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Re: [OM] ( OM ) Question for Dr Flash ?

Subject: Re: [OM] ( OM ) Question for Dr Flash ?
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 14 Sep 2010 20:37:19 -0400
Yes, with respect to the GN of the T28 Macro flash I accept that the 
guide number is accurate for its intended usage on macro subjects... at 
least within a tightly controlled range.  In fact, I suspect (but don't 
know) that the flash electronics are basically from the T32 and that the 
reduced guide number (28 vs 32) is the result of operating on macro 
subjects without walls and ceilings to reflect back on the subject as 
would typically be the case with the T32.

I still don't understand your confusion though.  You refer to page 108 
and ask "So why would ANY substantial adjustment for a non-neutral 
subject be required if the light output is fixed?"  I can only conclude 
that you haven't noticed that page 108 is describing the use of zooms 
and diopter close-up lenses along with macro flash.  The light output is 
fixed but the magnification of the image is not.  Magnifying the image 
spreads a fixed amount of light over a larger area and thus invalidates 
the guide number.  That should also tell you that the guide number is 
really only valid over a very limited range of distances and a narrow 
magnification range.  Trust but verify.  :-)

You also say: "His statement on pg 108 implies the direction and 
magnitude of the exposure adjustment is opposite to that for TTL or 
normal auto flash."  I'm not sure I'd have stated it exactly that way 
but in the TTL and autoflash cases you are trying to control the amount 
of light emitted from the flash.  In the case of manual exposure control 
the amount of light emitted is fixed and you are trying to control the 
amount of light admitted back into the camera.  I think you may be 
confusing yourself with thinking in terms of "direction and magnitude of 
the exposure adjustment".

ps:  I've never read the T28 manual before but just decided to do that. 
  Olympus should be ashamed.  But I did note that the guide number of 28 
is only valid for a distance of 1 meter from the flash.  Makes perfect 
sense but kind of restrictive wouldn't you say?  Trust but verify!

Dr. Flash



On 9/14/2010 7:52 PM, usher99@xxxxxxx wrote:
> 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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