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<blockquote data-quote="WayneF" data-source="post: 444692" data-attributes="member: 12496">15 seats per row, with 18 inches spacing, is 22.5 feet width.18mm DX at 15 feet is 19.7 feet width, field of view.Calling the width 20 feet, then at 41 feet distance (for some range depth), that computes 48 mm focal length.( <a href="http://www.scantips.com/lights/fieldofview.html" target="_blank">Lens Field of View Calculator (FOV)</a> )SB-910 DX Guide number for 50mm zoom is 147.6 ISO 100.&nbsp;&nbsp; At ISO 400, GN 295.Plugging these numbers (GN 147.6, ISO 400, 41 feet) into GN calculator at <a href="http://www.scantips.com/lights/flashbasics1c.html" target="_blank">Understanding Guide Numbers, including GN Calculator</a>at 41 feet shows f/7.1 with one SB-910 flash, maximum power (for this GN, Standard DX mode).&nbsp;&nbsp; Two equal flashes is f/10.41 feet at f/7.1 shows +/- 1/3 stop range is 36.90 to 46.49 feet, which is 9.6 feet range.&nbsp; I suspect four rows of chairs and aisles may be deeper than that?and assuming if centered at 41 feet, then +/- 1/3 stop means there is 2/3 stop difference, front to back.Calculator shows that at 18mm 15 feet, you could use 1/4 power level for f/10But the range of 15 feet f/10 is only 13.05 to 16.44 feet, +/- 1/3 stop.&nbsp;&nbsp; That is only 3.4 feet range (at +/- 1/3 stop exposure).</blockquote>

[QUOTE="WayneF, post: 444692, member: 12496"] 15 seats per row, with 18 inches spacing, is 22.5 feet width. 18mm DX at 15 feet is 19.7 feet width, field of view. Calling the width 20 feet, then at 41 feet distance (for some range depth), that computes 48 mm focal length. ( [URL="http://www.scantips.com/lights/fieldofview.html"]Lens Field of View Calculator (FOV)[/URL] ) SB-910 DX Guide number for 50mm zoom is 147.6 ISO 100. At ISO 400, GN 295. Plugging these numbers (GN 147.6, ISO 400, 41 feet) into GN calculator at [URL="http://www.scantips.com/lights/flashbasics1c.html"]Understanding Guide Numbers, including GN Calculator[/URL] at 41 feet shows f/7.1 with one SB-910 flash, maximum power (for this GN, Standard DX mode). Two equal flashes is f/10. 41 feet at f/7.1 shows +/- 1/3 stop range is 36.90 to 46.49 feet, which is 9.6 feet range. I suspect four rows of chairs and aisles may be deeper than that? and assuming if centered at 41 feet, then +/- 1/3 stop means there is 2/3 stop difference, front to back. Calculator shows that at 18mm 15 feet, you could use 1/4 power level for f/10 But the range of 15 feet f/10 is only 13.05 to 16.44 feet, +/- 1/3 stop. That is only 3.4 feet range (at +/- 1/3 stop exposure). [/QUOTE]

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