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Photography Question 
Marius Liebenberg
BetterPhoto Member Since: 11/21/2005
 

Strobe power & inverter output


Q1. What strobe Ws power rating is required to balance ambient daylight when used in umbrella or soft box? I do mainly portraits and fashion shoots.
Q2. How much inverter power (Watt) is required to adequately power the above strobe from a 12V DC source?


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10/30/2007 3:33:01 AM

 
Alan N. Marcus   Hi Marius,

Wattseconds = (Capacitance times voltage) squared divided by 2,000,000
Capacitance value is stated in microfarads.

We cannot convert Wattseconds directly to a guide number because we donít know the efficiencies of the circuit or the beam angle of the flash reflector combination etc. Once efficiency is derived this value is called the Effective Candle Power Seconds. For a test equation use an efficiency of 70% until this value is proven to be incorrect.

The Guide number (the more significant quantity) is calculated by taking the square root of (0.63 times ECPS X ISO.

Formulas are derived from publications of the American Standards Association (ASA).

Now you should be able to calculate the answers you asked for.
A trouble-free method to derive the Guide Number:

Set the flash lamp to subject distance at 10 feet. Shoot a sequence at various apertures. Place a placard in each trial exposure scene to label the aperture used.
Examine the images for best exposure. Multiply the aperture value used by 10. The result will be an accurate guide number for your flash unit.

You should know that in my technical coaching days I generally bored the socks off students. Well somebody had to do it.

Alan Marcus
ammarcus@earthlink.net


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10/30/2007 7:20:41 AM

 
Alan N. Marcus   Additional stuff:
One approach to manage flash fill under sunlit conditions: The lens aperture and shutter speed is set to obtain an accurate exposure based only upon the sunlight reading as taken with a quality light meter. The chosen shutter speed is one that will synchronize with the flash.

Now calculate the fill flash lamp position (lamp-to-subject distance:

Divide the flash unit guide number by the lens-opening f-value obtained by the meter reading above. The result will be the lamp to subject distance in feet that will cause the light from the flash to arrive at the subject plane equal to the sunlight value.

To achieve the desirable 3:1 lighting ratio: Multiply this calculated flash to subject distance by 1.4. Relocate the fill flash to subject distance to this revised more distanct footage value.

For a 5:1 (higher contrast) flash is again setback from the 3:1 position using the 1.4 multiplier.

For a 9:1 (theatrical high contrast) flash is again setback from the 5:1 position again using the 1.4 multiplier.

Alan Marcus (marginal technical claptrap)


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10/30/2007 2:54:35 PM

 
Alan N. Marcus   I reread question 2 and concluded I misunderstood. What you are asking is not how to build a strobe power circuit but how to power one from an automobile power outlet.

Read the amperage rating on the unit. It will likely state the voltage and amperage consumed. All you need is an inverter rated for that amperage. Best to oversize the inverter by 10% or so.

The aditional stuff I wrote above will allow you to derive what you want answered in question 1.


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10/31/2007 6:47:57 AM

 
Mark Feldstein
BetterPhoto Member Since: 3/17/2005
  OR...you could buy and learn to use a flash meter. Then set your lights up one at a time and by zone.

OK, now a Halloween math problem. A witch on her broomstick leaves Sheboyban Michigan at 3:12 P.M. headed for Janesville Wisconsin. THEN another witch on HER broomstick leaves Kansas City Mo for Lancaster, PA. Witch # 1 from Michigan encounters a headwind of 15 knots while witch #2 picks up a tailwind of 23 knots. How soon will it be before the first witch gets to her first trick or treat stop in Janesville and the second to Lancaster?

Take it light.
M/

=====================================
Oh noooooooooooooooo, not the BROOMSTICK problems !!!! Be afraid...be very afraid....BOO !!!!
t light.
M.


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10/31/2007 9:15:39 AM

 
Mark Feldstein
BetterPhoto Member Since: 3/17/2005
  OR...you could buy and learn to use a flash meter. Then set your lights up one at a time and by zone.

OK, now a Halloween math problem. A witch on her broomstick leaves Sheboyban Michigan at 3:12 P.M. headed for Janesville Wisconsin. THEN another witch on HER broomstick leaves Kansas City Mo for Lancaster, PA. Witch # 1 from Michigan encounters a headwind of 15 knots while witch #2 picks up a tailwind of 23 knots. How soon will it be before the first witch gets to her first trick or treat stop in Janesville and the second to Lancaster?

Take it light.
M/

=====================================
Oh noooooooooooooooo, not the BROOMSTICK problems !!!! Be afraid...be very afraid....BOO !!!!
t light.
M.


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10/31/2007 9:16:32 AM

 
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