[photo-3d] Re: flash guide numbers

[CanterMike@xxxxxxx]

In New Digets 40, George (responding to to Bob) writes, in part:

<< 50ft for ASA 25 -> 100ft for ASA 50 -> 200ft for ASA 100. >>

Sorry, boys, but that progression is wrong.  Guide numbers do not double with 
the doubling of the film speed.  With each one stop increase in film speed, 
the guide number goes up by 1.4x (in does double with each TWO stop 
increase).  As an example (off the top of my head, but one which many people 
on this list can independently confirm) the venerable and high respected 
Vivitar 283 has an official guide number of 60 at ASA 25, 120 at ASA 100.  
That would place the V283's GN at about 84 at ASA 200.

To more precisely address Bob's question I wish to ask one of my own: is that 
the only info on the flash?  Is there no table, chart or dial to use as an 
indication?

If the GN is 50 @ ASA 25, it would be 70 @ ASA 50 and 100 @ ASA 100.  Bob's 
original post is correct.

The expression "GN 50 ASA 25 DIN 15 in feet" doesn't quite make sense as 
quoted.  Although ASA 25=DIN 15, flash manufacturers usually express guide 
numbers for DIN ratings in meters.  (Example: the current Metz units use 
their metric guide numbers as model nomenclature.  The 60 series unit have a 
guide number of 60 (meters) @ DIN 21, but this is usually expressed in 
English measure as 198 (feet) @ ASA 100, power enough to stun small animals.)


Mike Canter


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