[MF3D.FORUM:387] Re: fl/30

["Bill Glickman" <bglick@xxxxxxxx>]

John B

      Sorry I did not properly quantify my question... I meant this... if
you only view MF stereo pairs in a viewer, no projection, then using your
formula, should I keep the deviation constant?  If so, should it be 2.7?
Thanks John...

I believe another poster already mentioned yes, keep it constant... but you
seem to be our resident Guru of all this... so I just wanted to be sure
before blowing a few rolls of good film :-)

Regards
Bill


----- Original Message -----
From: <Tloc54452@xxxxxxx>
To: Medium Format 3D Photography <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Monday, April 03, 2000 9:35 PM
Subject: [MF3D.FORUM:381] fl/30


> You guys are nothing if not timely; you ask questions
> just as memory starts to fade.  Great review for me.
>
> Bill writes:
>
> > Are you saying if we use your formula, that we should
> > vary the
>  deviation based on the fl lens used?
>
> In a way.  The formula assumes you will view from the
> perspective point.  So if you use a focal length which
> is twice normal, you will either sit twice as far from
> the screen or, what is equivalent, you will put longer
> focal length lenses on your viewer.
>
> Why this rule?  It's because tolerance for depth in a
> stereo pair is an angular tolerance.  As long as the
> angular difference between near and far doesn't exceed
> 1 in 30, the pair is easily viewed.  So just take your
> focal length and divide by 30 to find the on-film max.
> linear measurement of deviation.
>
> This really is an angle.  If you have a near object and
> a far object, and to your left eye, they are aligned,
> then your right eye shouldn't have to converge more than
> 1 in 30 to acquire the near object.  This is a small angle
> so it could be arcsin or arctan.  So arctan 1/30 = 1.9
> degrees.  Not much, huh?  If your eyes are 65 mm apart,
> then that nearest object shouldn't be any closer than
> 65/tan(1.9) = 2 meters.
>
> So to answer Bill's question, what I'm saying is that if
> you vary your focal length, you should vary your seating
> distance to match, in an ideal world.  What about the
> not-so-ideal world?  Well, that's where Mannle's way
> comes in - just keep the on-film devaition to 1/30th of
> the normal focal length of the format.  I've never
> experimented with this, never pushed the envelope, so I
> can't comment except to say that many people use it and
> they don't have trouble with viewability.
>
> Paul writes:
>
> > BTW, how is the 2.7 calculated?
>
> That would be the Mannle method: format's normal focal
> length divided by 30.
>
> >> On the other hand, if you are shooting for projection,
> >> you are planning on a longer distance from observer to
> >> screen so you should use a longer focal length and can
> >> use more on-screen deviation than if your viewing
> >> distance were for 50 mm or 35 mm lenses
>
> > Very surprising to read this!  I've always labored on
> > the impression that when shooting for projection one
> > should be conservative and aim for no more than 1mm OFD
> > (in R-format), as opposed to the 1.2mm figure usually
> > cited.  The rationale is that the image is magnified a
> > great deal in projection, and if you end up with a lot
> > more than 2.5 inches on-screen separation you can cause
> > viewing discomfort in the audience.
>
> Yeah, you need to keep the infinity separation down to
> 2.5".  So if you set infinity at 2.5" on screen, the
> near point reconstruction pops off the screen (depending
> on screen size/distance) if you have more than 1.2 mm of
> on-film deviation.  If the infinity points have extra
> separation in the mount,then the window also pops off
> the screen.  This is kind of cobbish, though.  Better to
> mask off the outer borders of the apertures in your
> standard mount and leave infinity separation in the mount
> at standard.  That way the projectionist doesn't have to
> make adjustments for infinity separation.  Too bad masks
> like this aren't offered.  Instead, the manufacturers
> opted to keep the aperture separation constant when making
> closeup masks.
>
> Anyhow, standard mounts are extremely conservative and
> I wouldn't feel the least bit bad about masking them down.
> What the heck, the window pops off the screen if you have
> a distant screen anyway.
>
> > If I'm following correctly, 1/fl is a quickie way to keep
> > from exceeding...
>
> OK, this is for the rule where stereobase doesn't exceed
> some fraction of the near distance.  You are trying to keep
> on-film deviation to the normal for the format.  That's
> Mannle's way.  You use 50 mm lenses, near point in the scene
> is 50 times stereobase; you use 100 mm lenses, nearest point
> in the scene is 100 mm times the stereobase.
>
> Note that you are trying to keep OFD at normal for the
> format.  So 1/fl doesn't work for MF.  There it's more
> like 2/fl because the format is about twice as big.
>
> If you do your geometry, you'll find that 2/fl means you
> are aiming for 2 mm OFD, which is ~normal for MF; and 1/fl
> means you are aiming for 1 mm OFD, which is ~normal for the
> Realist format.  If this isn't obvious, and you don't mind
> more math, I can make up a .pdf to illustrate the idea.
>
> > In MF, 1/fl in a normal scene that includes infinity
> > would also tend to keep OFD under 1.2mm, right?  And that's
> > well below the target maximum of 2.7, so 1/fl doesn't
> > directly help as a shortcut for MF.  It would need to be
> > scaled up to 8/3 * 1/fl.  With an 80mm lens, that puts us
> > back at 1/30.  Then to maintain a constant OFD shooting the
> > same scene with a 240mm lens, the new base would be (8/3 *
> > 1/240) or 1/90.  Am I warm?
>
> You're hot!  We could quibble over the last decimal place,
> but that would be silly.
>
> Oleg writes:
>
> >
>  1.2 mm is the most deviation you can squeeze between the
> > window and nominal
>  infinity when you mount your slide.  I
> > thought this held whether the slide would
>  be hand viewed
> > or projected...
>
> Yes, that's the correct figure for Realist format (RF).
> For MF, it's more like 2.7 mm.  35/30 = 1.2 mm; 80/30 =
> 2.7 mm.
>
> >> 2) base = near point/30 regardless of focal length
>
> >
>  This is what I started out with.  But I kept getting too
> > much depth in my
>  telephoto shots, so I started factoring
> > in focal length.
>
> Right, and that's because the audience was actually sitting
> too close for long focal lengths; on film deviation was
> also way too much for the standard mount.  Lots of problems
> here.
>
> >> 3) on film deviation = focal length/30
> >> ...Spicer's and my rule which can be back-derived from
> >> Ferwerda and Waack, at least their advanced sections
>
> >
>  Lost me here, John.  Could you give an example of how this
> > rule would be
> applied?
>
> Well, let's put it in terms of the discussion above.
> Let's say you shoot with the ideal focal length for RF
> and projected viewing, which is 70 mm.  Then your MAOFD
> should be 70/30 = 2.33 mm.  Now your rule becomes (2.33/fl)
> times distance-to-nearest-point-in-scene = maximum
> stereobase.  You also have to mask your mount apertures
> down to get 2.33 mm instead of 1.2 mm on them.  (Start
> with a wider-than-normal mask.)
>
> > I have been using 2/1 scaling between the 35mm and medium
> > formats, resulting in a 2.4mm maximum OFD.  Sounds like
> > that was way too conservative.
>
> Not really.  The normal lens for RF is 35 mm and the normal
> lens for MF is 80 mm.  80/35 ~= 2.  You could also use
> (80/30)/fl = 2.7/fl but then to be consistent, you'd use
> (35/30)/fl = 1.2/fl for RF.  Pick your poison.
>
> =========================================================
>
> OK, having said all that, I disagree.  8-)  I disagree with
> the idea of keeping a fixed OFD by varying FL and near point.
> Also, I don't like these rules because they imply infinity
> is in the scene and often it's not.  Certainly if it's not,
> you are limiting yourself by the 1 in 30 rule and no macro
> shooter uses this rule.  Instead, they use charts which can
> be made from the MAOFD spreadsheet or looked up in Waack or
> Ferwerda.
>
> I would always start with the perspective point of the
> observer.  If he sits 18' from a 6' screen, why would you
> use a 35 mm taking lens for RF?  That way lies extreme
> stretch.  Instead, use a 70 mm lens (good luck finding one)
> and thereby make the observer's seat the correct seat.
> This is absolutely nothing new; read Ferwerda.
>
> For macro, you want the same focal length.  So use a 50 mm
> macro lens (RF) and rack it out to 70 mm.  Your observer's
> perspective is correct.  Then you get to choose the amount
> of enlargement in the stereo reconstruction by varying the
> stereobase to whatever you like, within reason.  Don't make
> your stereobase so large that you have more than 70/30 =
> 2.33 mm of OFD, though.  A lot of times you will want less
> to make the enlargement of the object more.  And again, yes,
> 2.33 mm means masking your mount apertures down.
>
> If anyone read this far, he is nuttier than I am, almost.
> 8-)
>
> John B
>