How do you combine exposure values from a bright lamp and ambient light?

Asked 8/16/2015

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If a lamp contributes about EV 17 and the surrounding cloudy daylight is about EV 14, how do you determine the effective exposure value of the lamp area in the photo? In other words, how are two light levels combined when one source is much brighter than the other?

ev lighting metering exposure

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

10y ago

2 Answers

The math on two fstops is the square root of the sum of the two squares, but I don't know about EV.

If the lights were equal, and if they lighted the same overlapping area, twice the light is one stop additional. But you say the ambient is 3 stops down from the lamp, so it won't have much effect. This ambient adds less than 0.2 stop increase, not over 17.2EV.

Originally by user38978. Source · Licensed CC BY-SA 4.0

user38978

10y ago

AI Answer

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You do not add EV numbers directly, because EV is logarithmic. Convert the difference in stops to a light ratio first.

Here, EV 17 is 3 stops brighter than EV 14. A 3-stop difference means the brighter source has 8× as much light as the dimmer one. So the total light is:

8 + 1 = 9 units

Compared with the lamp alone (8 units), the increase is:

9/8 = 1.125×

That is only about 0.17 stop more light, so the combined exposure is about EV 17.2.

So in practice, the lamp still exposes at essentially EV 17, with only a very small boost from the ambient light.

A useful shortcut: when one light source is several stops brighter than the other, the weaker source has little effect on the final exposure.

How do you combine exposure values from a bright lamp and ambient light?

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