software – Creating a digital photo with an exposure of EV -2

EV -2 or any other Exposure Value value is not a particular brightness or illuminance level, it is a particular combination of sensitivity (normalized at ISO 100), aperture, and shutter time used to expose a photo. As such, you can’t really create a photo “with” an EV of -2. You can only create a photo “using” EV -2.

Two photos can be taken at the same Exposure Value and one could be totally dark while the other might be very bright. It all depends on how much light is in the scene during exposure. Exposure Value is an expression of the mechanics used to capture the light, not an expression of the quantity of the light captured.

Let’s use the old rain bucket analogy. Imagine you set out several rain buckets of various dimensions but with the same total volume. Those with larger diameters would equate to wider apertures. Those with more depth would equate to a longer shutter times (because it would take them longer to collect the same volume of water).

If one bucket has twice the surface area at the opening (the diameter would be 1.414 or square root of 2 times larger than the smaller bucket) we could call it f/2 @ 1 minute. Since the other bucket is twice the height we could call it f/2.8 @ 2 minutes. If both buckets are left out under identical rates of rainfall they would both have the same Exposure Value because they would both collect the same amount of rainfall with each combination of opening size and time left in the rain. The wide one would take one minute to fill, the narrow one would take two minutes to fill. Thus both are the same Exposure Value.

But let’s say when we put out the narrower bucket for two minutes it’s barely sprinkling. The bucket will only have a trace amount of water in the bottom which would equate to a very dark photo. When we later put out the wider bucket for one minute it’s during a heavy thunderstorm. The bucket will be almost full which would equate to a very bright image. Yet we used the same EV for both!

The rate at which the rain falls is not an Exposure Value, it is an Illuminance Value or Luminance Value. In other words, it’s how much light there is in the scene.

From the Wikipedia entry for Exposure Value:

Strictly, EV is not a measure of luminance or illuminance; rather, an EV corresponds to a luminance (or illuminance) for which a camera with a given ISO speed would use the indicated EV to obtain the nominally correct exposure. Nonetheless, it is common practice among photographic equipment manufacturers to express luminance in EV for ISO 100 speed, as when specifying metering range (Ray 2000, 318) or autofocus sensitivity.And the practice is long established; Ray (2002), 592) cites Ulffers (1968) as an early example. Properly, the meter calibration constant as well as the ISO speed should be stated, but this seldom is done.