# What's the relationship between flash power expressed as a fraction and as EV?

I'm looking around for a second flash to play around with for off-camera stuff. Some flash specs I'm reading say that I get an adjustment range of plus or minus 2 EV or similar, while others say I get ½, ¹⁄₄, ¹⁄₁₆, and so on as manual settings.

What's the relationship between these different measurements? How do I convert from one to the other?

+-EV is used to measure compensation of automatically determined flash power (using either TTL or light sensor on flash). Fractions are used in manual mode and refer to how much of maximum power the flash is using.

Since these measures are used in different modes and there is no easy way to determine exactly how much power your flash uses in auto mode, you can't directly convert from compensation value to a manual setting. First, you'll have to determine flash power for correct exposure.

When you have determined manual flash power that gives you correct exposure, you have the equivalent of 0 EV. Now, if you double the power, that is +1EV; similarly, if you halve the power, it gives you -1EV. The amount of EVs gained by such compensation can be calculated as

log2( current setting / correct exposure setting )
• +1 Thanks for the additional info - I hate logarithms. My point of confusion is that different manufacturers confuse terms. eg. Nissin and Canon
– Mike
Commented Jun 18, 2011 at 1:45

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

• +1 Thanks Jerry. It sort of makes claims of being able to dial your manual flash power down to 1/128th (or -7EV) rather pointless.
– Mike
Commented Jun 18, 2011 at 1:42
• @Mike: Not entirely. These are relative to the maximum amount of power the flash can supply, not relative to the exposure you're using. For macro work, in particular, you often need to dial the power down quite a ways. Commented Jun 18, 2011 at 1:47
• And if you want a really short burst, dialing down to the smallest possible fraction is the way to go. Commented Sep 15, 2011 at 21:40
• I've often used 1/128 on a flash to add a bit of gentle fill light, especially when using a very wide aperture. Commented Sep 15, 2011 at 22:18