How do flash power fractions relate to EV/stops?

Asked 6/17/2011

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2 answers

I’m comparing flashes for off-camera use. Some specs describe flash adjustment as ±2 EV, while others list manual power levels like 1/2, 1/4, 1/8, or 1/16. What’s the relationship between EV/stops and these fractional power settings, and can they be converted?

off-camera-flash ttl flash exposure manual-flash

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

Photography Stack Exchange contributor

15y ago

2 Answers

A difference of one stop means halving (or doubling) the power/amount of light.

-1 stop = 1/2 power.
-2 stops = 1/4th power.
-3 stops = 1/8th power.
-4 stops = 1/16th power

Originally by user603. Source · Licensed CC BY-SA 4.0

user603

15y ago

AI Answer

Generated from our catalog & community — verify before relying on it.

A 1-stop (1 EV) change in flash output means doubling or halving the light.

So in manual flash power terms:

1/1 = full power
1/2 = -1 EV
1/4 = -2 EV
1/8 = -3 EV
1/16 = -4 EV

Going the other way, each +1 EV is double the power from your current setting.

The catch is that these labels are often used for different modes:

EV compensation (for example ±2 EV) usually refers to flash exposure compensation in TTL/auto modes.
Fractions like 1/4 or 1/16 refer to manual mode, where you directly set a percentage of maximum power.

Because TTL/auto flash decides its base output automatically, there is not always a direct fixed conversion from “+1 EV” to a manual fraction unless you already know what output would be correct exposure at 0 EV. Once you know that reference point, -1 EV is half that power and +1 EV is double it.

In general, the relationship is logarithmic: EV = log2(current power / reference power).

How do flash power fractions relate to EV/stops?

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