[MF3D.FORUM:1537] Mike's method (was Re: folio comments...)

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

Paul,

(Tangent:  The black mounts are awesome!  Thanks!)

I thought I'd jump in to answer your questions about how I'm choosing
on-film deviation and discuss the rest of my method.

At 12:49 PM 2/16/01 -0600, you wrote:
>Bill Glickman wrote:
>
>> I think the 50% correction factor used in Mike Davis's formula
>> will correct this, I was to bold in the begining!

I'm sure Bill would agree that what he was saying here is this:  

"I think specifying a value of 50% in the %MAOFD variable Mike Davis's
formula uses will correct the problems I had when shooting at 100% MAOFD."

>Can't disagree with the "too bold" comment.  Shooting for the
>maximum deviation in every shot doesn't leave any room for error.
>But a 50% correction sounds possibly too conservative.  I'm not sure
>I know how the "50%" was figured, however.  Does the "correction"
>cut the MAOFD from about 2.7mm to about 1.3mm?  

Yes.  More precisely, the 50% figure Bill proposed would cut the on-film
deviation from 100% of MAOFD to 50% of MAOFD, where MAOFD is FL/30.  So,
yes, for an 80mm lens, 50% MAOFD would be 1.3mm vs. 2.7mm for 100% MAOFD.

>Would it be equivalent
>to changing the usual "7 ft to infinity" guideline for a normal stereo
>base camera to "14 ft to infinity?"

Yes, exactly.  


>
>> I have viewed some of
>> Mikes shots, and he used even less correction and his fused very well... so
>> I am convinced this is the problem....

I can't remember the context of this quote, but as it reads here, Bill
sounds like he's suggesting I had used bases which produced less than 50%
MAOFD.  That's not the case.  All of the slides I have shared with Bill
were shot very consistently at 80% MAOFD.  I have since made the decision
to reduce all my shooting to 70% MAOFD and I've just done enough shooting
at there, with various subjects, to conclude that I like it better than the
"bite" experienced at 80%.  

I'm not willing to decree that I intend to shoot at 70% for the rest of my
life, of course, but I'll make this subjective statement:  Having used a
calculator in the field for every shot, measuring distances with a Bushnell
Yardage Pro Compact 600 laser rangefinder (and a tape measure for Nears
that fell inside it's minimum range of 15 meters), taking the time and the
effort to position the cameras precisely each time, I can say that I have a
really good feel for what 80% MAOFD looks like across several different
Far:Near ratios.  That "amount" of stereo bite is extremely consistent from
one shot to the next.

80% fuses easily yet still garners a big reaction from people unfamiliar
with 3D.  The problem is that it just looks a bit too strong for my taste
and I'm sure that for some Far:Near ratios, a person who prefers shooting
ortho might find 80% MAOFD to be wholly unacceptable.   

In the interest of making the medium of this art less prominent, so that
the content can be appreciated without having to compete with the "special
effects", I have for the moment, settled on a deviation equal to 70% MAOFD.

On Sunday, I photographed a B-17G, named "Chuckie", inside a hanger at the
Vintage Flying Museum (at Meacham Field, Ft. Worth, Texas.)  I shot two
compositions.  In the first, I was 26 feet from the nose, out in front of
the right wing.  The Far Point in this shot was at the top of the far end
of the hanger (behind the plane), which measured 102 feet from the cameras'
position (M7 II's with 65mm lenses) at 10 feet off the concrete, atop a
Bogen 3058 tripod (for which I have to use an aluminum step ladder.)  

For this combination of variables, and compensating for the fact that my
Saturn viewer has a 78mm FL, not 65, the resulting base for 70% MAOFD was
296.3 mm (11.66 inches).  I just now recalculated that figure afresh - my
memory hasn't been THAT good for awhile!  The Far:Near ratio here was
3.92:1, well above the the 2:1 ratio at which I would abandon the "General
Solution", in favor of a modified version of the 1/15th rule (Near/15).

In the second composition, taken from much further back (49 feet from the
nose, but on the left side of the plane and not nearly as end-on), again at
a height of 10 feet, the Far Point measured 147 feet, which was a Far:Near
ratio of exactly 3:1 - a more 'shallow' subject than the previous
composition.   The base to deliver 70% MAOFD was, compensated by the Viewer
FL/Camera FL mismatch ratio, 625.5mm (24.6 inches).

Both shots came out really well and the idea I'd like to get across in
sharing all the numbers is that they both have the same sensation of stereo
bite (precisely the same) and I really enjoy the "predictablity" of this
approach.  Having gone through about 16 rolls of 120 (8 rolls worth of 2D)
at 70% MAOFD, I definitely prefer it to 80%.  On aesthetics alone, I don't
really feel moved to cut this down to 60%, but this is especially
undesirable since that would push my Nears further away from the camera still.

It's impossible for me to obtain bases less than 165mm with my equipment,
so at 70% MAOFD, again compensating for viewerFL/cameraFL mismatch, I can
not shoot subjects closer than 19.5 feet, when the Far point is at
Infinity.   If I allowed the deviation to increase to 100% MAOFD, my
minimum base of 165mm would permit Nears as close as 13.6 feet, with my
equipment.

Taming every last variable is a process that becomes ritual after awhile.
Measuring Near and Far distances takes me only about a minute if it can be
done without the tape measure.  The math for base takes exactly one minute
(yes, I've timed it).  I also calculate best focus distance, widest
aperture that will acheive sufficient DoF,  going for a Maximum Permissible
Circle of Confusion diameter of 0.03115 mm, on-film, at the Near and Far
sharps. This is equivalent to 7.6 lpmm after the 4.21x magnification
provided by the Saturn Viewer.  7.6 lpmm  is fully 50% more resolution than
the 5 lpmm which can be resolved by the average human eye at 25 cm (9.84
inches) - closer than my eyes can focus.  My choice of 0.03115 mm CoC's for
DoF calculations ensures that my Near and Far sharps are completely
uncompromised after magnification and leaves a margin for compensating the
possiblity of poor film flatness.

The last calculation I do (on an HP 48G+) is to determine the smallest
aperture at which diffraction's Airy disks will reach the diameter of my
chosen Maximum CoC's.  This prevents me from using a stop that would cause
Airy disks to exceed the 7.6 lpmm resolution goal.  To focus, I use the
laser rangefinder to find a target that resides at the calculated best
focus distance, even if that target is outside the composition, behind me
for example.  After focusing on that target, with both lenses, I restore
the intended composition.  (My thanks go to Bill Glickman for this tip!) 

To meter, I use a Pentax Spotmeter V to place the highlight detail at 2.5
stops above 18% grey (placement at Zone 7.5), then check the shadow detail
to see if it's more than 5 EV below the highlight.  If it is, I elevate the
shadows by pre-exposure using a home-made diffuser (two layers of white
plexi from a 5000k light box, measuring 6 inches square, separated by a
frame of 1/4-inch balsa wood. Nothing can bond acrylic to balsa better than
SuperGlue, by the way.)  

Having determined the main exposure (using the calcuated f-stop at whatever
shutterspeed will place the highlight at Zone 7.5 and freeze subject motion
hopefully), I stop down either 2, 2.5 or 3 stops for the pre-exposure on
each camera (M7 II's permit multiple exposures.)  I just hold the diffusion
panel flat to the front of the lens and fire the shutter, first on one
camera, then the other - they don't have to be sync'd.  :-)   A -2 stop
pre-exposure will lift Zone 0 nearly 3 EV's but this is about the brightest
pre-exposure you can make without elevating the midtones a wee bit too and
running the risk of large areas of shadow appearing "milky."  The -2.5 stop
pre-exposure is nearly always safe, elevating Zone 0 about 2.5 EV's.  A -3
stop pre-exposure is guaranteed to be "undetectable" even with large areas
of shadow - it will elevate Zone 0 only about 2 EV's.  The neat thing about
pre-exposure is that the higher the original luminance of a given subject,
the less it will be affected by pre-exposure.  Your darkest shadows get
lifted more than the higher zones.  At -2.5 and -3, midtones and, obviously
the highlights, are completely unaffected.  The main exposure (made after
the pre-exposure) is done at the calculated settings, as if the
pre-exposure was not applied.  

In these shots, I didn't have to do any pre-exposure because the luminance
range was well within the film's latitude.  I did have to compensate for a
filter factor - I was using Nikon A2's (the most wonderful warming filter
on the planet - I can't stand the look of 81A's now.)

The last step in exposure determination is to make sure the calculated
optimum aperture for sufficient DoF is not smaller than the calculated
diffraction-limited aperture, in which case, I have to back up, to increase
the distance to the Near Sharp so that I can maintain the 0.03115mm on-film
CoC and Airy disk maximums at a wider aperture, one that won't induce
visible diffraction after magnification.

All the math takes 4 minutes.  Focusing, including finding a target at the
calculated best focus distance, takes about two minutes.  Exposure
determination takes about two minutes.  Pre-exposure can slow things down
by another three minutes or so.   Overall, once the point of view has been
selected it takes me about ten to twelve minutes at the very least to set
up a shot, with some taking nearly 20 minutes, if I have to back up and/or
do pre-exposures.

>What does "even less correction" mean?  Would 30% be an example
>of less correction, and would it mean targeting an OFD of about
>1.9mm?

Where were we?  I think Bill was saying that he would like to use less base
than what he saw with my 80% MAOFD shots.  

>
>> > Do you see cardboard cut out look even though you don't see
>> > miniaturization?
>> 
>>             YES!  Isn't that odd, I notice a lot of cardboard cut out
>> things, but I rarely ever see any miniturization.... I still do not have a
>> clear understanding of what causes this... do Spud and Rolleidoscope users
>> see this too?  I think it is related to stereo base also...???
>
>Someone just asked about cardboard cutout effect over on P3D.  Here
>is a reply from DrT:
>http://www.pauck.de/archive/mailinglist/photo-3d/mhonarc/msg47491.html
>
>The times that I have seen cardboard cutout effect I felt it
>was due to low resolution imaging.  I've seen it in lenticular
>prints, and printed stereo pairs.  I can't recall ever seeing
>it in a slide, though I might have.  Stereo base can contribute
>to the cardboard cutout effect.  How about mailing me an example
>of an image in which you see cardboarding?  I'd like to compare
>notes.
>
>> > retinal rivalry.
>
>>         NO, I should have explained this one...it has been my experience
>> that the wider the base, the more potential for different effects in each
>> chrome.  For example, the sun will be cracking through the leaves in the
>> left chip at a slightly different location than on the right chip...
>
>Oops, yes you are correct; I'd forgotten about that effect.  Note that
>it does also happen in normal base shots, but as you mention it is more
>of a problem in wide base shots.

Yes.  I have a couple of shots where thin, leafless branches appear to be
unattached to larger limbs of a tree because strong backlighting has
refracted around both sides of the branch, blowing out the density to clear
film base - and usually by differing amounts, comparing one chip to the
other.  Yuck!

Mike

>
>> Also the wider the base, the more risk
>> of getting something in one image but not in the other...for example, one
>> chip showed a telephone pole from the ground up, the other chip had a
>> building blocking the base and only showed it from the top of the building
>> up...more retinal rivalry...
>
>That's also a good example...and another one that I've also seen
>in images from a normal stereo base shot (the nearest object was
>too close to the camera).  This one too is probably more likely
>in wide base shots.  But I hesitate on this one, because I wonder
>whether the cause might have been an incorrect application of the
>stereo base calculation.  For example, perhaps the real "closest
>object" in the scene was overlooked, causing the calculated base
>to be too large.
>
>Paul Talbot
>
>