Bracketing DrT Style (Part II)

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Bracketing DrT Style (Part II)

I realized I made a mistake in my previous note. I mentioned that by
bracketing in full f-stops symmetrically around the guessing point one can
cover a range of 2 f-stops within 1/2 f-stop accuracy.  That's not correct.
 It should be within 3 f-stops accuracy!!!  That's as much as most people
would ever need and will be hence called "Survival Strategy".

I have expanded these ideas and have created the following "Dr. T's Guide
to Full Stop Bracketing" Table:

Case #       Title           Pictures Taken   F-stop range covered 
------   ------------------  --------------   --------------------
  1        No Bracketing           1                  1
  2        DrT Bracketing          2                  2
  3       Survival Strategy        3                  3

F-stop range covered is the range of full f-stops around the center of
exposure range, in which you have 100% probability to be within 1/2 f-stop
away from "perfect" exposure.

Interesting observation: In all three cases there is 66% chance
(mathematically proven) to be within 1/3 f-stop from "perfect exposure". 
Of course, these probabilities assume that the "perfect exposure" is inside
the f-stop range covered by bracketing!

It appears that by taking more pictures (equally spaced) one increases the
sampled exposure range but does not affect the closeness to perfection,
assuming that the initial guess is sufficiently close.  If more attention
is paid to the "initial guess" equivalent results can be obtained with less
film waste.  If bracketing is to be done, wouldn't be better to bracket the
range (just in case the initial guess is way off) instead of the
closeness???  

That was the math part.  Here is the editorial comment:  When I tell people
that I bracket full f-stops, their reaction is that 1 f-stop off exposure
makes a big difference.  However, if the "perfect exposure" is within the
initial guess range, I will never be within 1 f-stop from my targeted
exposure. As I explain, most probably will be within 1/3 f-stop.

For years I have been either not bracketing or bracketing full f-stops,
despite advice from "perfectionists".  My exposures are working OK. 
Finally!!!  It is good to know not only that your strategy is working OK
but also WHY is working OK.

Thank you! -- George Themelis

Final note:  Some people will say that this is theory and in practice
better results are achieved via 1/2 stop bracketing.  I do not know if this
a psychological effect (sense of security?) or a fact.  I am the last
person in the world to talk about saving money on film and processing.  If
I felt that 1/2 f-stop bracketing could make a difference, I would be doing
it.  But in most cases I feel that a different angle of view, composition,
etc. would make more difference and I go for that instead of a different
exposure.  So, I'd rather "bracket" composition.  Or, if I am going to
bracket exposure, I'd rather bracket the initial guess (to be safe) than
the closeness to perfection.  

Perfection is a dream.  
Good photography is reality. 
This is, the DrT philosophy!  
(quote No. 4532) ;)


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