[MF3D.FORUM:1482] Re: NSA MF slide show

[Tom Hubin <thubin@xxxxxxxxx>]

Bill Glickman wrote:
> 
> Tom
> 
>        I do agree with your math....the inverse square law is
> indisputable... but I think this is where we may have been confusing each
> other.....  With the electronic flashes, the light leaving the flash alters
> based on the fl you are shooting at.   So for longer fl the beam is very
> narrow, and for wider fl the beam is very wide....

<big snip>

Hello Bill,

I'm not familiar with flash accessories. For that matter, I don't even
do photography. So I was just applying some optical theory.

If I have this correct now, you are telling me that the 50mm focal
length you are referring to is for a lens or reflector used on the flash
and not the lens used to produce the image for the film. Or are there
two lenses of the same focal length, one for the flash and one for the
film?

Then would you not also need the f# for this flash lens to determine the
light collected by the flash lens and the f# for the imaging lens to
determine the light collected for the film? 

This also does not change what I said about multiple flash lamps or
multiple flashes of one lamp where the physical setup (camera and lamp
locations) and optical setup (focal length, fStop, exposure time, flash
duration) are unchanged. A subject which is twice as far away receives
and backscatters one fourth as much light so four flashes will be
required to make up for doubling the distance. 

It's party time now. No more Email for me until Sunday afternoon. Enjoy
your evening.  

Tom Hubin
thubin@xxxxxxxxx