Re: Watt-seconds in flash

Tomo Tanaka <ttanaka@xxxxxxxxxxxxx> · Fri, 09 Feb 2007 10:39:59 -0800

Hmmm... this is making me think too much for a Friday. See below.......

Herschel Mair wrote:

  OK guys...

  Well perhaps I have it wrong...
  Correct my arithmetic:

  1 joule per second is a watt.

  1 watt second is 1 joule per second  for 1 second

  500 watt seconds is 500 joules per second for 1 second 
  (or in the case of a theoretical flash unit,  its 500 000 joules
for 1/1000th of a second)

  A 500 watt lamp, burning for one second is 500 joules too so a
1-second exposure should give the same amount of  energy (light) to the
sensor/film as a single burst from a 500 ws flash.... 500 joules????

The "single burst" is actually a 500 000 W light for a duration of
1/1000 sec which equals 500 joules

  Where did I go wrong?

  My point was:

  In a perfect world, a single burst on a 500 WS flash unit should
require the same aperture as a 500W tungstren light at 1 second
exposure which is also 500 WS.

  (Not allowing for the inefficiency of the tungsten lamp)

  A good part of the energy is given out as heat in both systems so
it becomes impossible to actually relate watt-seconds to light output.

  Herschel Mair

  Head of the Department of Photography,

  Higher College of Technology

Muscat

Sultanate of Oman 

  Adobe Certified
instructor

  + (986) 99899 673

  www.herschelmair.com

  -----
Original Message ----

From: Qkano <wildimages@xxxxxxxxxxx>

To: List for Photo/Imaging Educators - Professionals - Students
<photoforum@xxxxxxxxxxxxxxxxxxxxxx>

Sent: Friday, February 9, 2007 2:58:09 PM

Subject: Re: Watt-seconds in flash

  >Ah Roger... read it again:

>

>>No!  Watt-seconds is just that.  Watts times seconds.  It's a
measure

>

>That's exactly what I said... allow me to do the math for you:

Hershell

I thought you had it wrong too I'm afraid.

I understood the correction watts.seconds was much clearer.

Bob

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