[MF3D.FORUM:1130] Intro, Deviation and Viewers for a 43mm taking lens

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

Hi!

I'm new to the group.  Bill Glickman has told me how much fun everyone's
having in this forum, so here I am.  I've been reading the archive off and
on for two days and have already found answers to a lot of my questions.  I
do hope to be a contributor as a well as a beneficiary!

Having studied John Bercovitz' General Solution for calcuating separation
and created a spreadsheet so I could play with the numbers, I have modified
it a bit to borrow from David Lee's formula which takes the ratio of viewer
lens to taking lens into account.   So, I feel like I've got that piece of
the puzzle down with the exception of being a little bit wobbly about the
value for d - deviation.  Some articles throught the archive suggest a
deviation of 2.5 for MF, one specifies 2.4 to 2.7, and others a value of
2.7 (of those I've read so far.)  I've noticed that going from 2.7 to 2.5
causes a HUGE shift in the resulting separation calculation, though.  

I had previously decided to leave it at 2.5, since this seemed to be about
the mean value of those quoted, when I read an article where Bill Glickman
agreed that he too had calculated a base of 5.3 inches (using John's
formula with a Near of 8 feet and a Far of 20 feet, assuming the plane of
best focus was half way between Near and Far, at 14 feet, using an 80mm
lens.)  

I was coming up with 4.78, using my formula, where Bill's viewer lens focal
length of 78mm is taken into account (ie. not equal to 80mm) so when I set
the viewer length to 80, like the taking lens, I expected to get the 5.3
inch separation that Bill and the other party had calculated.  Nope.  I got
4.91 - still well shy of 5.3.  Then it occured to me that they must be
using a different deviation - not specified in the original article.  When
I changed my deviaion to 2.7, from 2.5, I got their calculated value of 5.3
inches.

So, my first question is:  How does one determine the correct deviation
value to use for the separation calcuation?   Going strictly on
proportions, if 1.2mm is appropriate for 35mm (it seems to be very constant
at this value throughout the archive), can we use the ratio of 36mm (35mm
is 24x36) to 50mm (RMM mounts for MF are 50x50) to conclude that the
deviation value for 50x50mm should be 1.667?  That's so far off from the
values 2.4 through 2.7 that a straight proportion seems inappropriate.  Is
1.2mm used for something other than a 24x36 mount?

Also, I found the articles about the disparity between going for realism
vs. going for stereo effect very interesting.  Not having any experience to
speak of, I think I will lean toward the realism side of the fence,
although not necessarily all the way to pure orthostereo.   A major
attraction of stereography for me is the hope of reproducing the experience
of a natural scene as closely as I can and I consider the miniaturization
effect of a wide base to be a departure from that line of thinking - even
if it's not as readily apparent as it would be with man-made subjects that
provide a better sense of scale than the subjects of most scenics.   Along
those lines, and perhaps as an excuse for not wanting to buy a second
camera and lens, I am very inspired by Oleg Vorobyoff's comments in one
article where he states the merits of using a single camera on a slide bar.
 At the very least, the single camera approach allows one to achieve a 65mm
base that might be prohibited with two cameras.  Time will tell if I can be
as patient as he is, waiting ten minutes for a lull in the breeze!  I do
admire your perseverance Oleg!  I'm going to be right in that same boat
with you.  Do you like to fish?

Using a 43mm on 50x50 format is like using a 24mm FL with the 35mm format.
I have read of several people using 24 and 28mm lenses in their 35mm
stereography, so I'm wondering what pitfalls there are other than keeping
far enough away from the nearest subject, preventing odd subjects from
intersecting the window frame and finding a viewer with the right lenses.
Someone wrote that Don Lopp uses 45mm lenses in his MF viewer.  Is Don
using a viewer of his own design or can something be purchased with lenses
of that length (near 43mm)?

Lastly, given the FL of a viewer's lenses, how does one calculate the
degree of magnification?  I understand that shorter lenses yield greater
magnification, but don't know the math.  If a 78mm lens gives 4x
magnification, would a 43mm lens produce something like a 7x magnification?

Thanks!

Mike