Formula
The shutter is entered as the denominator of a one-over fraction:
EV = log₂(aperture² × shutter denominator)
This is the camera-setting value, not a light meter. Depth of field uses the aperture for sharpness and does not use the shutter.
Assumptions
- The shutter field is the denominator. 125 means 1/125 second.
- ISO is 100, because this EV definition leaves sensitivity out.
- The aperture is the f-number.
Worked example
f/8 at 1/125 second is 12.97 EV.
Input definitions
- Shutter denominator
- The bottom of the shutter fraction. The exposure time is one divided by that number.
- Exposure value
- A log scale on which doubling the light changes the value by one.
Limitations
This is a planning figure from the numbers you type. It is not a measurement of a network, a camera, a printer, or a power supply.
Questions
Where is ISO?
This EV is defined from aperture and time. Changing ISO would be a different exposure for the same light, and it is not an input here.
Why log base 2?
Each step of one EV doubles or halves the light. Base 2 counts those doublings.