[MF3D.FORUM:374] Re: 1/fl

[Paul Talbot <ptww@xxxxxxxxx>]

John B wrote:

> However, Oleg came over Friday to take pictures of the hills behind
> my house and he uses a different cheat sheet for each focal length.
> Then it dawned on me that he's keeping the on-film deviation constant
> at something like 1.2 mm regardless of focal length.  And that's what
> generates the 1/fl rule.  [snip]
>
> 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.

> So it looks like we have three rules going:
> 1) Constant maximum on-film deviation regardless of focal length
> (but dependent on format: 1.2 for 35 mm format, 2.7 for MF, etc)
> 2) base = near point/30 regardless of focal length
> 3) on film deviation = focal length/30

So is your 2.7 figure for MF based on (80mm fl/30)? (2.667)
If so, are you suggesting that because R-format is typically
viewed with 44mm FL lenses, instead of the 35mm typical shooting
lens, an OFD of 44/30, or 1.467mm, should be easily tolerated
in 35mm stereo images?

> #1 I attribute to what's-'is... Aha!  Good old Bob Mannle!
> #2 is the common rule from Ferwerda, for example
> #3 is Spicer's and my rule which can be back-derived from
>    Ferwerda and Waack, at least their advanced sections

If I'm following correctly, 1/fl is a quickie way to keep from
exceeding 1.2mm OFD in 35mm, given normal scenes that include
infinity.  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?

Paul Talbot