[MF3D.FORUM:376] Re: 1/fl

["Oleg Vorobyoff" <olegv@xxxxxxxxxxxxx>]

John B. wrote:
>I didn't study it really closely, but I didn't see 1/fl right off in there.
>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.

Well, 1/fl is a good approximation of those cheat sheets, which were calculated
using 1/30 x 35/fl.  Note that the 35 numerator nearly cancels the 30 in the
denominator, leaving 1/fl.  For MF I need to make up another set of cards based
on 1/30 x 70/fl.

>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.

Yes, that is what happens in the background.  From my point of view, though, I'm
simply getting a conservative (somewhat short) estimate of the stereo base to
use.

>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.

I still do not quite understand projection.  All I know is that I have stereo
pairs taken with 14mm through 200mm lenses under (essentially) the 1/fl rule
that project beautifully.

>On the other other hand, I
>think Oleg said he shoots for the hand viewer, and slide competition
>is incidental to his main aim (correct me if I'm wrong).

Right.  I feel that too much is lost under projection.  To me a projecting a
slide is sort of like scanning a fine print and printing it at low resolution.
A good photograph that depends on detail or subtle color distinctions is liable
to end up looking awful.

>In this case [hand viewing],
>going with the 1.2 mm limitation sounds right, though I've not tested it.


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, but it sounds like there is some flexibility under
projection.  However that stands, it must be emphasized that in many situations
having part of the scene in front of the window is not a sin, and may actually
be laudatory.

>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)

Yes, 1/fl, 1/30x35/fl, as well as the general formula in
http://werple.net.au/~kiewavly/bases.html, all suggest stereo base under this
principle.

>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.

>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?

Oleg Vorobyoff