Re: [photo-3d] Multiple flash...

["David W. Kesner" <drdave@xxxxxxxxxx>]

Oliver Dean writes: 

> Since two flashes of the same flashgun would be doulbling
> the amount of light on the subject, which is similar to
> using a shutter speed twice as long in normal light, why
> doesn't it translate into a 1 f/stop difference?  Is there
> some sort of reciprocity effect at work here because of the
> short duration of the flash? 

I don't claim to know or understand the reason, just what I 
have  been told and that is two flashes from a flash with a GN 
of 100 is  not the same as one flash from a flash with a GN of 
200. It  probably has to do with all that stuff about the 
square of the  light divided by the distance to the subject 
minus the thin film  of dirt on the lens plus the square root 
of the reflective value  of the surface being imaged *{;-) 

> Your detailed answers were articulate and informative! 

I guess that means I should give you the correct answer (this 
is  taken directly from the Metz web site: 

<http://www.metz.de/mecablitz/eng/Blitztips/Welcome.html> 

"Multiple flash can also provide the answer to the problem of 
shooting a picture of a painting on the ceiling of a 
cathedral.  Under normal conditions the capacity of your flash 
unit will prove  to be insufficient to produce a correct 
exposure of the ceiling  painting from a distance of 12 
meters. However, the drawback of a  small guide number can be 
offset with static subjects by firing a  series of individual 
flashes to "build up" the correct exposure.  

Let us assume that your electronic flash unit operates with a  
guide number of 32 and that the speed of the loaded color film 
is  ISO 100/21°. First determine the guide number that would 
be  necessary to produce a correct exposure of the painting at 
a  height of 12 m. Select f/5.6 as the initial aperture: f/5.6 
x 12 m  = a guide number of 67. Now use the following formula 
to calculate  the number of flashes to be fired: 
 
Number of Flashes = required GN/existing GN = 67sqrd/32sqrd = 
flashes 

4 full-power flashes must be directed at the ceiling for the  
exposure of a single frame. The shutter must remain open and 
the  camera must be mounted on a tripod to eliminate camera 
shake." 

Most, if not all, of my photographic "talents" have been 
gained by  trial and error and not by knowing what I am doing. 
Once I learn a  new technique or piece of equipment I tend to 
stick with it. That  is one of the main reasons I use Vivitar 
283 flashes - I have  gotten to know them very well. I also 
burn a lot of film, but not  quite as much as Dr. T. That is 
how I have been able to go from  "not even knowing what the 
stereo window is" to having images  accepted in exhibitions in 
such a short time (just over four years  from my very first 
stereo image).

That's all for now,

David W. Kesner
Boise, Idaho, USA
drdave@xxxxxxxxxx

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