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| Re: Group Photo Lighting Advice Needed |
The 600ex specs out at GN 85 for use @ 20mm setting (100 ISO)
The problem is going to be getting the light to cover the FOV needed for the width and depth of the subject.
Shooting @ f4 will provide yield a 20 foot flash to subject distance. BUT ... at 20 ft subject distance @ 20mm flash setting, the coverage will be approximately 30-35 feet wide not taking into account falloff from center to edges.
Arranging 150 people in 6 rows of 25 shoulder to shoulder will be stretch kinda wide. Angling everyone so they are closer together will help, but then that makes the depth of the rows go deeper. Reducing the number of people wide, increases the depth of the rows as well.
Change that to 250 people and you are looking at 25 people, 10 rows deep. Now you are trying to illuminate the front row from 20 feet away to get the width, but the back row is 40 feet away. If you move your lights back to matching Brian's 55 ft shooting position, you've got good coverage but you've got to bump your ISO (which is okay) to offset the distance to the back rows.
Having the two speedlights can offer assistance in getting broader coverage, but there is still the issue @ depth falloff. If you can get your lights higher and/or stagger them for front to rear coverage, you can offset some of the falloff.
Of course, the bump in ISO will assist with exposure & aperture for lens DOF ... but it doesn't address the falloff or width of coverage needed depending on arrangement.
I might take a look at orienting the flash portrait (so it spreads the light deeper than wide) and set the two speedlights @ 1/3 & 2/3 of the FOV width.
I know I didn't say that very well, but it will be challenging to contend with the fov coverage of the speedlight vs. the GN vs. falloff.
You might see if you can run some test shots prior to the event to get a feel for what your 600EX's can do for you, so you'll have a better idea @ how to arrange your groups & lights. I don't think exposure is the big challenge here ... I think it is coverage vs. falloff.