[MF3D.FORUM:1138] Re: Deviation and Viewers

["Michael K. Davis" <zilch0@xxxxxxxxxxxx>]

Abram,

At 09:27 PM 8/22/00 +0200, you wrote:
[snip]
>Beware of discussing General Solution!  Some lists have
>gone very nasty doing it. :-)

Lest I stir sleeping dogs, I'll seek the truth while tip-toeing in my hush
puppies!

Seriously, I appreciate the heads up.  I suspect the differences in opinion
are due more to a difference in goals.  Just three days into this, I can
see that one's choice of math (or no math at all?) can be completely
justified by subjective preferences.  You just can't argue style.  

If, on the other hand, there is some debate about the facts affecting
stereography, let me bang some pots and pans together - I want to wake
those weary hounds! 

HEAR YE!  HEAR YE!  HEAR YE!   
ALL RISE TO THE HONORABLE AND NOBLE MISSION OF FINDING THE TRUTH! 
LET NO MYSTERIES SURVIVE OUR CRUSADE!

>I could write a book (O.K., a slim one) on base calculation 
>and it presumptions. For one thing, the so called Bercovitz- 
>Spicer formula goes back to _very_ similar formulas and 
>principles more than a century old, at least five names
>should be attached _before_ Bercovitz in order to get a
>justifiable name for that formula or equation.

I kind of figured John wasn't breaking entirely new ground, but that sure
is a nice job he did putting it altogether on his web page.  I don't doubt
its origins go way back.  I guess we should call it "the formula John
Bercovitz uses."

>Leaving that for now, I believe you are heading in the
>right direction. The only, repeat only, rational for
>limiting deviation is a _viewing_ constraint.
>
>It is believed that sticking to a swing or _difference in 
>convergence_ of two degrees (between looking at far points and 
>looking at near points) generally is a wise limit for
>_viewing_. 
>As the tangent of two degrees is about 1/30th, the deviation 
>limit should be about 1/30 of _viewer_ focal length.

I wondered where this 1/30th rule had come from -- thanks!

>
>The funny thing is that in several rules and equations the 
>viewer focal length isn't even mentioned. 

Yeah, I saw that David Lee does accomodate the ratio of viewer focal length
to camera focal length - that's why I felt certain I should modify the
formula John Bercovitz uses by mutliplying his results by the ratio of
viewer focal length to camera focal length.

>Freeviewers however 
>know that freeviewing of images with larger deviations is 
>easier at a longer distance, which is easy to understand when 
>you realize that the swing in convergence from far to near 
>point becomes smaller at larger distances.

I'm with you...

>
>The next point is that the two degrees limit isn't a law. 
>Experts seem to agree that in a series a few slides requiring 
>_four_ degrees swing will do no harm, especially when they 
>contain subjects at many different distances, gradually 
>receding in depth. Earl Krause and Koo Ferwerda were in favor 
>of (although limited) use of "double depth" mounting, placing 
>the window at half the normal distance. 

Which variable(s) actually control the placement of the window? Just
deviation, or is there something else as well?

>I believe that for some selected subjects, seen in viewers, 
>even some eight degrees can be tolerable. But I agree to two 
>degrees as the standard. But it seems not necessary to 
>compute deviation to a hundredth of a mm :-).

I can see that there's plenty of room for slop, and again, because of
subjective prefrences, there is NO right answer.  Still, I would like to
get a handle on ALL the variables so that my choices are actually CHOICES
and not accidents born of ignorance.  As obsessive as I am about unearthing
all the variables, on this issue, my subjective preferences are already
leaning heavily toward using a shorter base than the formula's David Lee or
John Bercovitz are using.   

My preferences aside, I want to use a 43mm lens on a Mamiya 7II
(single-camera) and would like to get all the math worked out before I buy
a viewer and a slide bar, for example.  I know I'm headed for a much
greater separation with such a lens.  By the way, cropped to 50x50, the
43mm would be like a 24mm for 35mm - not as ultra-wide as it is for
fullframe 6x7.

>Now for some esthetics. From the viewing swing limit you can 
>calculate a deviation, from there with camera focal length, 
>near- and far point distance you get a base value. 
>Remember this is a maximum recommended value. When you use 
>near point 8 feet and far point 8.5 feet, the computed value 
>will be almost always too large, from an esthetic point of
>view. It will cause unpleasant deformations in viewing.

I hear you...

>
>Indeed, the shallower the depth interval in object space is, 
>the more absurd the base values which the A.-B.-C.-D.-E.-
>Bercovitz-Spicer equations give. That's why several people 
>recommend to stick to the 1/30 rule for everyday subjects. 
>
>Abram Klooswyk

I understand - and that is SO much easier, too.

Thanks for all your feedback!

Mike