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[OM] ( OM ) Question for Dr Flash ?

Subject: [OM] ( OM ) Question for Dr Flash ?
From: usher99@xxxxxxx
Date: Tue, 14 Sep 2010 19:52:27 -0400
Thanks Dr.Flash,

I  agree with your thorough analysis and appreciate the final
suggestion but still perceive a conflict in the JS advice.

  I had the GN of  the T28 Macro flash in
mind with this question which I assume was
determined by Oly to be reasonably accurate in its macro  application
and they clearly used it for the calculator panels.

A repeat exposure test for a mid-toned subject would essentially
provide another real GN again, No?.  So why would ANY substantial 
adjustment
for a non-neutral subject be required if the light output is fixed?
His statement on pg 108 implies the direction and magnitude of the
exposure adjustment is opposite to that for TTL or normal auto flash.
Perhaps he really didn't wish to imply the magnitude of the exp
compensation would be the same, but it reads that way.  I can see a 1/2
stop or 1 stop tweak  less  light on say a white flower with manual
macro flash to avoid losing the texture and details but a full 2 stops
if if 2 stops lighter than mid-toned seems excessive.

   The calculator panels with the Zuiko 135 macro and T28 seem about 
correct at least
within the exp latitude of Ektar 100 or UC100.  I don't remember trying 
that with Velvia.

A Still Puzzled , Mike



Dr. Flash states,

"I don't think there's any conflict of advice here.  Note that the
advice
on page 108 says absolutely nothing about use of guide numbers.  His
advice is strictly on using testing to determine actual exposures.  His
advice to do the reverse of what you do with TTL or autoflash when
using
manual exposure is in how you interpret and adjust the results of the
exposure tests.

If you go back to pages 80-85 (flash theory) you will see a description
of manufacturers guide numbers as being based on an assumption of use
in
a moderate size room with an assumed reflectivity of walls and ceiling.
  The accuracy of the guide numbers depends on assistance from those
reflective surfaces.  When used outdoors you don't have those
reflective
surfaces and the guide numbers are therefore... invalid.  In that same
section he proposes that you conduct a series of exposure tests to
determine the real guide numbers for the conditions where you're using
the flash.

Jumping ahead to page 108 again notice that the section is labeled
"Adding flash to zooms with diopters".  In this section he's advocating
another whole series of exposure tests because the different optical
configuration has presumably invalidated your first "real" guide number
calculations.

I suggested a comparison of the flash output (a constant) with an
incident reading not because they are the same but because both are
independent of the reflectivity of the subject.

Do your exposure testing with a digital camera.  You could also use a
flash (incident) meter but that's complicated for use with macro and
filters."

Dr. Flash



On 9/13/2010 11:23 PM, usher99@xxxxxxx wrote:
> Well,
> Perhaps the exposure suggested by use of GN and distance is not really
> the same as an incident reading
> and gives a "correct" exposure for a mid-toned subject. To quote JS in
> his first book,
> "When you use manual flash, you have to compensate for nonneutral
> subjects by doing the reverse of what you do
> with TTL or autoflash." (pg 108)  He recommends of course to determine
> the output of the flash by a series of exposures.
> It goes with out saying that other concerns regarding how to depict
the
> scene of the photog should trump what should be "correct."
> Dr. Flash suggested this exact book on manual macro flash a number of
> years ago and I followed his suggestion.
> Nonetheless, if the midtones are placed properly and the light output
> is fixed, why should the tonality of the subject matter?  Perhaps as
he
> largely used
> Kodachrome 25 in the book, there was too much risk of blowing out
> highlights or muddying up dark subjects if not adjusted.  I was hoping
> you notice this
> issue and clalrify it. I know that bothered me when I read it too.
> A Student of Dr. Flash, Mike
--
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