What is the relationship between watt secs and subject matter in photo lighting?

Answer 1

Watt-seconds, or joules, is a measure of the power output of a flash unit. Usually, only studio lighting equipment is of sufficient power to be rated this way, and the numbers are best used to compare one unit with another as opposed to relating to the subject matter. Since they are often used with reflectors of all sorts, I personally wouldn't make a connection with a rating and the subject matter at hand. Lumens, which can be measured by a light meter at the subject's position, would be a more meaningful measurement. However, I would not be too concerned with Lumens either, preferring to rely on the meter reading expressed as an F number and shutter speed combination as long as I had tested all the equipment first.