BetterPhoto Q&A
Category: New Answers

Strobe power & inverter output

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October 30, 2007

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 don’t 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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October 30, 2007

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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October 30, 2007

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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October 31, 2007

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.

To love this comment, log in above
October 31, 2007

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.

To love this comment, log in above
October 31, 2007