P3D RE: Stereo Flash Photography

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

In Photo-3D Digest 2958 Larry Haines asks about flash photography and 
his RBT X3.

It just so happens that I also have an RBT X3 and a large part of my 
photography is done with flash (cave stereo).

First off the built-in flash of the X3 has a guide number of 11. That 
means that a subject at ten feet would need an fstop of f1 if you 
are using 100 speed film. Basically the flash is usless for stereo 
shots where you want to use f8 (1.3 feet max) or f11 (1.0 feet max). 
The one thing that it is good for is triggering slave flashes (see 
below).

You have two ways to go with this camera.

The first is to get a dedicated TTL flash. The RBT X3 is based on the 
Ricoh XR-X3PF so get a Ricoh module. This way all your flash is 
automatic and you don't have to worry about any settings or changes 
in distance or fstop (provided you don't exceed the flash's maximum 
output). The Promaster 5700 TTL flash that I have has a guide number 
of 130 and has produced some great results.

The other is to get a semi-automatic flash such as the Vivitar 283. 
These flashes allow you to set various ranges (f4, f8, f11, etc.) on 
the flash and then set the camera aperature the same and shoot. The 
flash will automatically cut the light when it reads enough. Once 
again you do not want to exceed the flash's output. The Vivitar 283 
has a guide number of 120.

Flash output is usually referred to as the guide number. This guide 
number is dependent on film speed, but to make things easy most 
published charts are based on 100 speed film. A low power flash is in
the 40-60 guide number range, a medium 60-90, and a high 90-130. In 
order to get in the 130-150 range you will need to buy one of those 
big "potato-mashers" or go to flash bulbs.

Now that you know what the guide number is how do you use it? WARNING 
- math formula coming *{;-)  GN/D=f or the guide number divided by 
the distance from the flash to the subject equals the fstop. So if 
you have a flash with a guide number of 120 and your flash to subject 
distance is ten feet then the fstop would be f12 which would 
translate to f11. Working the other way GN/f=D. If you want to use f8 
and your guide number is 120 then the flash would have to be 15 feet 
away from the subject.

Have a flash and don't know what the guide number is? To find out go 
outside at night in the back yard. Place a subject in a brightly many 
colored shirt exactly ten feet away from the flash/camera. Have the 
model hold a card with an fstop number on it. Take a flash shot at 
that fstop. Do this for the full range of your lens. When you get the 
pictures developed (use slide film so processing is not a factor) 
choose the one where the subject is evenly and well lit with no 
fading or washing out of the colors. Then simply mulitply that fstop 
by ten and you have the guide number. If f8 was the best then the 
guide number is 80 and if f11 then GN=110. It is always a good idea 
to test a flash this way even if you know the GN as manufacturers 
tend to over-rate their flashes. Besides, what matters most is how 
the flash works with your camera/lens setup. In my case I have to 
compensate depending on whether I am shooting in a black 
light-sucking lava tube or a white reflective limestone cave.

The reason I have emphasized flash to subject distance is because if 
you are going to use a slave flash off to the side of the camera or 
even behind the subject you must measure that distance and not the 
camera to subject distance. A subject that is ten feet away from a 
flash will be illuminated the same on film whether the camera is 
ten feet or fifty feet from the subject.

Using a slaved flash is an easy way to get the flash farther or 
closer to the subject without also having to move the camera. The 
built-in flash on the X3 is great for triggering these slaves without 
adding too much light of its own. Another trick is to cover your on 
camera flash with a piece of exposed film. This allows only infra-red 
light to pass though which will trigger the slave but not light up 
your subject.

One last thing Larry - I will be in Seattle and free Friday night 
September 25th. I will have my X3, Promaster TTL flash, Vivitar 283, 
and several FireFly slave units, and maybe even some M3B or Press 
25B flash bulbs. We can go flashing downtown Seattle with dual RBT's 
- what a sight!

Sorry if this post is too long, but I did stop short. If you want 
to know more about stereo flash photography just ask.

Thanks for the time,

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


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