I received the following message from a professional commercial
photographer in Edmonton, Alberta in response to my initial enquiry
about studio strobes. It is being submitted to the list with permission
of the sender. My thanks go to list member Garth Wood in Edmonton who
passed on my initial message to his friend who in turn provided me with
what is written below. Hopefully this will add to what has been sent
directly to the list for all of which I am extremely grateful.
I wrote originally:
>>>I was told today that any two different brands of studio strobes each
>>>rated at 400 watts / second [or any other rating for that matter] might
>>>not deliver the same intensity of light output. By way of analogy my
>>>source said that in and of itself any particular strobe rating was like
>>>saying that an automobile's engine had four, or five, or six cylinders
>>>and so many valves but did not imply or specify the actual power output
>>>other than suggest, for example, that a 800 w/s rating was likely to be
>>>twice as intense as a 400 w/s rating.
>>>
>>>Can any one out there explain just how you measure the intensity of
>>>light output by reference to the watt per second rating and how this can
>>>be converted into a guide number for any particular ISO film speed?
Here's the scoop. If you like automobile analogies, WS (or more properly
Joules) are more like Horsepower. Assuming it's rated accurately and
truthfully, an 800 J unit does release twice as much energy as a 400 J
unit.
This means that it should be one stop brighter, all else being equal. A
1600
J unit should be 2 stops brighter, etc. This enables you to compare
units.
However, that doesn't translate directly into guide numbers. For one
thing,
some flashtubes may be more or less efficient than others. It Brand A
dumps
10% more energy as heat rather than light, it won't be quite as bright
as
Brand B, which only dumps 50f its energy as heat.
Of course, *MY* brand is far more efficient and brighter than *YOUR*
brand
of the same rated power, right? Everyone's is. In truth, I wouldn't
waste my
time fretting about it. It takes a 100 0ncrease in light energy to be a
stop brighter. Electronic components don't usually vary by a tenth of
that.
Now how does this relate to guide numbers? (I'll state right here, if
you're
serious enough to buy a studio flash, you're serious enough to buy a
used
flashmeter or Polaroid back. I haven't used guide numbers in the 25
years
flashmeters have been easily available.)
The reason pro flashes don't have guide numbers is funnily enough the
same
reason that there are pro flashes - they exist to supply and control
light
through different diffusers, softboxes and reflectors.
Let's use one of my Bowens heads, and set the pack to put 1000 J into
it,
which is my general purpose setup. If I put on a narrow reflector, I
might
get f/32 out of it. No reflector, bare bulb, it might be F16.5, with a
small
soft box, 22, with a huge soft box 8, and with an umbrella (white or
silver?
large or small?) f22 or f11. All at the same distance.
So the intensity of the light (same power, same distance) varies
according
to how concentrated or diffused the light is. Which is why guide numbers
are
of little utility with a modifiable light source - a studio flash.GN is
of
(some) use for an on-camera flash which _does not change_ reflectors.
The GN
would be different for every different umbrella, pan, box, or reflector
I
used. I'd go nuts trying to get a photo made. The ice cream would melt
by
then. So just use a flash meter or better yet a polaroid, and if you
need to
change the light by a half stop, turn it up or down to taste.
Hope this explanation makes sense - feel free to post it to the list if
it'd
be of help to anyone.
- Alan
Alan Schietzsch
____________________
ARS Technical Images
13744-115 Avenue NW
Edmonton AB T5M 3B3
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