Wattage to light output and stops

Canon RF 24-70mm f/2.8 L IS

12/21/2009 08:24:32 PM · #1

Hi. I don't know if this question can be answered specifically, but even a general idea would be helpful.

I'm using some homemade lighting equipment for a variety of things. I have used it for simple portraits and used it for these photos:

It does what it needs to, and I learned a lot about lighting when building it. I use up to two, 100W bulbs on each pole. I learned very quickly that 400W total is not a lot of light when taking photos of people, especially squirmy kids.

So, my local photo store is selling the Westcott uLite 3 light set which includes two 500W flood lamps and a 100W backlight.

Yes, I know those will be hot, but I don't do very much portrait work at all. My question is this, if I had the same setup as I do know with two 100W bulbs on each post (400W total), how much light gain would there be with 500W on each (1000W total) in terms of stops? Or, is there some practical way to explain the light gain?

Thanks,
Paul

kirbic

Canon EOS R5

12/21/2009 08:39:10 PM · #2

Well, if we make the assumption that the light is being spread over the same area in both cases, then 1000W will get you a little over 1 stop. Remember that 1 stop more light means "twice the light," which really does mean "twice the power" for continuous lights, or "twice the energy" for flash.

ETA: to be exact: 1.3219 stops :-)

Message edited by author 2009-12-21 20:48:12.

Samsung Galaxy S24, S24+, S24 Ultra

12/21/2009 08:43:34 PM · #3

Yes, I'm making the assumption that the spread of light would be the same. I wasn't sure if the light output was linear with respect to wattage. In this case 1000/400 = 2.5/2 = 1.25 stops. Is it really that linear?

Bear_Music

Sony 24-70mm f/4 Vario-Tessar T* FE OSS

12/21/2009 08:48:20 PM · #4

The problem here is that wattage is a measure of power consumed, not lumens produced. And, typically, the more lumens produced by and incandescent bulb of a given wattage, the shorter will be its life. 100-watt photofloods are MUCH brighter than 100-watt household lightbulbs, but they don't last anywhere NEAR as long.

Assuming you're using bulbs of the same general specification, two 250w photofloods should produce the same light as one 500w photoflood, and half as much light as a single, 1000w photoflood. So the "stops" are linear, double the wattage and halve the exposure time.

R.

Bear_Music

Samsung Galaxy S24, S24+, S24 Ultra

Sony 24-70mm f/4 Vario-Tessar T* FE OSS

12/21/2009 08:49:35 PM · #5

Originally posted by PGerst:

Is it really that linear?

Yes, if the bulbs themselves are directly comparable. If you have been using four 100w household bulbs and you switch to a 1000w photoflood, it's gonna be a LOT more than twice as bright.

R.

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

12/21/2009 08:53:15 PM · #6

Originally posted by PGerst:

Actually, the math goes like this:

P = 2^f (where P is the change in power and f is the change in f-stop. P equals two raised to the power of f.)

Solving for f, we get:

ln(P)/ln(2) = f (that's natural log of P divided by natural log of 2)

so for P = 1000/400 = 2.5 we get:

ln(2.5)/ln(2) = 1.3219

Make sense?

Message edited by author 2009-12-21 20:53:48.

12/21/2009 08:54:17 PM · #7

I would be going from four 100W households to two 500w photofloods.

Originally posted by Bear_Music:

Yes, if the bulbs themselves are directly comparable. If you have been using four 100w household bulbs and you switch to a 1000w photoflood, it's gonna be a LOT more than twice as bright.

R.