Exposure Value (EV)
The exposure value represents a combination of shutter speed and
aperture setting (f-number), independent of sensitivity (ISO)
setting. Since the camera settings for a given EV takes no account
of the sensitivity setting, it is normal to adjust the EV to take
sensitivity into account, and the Readout value is an EV adjusted
for an ISO speed of 100.
The combination of camera settings that result in a particular
exposure value adjusted for a particular sensitivity is given by the
equation:
EV = log2 (F2/(ISO * s))
Where:
F = f-number
ISO = ISO sensitivity.
s = shutter time in seconds.
The measured EV value is an exposure recommendation based on either
the incident light (ambient illuminance measurement) falling on the
subject of the photograph,
or the reflected light (spot emission luminance measurement) from
the subject towards the camera,
both of which are computed according to the ISO 2720 method. The
appropriate calculation is chosen automatically based on the
instrument measurement mode.