I have seen an "EV" (exposure value) scale that's sometimes used to express a camera's exposure settings, or scene brightness. How exactly does this scale work?
We know that for any scene (really, any light meter measurement) of a particular brightness (and particular sensor sensitivity) there is usually more than one "correct" set of shutter speed and aperture settings. A scene that wants f5.6 and 1/125 will also be correctly exposed at f4.0 and 1/250 and so on.
EV numbers are a way to express the brightness of a scene in a scale that combines the shutter speed and aperture settings into one number -- letting the photographer choose what combination of shutter speed and aperture settings to use. Each EV number equals one stop of brightness, so a scene with an EV of 6 is one stop brighter than a scene with an EV of 5.
The EV values are used generally in the following ways:
For all the technical details (including the formula), look at Wikipedia's "Exposure value" entry.
The exposure value (EV) is usually defined as
This means that EV 0 equals an aperture of f/1.0 and a shutter speed of 1 second. If you increase the aperture by the square root of 2 or decrease the shutter speed by a factor 2 you increase the exposure value to 1.
More generally an increase or decrease of the exposure value by 1 corresponds to what's called a "stop".
The exposure value is calculated from the camera's settings and all shots (of a given scene) taken with different settings that correspond to a certain exposure value will yield the same level of exposure. This is useful since you can alter the shutter speed and the aperture, while maintaining the same exposure level to accomplish different effects such as shallow/deep depth of field or freezing of movement/motion blur.
If you've metered the light you can calculate the different sets of settings that will give you correct exposure prior to taking the shot. This is what all modern cameras do automatically, but by knowing how this is done you can do a better job yourself and you'll also get a deeper understanding of photography and means to be more successful as a photographer.
Exposure Value (or, EV) is a way of representing the brightness of a scene in a simple numeric scale. The scale is measured in "stops", a standard concept in photography where each stop is a doubling or halving of the amount of light.
This is convenient, because we also work with the fundamental parameters of exposure — aperture, shutter speed, and ISO sensitivity — in stops. That is, each step on the standard aperture scale (f/1.4, f/2, f/2.8, f/4, f5.6, etc.) represents an opening half the area of the previous step, which of course means half of the light reaches the sensor. And we typically work with corresponding doublings or halvings of shutter time and ISO.
That means that if you know the Exposure Value of a scene, either by metering or even by guessing, you can easily set your camera's controls to get the right exposure by using any combination that fits. And, if you know that you want the scene to be brighter or darker than the meter (and therefore automatic program-mode camera settings) say, you can use EV compensation to tell the camera you want something different. This is typical for snowy landscapes — the meter is dumb and doesn't know that that white snow isn't typical medium gray, so you can dial in EV compensation to compensate.
I'll get to how one might adjust aperture, shutter, and ISO in a bit, but first, a depiction of the EV scale itself. EV 0 corresponds to a scene which is correctly exposed with a 1 second shutter and a theoretical f/1.0 lens. Brighter scenes go up from that, requiring either a shorter shutter speed or a smaller aperture (that is, higher f numbers). Darker scenes require longer exposures and larger apertures — or, the right combination, since lenses as fast as f/1.0 or even faster are extremely rare beasts.
Technically, EV is just shutter speed and aperture without regard to sensitivity, but we typically assume it to mean EV₁₀₀ — that is, Exposure Value at ISO 100. That allows us to make a chart correlating EV to the real world. Here's a simplified version of a table from Wikipedia (which was taken in turn from the ANSI standard):
That's a nice enough chart, but I find pictures even more helpful, so here is the above as a series of concentric circles. If I did every whole-number level, each ring would have the same area as all of the rings it contains, but to keep it relatively uncluttered I'm skipping some steps. It is, however, to proper scale — the relative sizes correspond to the increase in scene brightness.
(Note that these are overlaid — the area of the whole circle is what matters, not the separated rings. And, yes — sunlight is 500× brighter than a typical home interior.)
So, how can one use this practically? It's pretty simple: once you know the EV of the scene, start at EV 0 (ISO 100, 1 second shutter, f/1.0) and count stops until you get to the desired number. For example, to get to EV 13 — a cloudy day — you might count aperture first: "f/1.4 — one; f/2 — two; f/2.8 — three, f/4 — four, f/5.6 — five, f/8 — six", and then shutter: "½s — seven; ¼s — eight; ⅛ — nine; ¹⁄₁₅th — ten; ¹⁄₃₀th — eleven; ¹⁄₆₀th — twelve; ¹⁄₁₂₅th — thirteen!"
Or you can just let the camera's automatic metering handle it for you — but it helps to know what's going on under the hood.