Re: BOGUS - 1/30 Rule! (fwd)

[Yiing Lin <ylin@xxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 13 Dec 95 18:50 PST
From: Bob Mannle <threed@xxxxxxx>
To: photo-3d@xxxxxxxxxxxxxxx
Subject: Re: BOGUS - 1/30 Rule!

I'm sorry but I have to jump in here and clarify soemthing. This
1 in 30 rule has been floating around again, and in my opinion
is a rule that only applies in a particular circumstance... Certainly
not in every instance of 3-D photography. Using it for dual 35mm
camera work will rob you of maximum depth in your images.

WHAT DOES IT MEAN?

The One in Thirty Rule states that the lens separation, or
stereo base should equal 1/30th of the distance from the
camera to the nearest object in the scene. It doesn't matter
if that object is a rock, a piece of dirt, a twig, or the
main object... just whatever is the closest object you see
through the viewfinder. (See Ferwerda pg 104)

WHAT IT DOESN'T SAY?

It doesn't say anything about any particular camera equipment
being used, lens focal length, film chip size or film format
being used. It doesn't mention far point distances... 
or whether infinity is... or is not in the image.

Basically... it is VAGUE! and because it is vague... it's
very misleading.


SO WHAT IS THE RULE?

The rule is the amount of "on-film" deviation ones eyes
can accept when viewing stereo images.


WHAT THE HELL IS "ON-FILM DEVIATION"?

Take your left film chip, and physically measure the distance
from the near point in the image... to the far point in the
image. Lets say it measures 6mm.

Now measure the distance between the same two points on the
right film chip. Let's say this measurement is 7mm.

Your on-film deviation is 1mm (The difference between the two)


HOW MUCH ON-FILM DEVIATION CAN WE HAVE?

Here again... the rule is vague. There is no mention of what
medium we will be using... anaglyph, hand held viewers, projection,
etc... nor any mention of final size of the image we will be
viewing.

Nevertheless... I have taken Ferwerda at his word and accept his
on-film deviation limits.(Ferwerda pg 238)
               
                Realist Format - Max = 1.2mm
                Full Frame 35mm - Max = 1.5mm

Remember... this realtes to the deviation on the film chip you
are using... not a print or projected image.


WORKING IT BACKWARDS....

Being I accept these limitations as being true... then we have
to work the calculations backwards to find out what stereo base
should be used for what lenses... at a particular near/far subject
distance.


We need to know a few things..

Camera or film format
Lens focal length
Distance to nearest object


HERE'S ONE SCENARIO..

35mm dual cameras... full frame... max deviation - 1.5mm
Lens focal length = 35mm
Near subject/object distance 8'-0" = 2438mm
Assuming that infinity is in the image.

Try this formula to calculate the stereo base:

           (distance-focal length)/(focal length/deviation)

           (2438-35)/(35/1.5)
     
           (2403/23.33)= 103mm = 4.06"

The one in thirty calculation would have been:

           2438/30 = 81.27mm = 3.2"


Had you used the 1/30 rule in this situation you basically
would have been robbed of maximum depth in your image. Perhaps
this doesn't appear to be very important to some of us... it is
important if you are trying to use a meager 1.5mm of space to
it's best advantage.

Obviously this doesn't do Realist photographers, or fixed lens
photographers a lot of good... but for those using dual cameras
or other special techniques...PLEASE FORGET THE 1/30 STUFF !

FINAL THOUGHT then I'll be quiet.

The above scenario is a realistic one... but can change dramatically
as the focal length of the lenses, and near distances change. There
is another important factor I haven't discussed at this time which 
involves "whether infinity is in the image... or not".

In many cases infinity is not in our images... which means that
the stereo base could go much higher... and in many cases... TWICE
as much as this formula implies... while still staying within
the deviation limits.

I classify the above calcualtion to be a MINIMUM stereo base to 
be used... and perhaps even bracket you great images with stereo
base... as you would with f-stops.


There... I feel much better now 8-)

Best Regards,





Bob Mannle

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