In my previous post I commented on the difficulties of taking action shots (horse jumping over a fence) in limited light. It seemed gloomy (it was noon in midwinter on a heavily overcast day) but how much darker than on a typical sunny day?
You can get an objective measure of the ambient light level by calculating the Exposure Value (EV) from the aperture, shutter speed and ISO setting required for correct exposure. EV0 corresponds to an exposure of 1 second at f1 with ISO set at 100. From there, each f-stop narrowing of the aperture corresponds to an increase in light level of 1EV. Similarly a halving of the exposure time or halving of the ISO setting, also correspond to +1EV.
A typical cloudless sunny day would require 125th at f16 and ISO 100 – a rule of thumb known as “sunny 16″. This corresponds to EV15. Relative to the definition of EV0, we have kept the ISO unchanged but narrowed the aperture by 8 f-stops and halved the exposure time 7 times.
Looking at the EV for the day at the stables, the shot of Bazil jumping was taken at f3.5 and 1/400th with ISO at 400. That works out at EV10.3, in other words nearly 5 f-stops shy of the light levels on a “sunny 16″ day. That’s 2 stops less light than suggested under a simplistic application of the Sunny 16 Rule.
The general formula for EV is:
EV = log2 (aperture x aperture) +log2 shutter – log2 (ISO/100) – expadj
where log2 means the logarithm to base 2, the aperture is the f-number, the value for “shutter” would be say 500 if shooting at 1/500th and expadj means any exposure adjustment applied in post, in say Aperture or Lightroom, expressed in stops.