Thursday, January 18, 2007
January/February 2006 HelpLine
Q) I've been in and out of photography for a number of years, and it seems that each time I get back into it, they change the rules. I got through the change from ASA to ISO. I've even gotten through the digital hurdle. But can you please explain EV to me and why I should care about it? It keeps popping up.
A) EV has actually been around for quite a while and isn't a digital-specific term. EV stands for exposure value and is just another way of talking about exposure. Unfortunately, it's not quite as simple as the film-speed name change from ASA to ISO. Though the math is a little complicated, EV is derived from a combination of lens aperture and shutter speed.
There's an explicit value for EV of zero—a shutter speed of 1 second with an aperture of 1.0. But it's more useful to consider exposure value in terms of how you might change your exposure.
Start with the understanding that an increase of 1 EV is equivalent to halving the light allowed to reach the sensor. A decrease of 1 EV is the same as doubling the light. (If you must know the math, EV is measured on a negative logarithmic scale.)
In terms of f-stop or shutter speed, an increase of 1 EV is the same as either decreasing your ƒ-stop by 1 (say, ƒ/8 to ƒ/11) or shortening your shutter speed a whole step (say, 1/125 to 1/250 sec.). Realize that you're changing one parameter or the other-either one reduces the light by half. If you decrease your ƒ-stop from ƒ/8 to ƒ/11, but also increase your shutter speed from 1/125 to 1/60 sec., the exposure will stay the same, resulting in no change to your EV or the light reaching your sensor.
A decrease of 1 EV comes from the opposite change in your f-stop or shutter speed-either increasing your f-stop by 1 (say, ƒ/11 to ƒ/8) or increasing your shutter speed a whole step (say, 1/250 to 1/125 sec.). Again, you'd change one parameter or the other-in this case, either one increases the light by two.
And that's where EV comes in handy. It's useful for defining a change in exposure without explicitly calling for a change in aperture or shutter speed—you get to choose. If I say, "You overexposed by 1 EV," you can decide whether to correct that by a change in your ƒ-stop or your shutter speed.
Now here's a question for you: If the explicit value of EV=0 is ƒ/1 at 1 sec., can you think of another value for EV=0? Next month, I'll give you an answer or two.
Page 4 of 4