I'd ask this on pdn, but after my recent barrage of lighting questions there, I figured I'd play it safe and ask here, even though it's not per se a people photography question.

I'll be using a metz 60ct and that mecatwin thing i put aside at Alt Camera. I've been having great luck with GN calculations (meter is next on my list). But how do you calculate the GN with 2 heads? If they are both aimed at the subject, do I just add the total GN, or subtract the distance of one from the other, or what. For exmple, say I have 1 light on the left side and one on the right, with some of the light from both hitting the front of the model, is the GN doubled? This will be a painful post, I'm sure...shawn

-- shawn gibson (shawngibson_prophoto@yahoo.com), February 13, 2000

Answers

If the units have the same GNs, and are used from exactly the same position, you would multiply the GN by 1.414.

Using them in different positions, only partly overlapping on the model, the concept of GN is pretty useless. This is what flash meters were invented for.

-- Alan Gibson (Alan@snibgo.com), February 13, 2000.

Wow it couldnt'a been said in an easier way to understand. So until I get a meter, it's 2 heads for power in the same position at your above multipilcation; or using one on the subject and one for separation/background, each calculated with it's own GN only. It will probably be a guessing game for spill.

Thanks Alan.

shawn

-- shawn gibson (SeeInsideForever@yahoo.com), February 14, 2000.

Wait a second: 1.414, that's 1 stop. I knew if you take a tungsten head and add another one of the same wattage, you are adding one hundred percent more light, which equals 1 stop. Kinda pathetic, but this is a mini-eureka moment...

-- shawn gibson (SeeInsideForever@yahoo.com), February 14, 2000.

Yup. One stop.

-- C Terry (yeti-man@webtv.net), February 14, 2000.

Good thought. It might work. But experience says that one light will always be just outside the one stop range. Therefore the "Dominant Light" rule applies. This rule works almost everytime and goes like this. 1, meter the subject with both lights but with one light off, then do it again with the opposite light on and the other off. Do not change the location of the meter. If you get two different meter readings, the one that gives you the most light is the dominant light. You can use either light as the base and adjust accordingly. If you have two 150 watt lights, one (the main) directly in front of the subject and one at a 45 degree angle (the fill) and both are the same "distance" from the subject, the GN for the exposure will be the same. If you move one light (the fill for instance) closer to the subject the GN for the fill only will change (It will now be the dominant light). The areas lit by the fill will have brighter highlights than those lit by the main. While not hard, this stuff is literally Physics. I suggest you experiment with some film and keep good notes.

Tow quick tips on meters. Spend the bucks and get a good one. When you doubt your meter, use polaroids.

Marcus J.

-- Marcus J. Wilson (marc53@doubled.com), February 14, 2000.

o fuckit im buying a Seikonic. i cant think straight with this stuff

-- shawn gibson (SeeInsideForever@yahoo.com), February 14, 2000.

Thats what I did. I don't like playing with gn numbers and all. Makes it easier. btw, I tested two canon 540ez's side by side in manual mode, and again with TTL (the slave is on an ikelite TTL slave) with the Sekonic L308BII when I got it. It measures precisely One stop difference.

-- C Terry (yeti-man@webtv.net), February 14, 2000.

If you want the GN for two different heads at the same distance here goes. You squarre their GNs and add them up then you extract the squarre root. e.g. GN 54 and GN35 (in meters). sqr((54X54)+(35X35)) =64,35

-- Jacques Desjardins (jac_desjardins@hotmail.com), February 25, 2000.