in a photograph, a certain amount of light must reach the sensor (or film) regardless of the brightness of the scene. Consider these two images; the one on the left is taken on a bright day, the one to the right at dusk:

F22 F4.5

Since the level of light (also called luminance or brightness) was different in these two cases, we must adjust the camera differently between the two in order to ensure that the same amount of light hits the sensor in both cases. 

Note: if two scenes are lit differently, they will look different in your photographs even when both are properly exposed.  This is especially true with respect to shadows and highlights, as can be seen above.

What is Light Value (LV)?

The brightness of a scene can be measured by a light meter and is expressed as a number called "LV" (Light Value). An external light meter as shown to the right will measure the brightness in LV. Do not despair and rush out and buy a light meter, as we do not need to know the absolute LV of a scene these days.  Still, the concept is of use since we need to adjust the exposure settings of our camera (exposure value or "EV") to match the LV of the scene. The meter in our camera is all we need to get this set correctly.

F22 F4.5
LV = 14 LV = 9

The scene to the left has a brightness of LV = 14 and the one to the right a brightness of LV = 9, so different exposure settings will be needed for each. PentaxForums @PentaxForums News | Reviews | Forum

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