This picture was taken mid afternoon in Dubai, with the sun about half high in the sky. The flash power needed in this circumstances is no mean feat, if an umbrella is used. The aperture was relatively open at 6.3 because I used a Polarizer to soak up light and smoothen the skin tones. The main light comes from a shoot through umbrella with three Metz 45 at manual full power. The fill light was an on camera SB 900 set to manual 1/32 which also triggered the Metz Set up.
The three flashes assembled with their brackets. All of them had the diffuser on, and were set to full Manual Output.
I used three photocells to trigger them individually, because I used three different models. CT-1, CT-4 and CL-4 digital. I did not want the three different models with very different Voltages to be wired together. Therefore one cell for each.
!!!! Caution !!!! If you connect a CT-1 and a low voltage model to the same cell electronics might be damaged!
The main question I want to explore today is the one of multiple flashes used to get a stronger output. Joe Mc Nally's Christmas Tree like set up with many SB-800 and SB-900 are legendary.
However there is physics to be considered. Combining two flashes does NOT result in double the guide number. Because light fall off in square relation to distance one needs a the power of 2 of number of flashes. Therefore to double the guide number (effectively double the distance) FOUR units are needed. To quadruple the guide number SIXTEEN units are needed.
There is a German website http://www.plainpix.com/fototools.php which has a number of tools to easy calculate some photographic relations. On the bottom of the page is a tool where one can give in the guide number of combined flashes and the overall guide number is calculated.
Below is a table with Guide Numbers for additional flash units used. (Guide Number for ISO 100 in meters at 35 mm)
|Flashes||SB 800||SB 900||Metz 45|
Looking at the table there are a few interesting observations.
1) For each additional unit added the actual increase in guide number gets less and less. The second SB 800 adds 16 to the overall guide number. The eighth adds only 6. The conclusion is that the "many small light" build up has its limitations and the point where it becomes really impractical is quickly reached.
2) Comparing models in the table one realises that maximum initial power is desirable in such a set up. For a guide number of 90 one needs 4 Metz 45ers compared to 7 SB900.
3) Big Stobes are really more practical from a point. 500 Ws = GN 85, 1000 Ws = GN 120, 1500 = GN 150. Comparing price and bulk the big ones are worth a consideration if a high power light output is needed. To replace one 1500 Ws big flash 16 SB900es would be needed.
Huh? Isn't that contrary to the whole carry less equipment story that started the strobist movement?
Well a full fledged big flash set up with 2 to 3 lights is much more bulkier than 3 or even 8 small flashes. The many small flashes are also more versatile in distributing them. And most of the time when full power output is not needed the small flashes do the same job as a big one with a weight advantage comparing light to light.
However when Raw power is needed the big ones come to the forefront.
Back to the picture above. The three Metz 45 have a raw GN of 78. With 200 ISO , 1.7 meter distance that would amount to an Aperture 64. The light loss in the shoot throough umbrella of roughly two stops brings us to Aperture 22. A further two stops of loss for the polarizer and we arrive at 11. The wide angle diffusers bring us to about 8. Close enough to the aperture found by test shots.
The sky is in the direction against the sun and is still very, very bright. Closing the aperture further to get the sky darker would have needed quite a bit more light.
So how far can I go by using all my available flash units fired together? 2 x SB900, 4x SB800, 2 x Metz 54-4, 6 x Metz 45 Total =152 I have not tried that yet. There will be another sunny weekend in Dubai.