There is an equivalence, but it involves exposure time and the differing efficiencies of the two technologies. Their spectra differs too; films respond poorly to the red-heavy light from tungstens.
Originally Posted by John Koehrer
If they had the same efficiency, a 500W-s (that's watt-seconds, i.e. Joules) flash would give the same amount of light as a 500W light for 1s, or a 2000W light for 1/4s. Flashes have far higher efficiency (about 2x to 3x depending on the power levels) though so you get as much light from a single pop from a 500W-s flash as you do from many seconds from a 500W worklight. Flashes are far easier for photography (short exposures! no cooked models!) when there are people involved, except they're harder to preview the penumbra, hence the presence of modeling lights on large strobes.
See Luminous Efficacy for approximate values on the differing efficiencies.
Last edited by polyglot; 04-28-2013 at 10:32 PM. Click to view previous post history.