Exposure Value (EV) is simply the combination of all shutter speeds and apertures that derive the same exposure. We probably don’t think about it very much but it’s out there all the same. The shutter speed 1/250th at f/8 has an identical EV to 1/125th at f/11. While these two groups of settings may define the same exposure, you might be tempted to ask “Well – WHAT exposure?” And that’s a good question that gets to the math of the problem. We’ll explain it but then we’ll move on – because it can be useful. Here’s the ugly formula – which after you read it – you can forget it: EV = log2 (N²/t), where N = relative aperture and t = shutter speed. Okay – now forget all that – here’s the part that matters. The formula indicates that a 1 second exposure at f/1.0 yields an EV of 0. You can derive everything else from there since it’s all about doubling and halving. For example, 1/250th at f/8 really gets you an EV of 14. WOW! 14! Yeah – so what?
That’s right, want to shoot Christmas Tree lights (last value in the table one)? It says you need an EV of 4-5. Find those rows in table two and you’ve got your shutter speed/aperture combinations that will render it best at ISO 100. Want to go to ISO 200 – simply add one to the value in table one. ISO 400? Add two. And so on.
To photograph outdoor night sports at ISO 400, find the table one value of 9 and add 2 to get EV400 = 11. Then go to table two and get your values for aperture and shutter speed. Oh – and bring a fast lens.
Most cameras don’t let you actually set an EV value for an exposure. You simply set the values indicated in the second table. What if those values are off slightly? There are numerous things that can fool your camera’s light meter – and sometimes you want to apply a universal compensation to your images. Well, you can always dial in a little exposure compensation – which is conveniently graded on the EV scale. Your indicated exposure is too dark – try dialing in +1 EV of exposure compensation. This will brighten up your exposure to a uniform +1 stop. Too bright to start with, dial in -2/3 EV (for example) and see how that works. So, compensation can be either positive (additional exposure) or negative (reduced exposure), and is commonly available in third- or half-step increments, usually up to two or three steps in either direction. Just find that little +/- button somewhere on your camera (usually on the back of Canon bodies and near the shutter release on Nikon bodies) – and dial in what you need. How does it work in reality? Exposure can be adjusted by changing either the aperture or the shutter speed and is often a factor of what exposure mode you are operating in. If you’re working in shutter priority, the camera usually compensates by changing the aperture. In aperture priority the opposite is true – the shutter speed is usually changed to accomplish the compensation. In aperture priority at f/4 – and shooting at 1/250th? Dial in +1 EV via exposure compensation, and you’re shutter speed will automatically drop to 1/125th to make it happen.
Now just keep this in the back of your mind while you’re out shooting. I think if you take the charts with you – you may miss a shot or two.
- Charts courtesy of the Wikipedia article on Exposure Value. You’ll find all the math there. You can also find the footnotes for table one.