f/6.3 or f/6.8? & Three R's

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Actually, f/6.3 is _exactly_ in-between f/5.6 and f/8 exposure-wise 
for the same reason that f/8 is between f/5.6 and f/11 (instead of 
f/8.3).  We are dealing with sqare roots here.

One way to remember the full stop aperture scale is to remember 1
and sqare root of 2 which is 1.4.  Write 1, 1.4 and then double 1, 
then double 1.4 and proceed to get:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44...

Here is what's written about the red dots in the "Stereo Realist 
Manual Book" (from an ealier DrT posting):

"If you don't want to bother focusing or using an exposure meter, 
you can use the following settings for all average outdoor pictures 
in daylight:

a) Shutter speed at 50
b) Focusing at 20 feet
c) Aperture at f/6.3 (midway between f/5.6 and f/8)

You will get good pictures all the time at these settings if it is 
an average sunny day and you keep the sun at your back... 
These simple settings have become known as The Three R's in Outdoor 
Stereo Photography ever since they have been marked in red on recent 
cameras.  Anyone can use The Three R's for foolproof pictures taken 
outdoors in good weather."

A couple of comments here:

1. Equivalent exposure at f16 will need a shutter speed of 1/10.  This 
"sunny day rule" was aimed at ASA 10 Kodachrome.

2. Given that the recommended aperture on a sunny day was f/6.3, it's 
no surprise that so many pictures from the 50s have out of focus 
backgrounds or are blurred (what do you do when it is not sunny???)

3. For a modern ASA film the sunny day rule of f/16, 1/100 and focusing 
at 10 to 15 feet is much easier to work with!

George Themelis


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