[MF3D.FORUM:51] Re: Apparent image size, MF verus

[Paul Talbot <ptww@xxxxxxxxx>]

Alan Lewis wrote:

> I was first trying to sort out the
> immersive effect (apparent image size) vs. format size.  So I chose a
> common starting point for all formats to be fair.
> What happened is that I realized that it is not really the format size
> that makes for an immersive feel, it is only the apparent image size.  I
> know this sounds so easy to figure out, but I (like others) never really
> could quantify whether the format size made the most difference or
> whether the magnification did.
> 
> I can say now that magnification plays the larger role, and your format
> choice will be guided by grain size and viewer lens choices.

Bill G. suggested using the diagonal to do the apparent image
size calculations, so I've made a new version of the table.
This time I've used the 78mm FL of the lenses of the current
version of the SaturnSlide.

Format     Size       Diag      Viewer Mag       Apparent size
------     ----       ----      ----------       -------------
5P         23 x 21    31.14     5.68 (250/44)    176.93
7P         23 x 28    36.24     5.68 (250/44)    205.88
MF         50 x 50    70.17     3.21 (250/78)    224.90
3Disc      24 x 36    43.27     5.51 (250/43)    251.57
MF-W       56 x 58    80.62     3.21 (250/78)    258.41
6x7        60 x 70    92.20     3.21 (250/78)    295.51

(MF-W is my estimate of the widest feasible aperture size for
mounts to be used in the existing SaturnSlide viewer design.
I used 56mm as an estimated maximum height to allow for some
loss of image area in mounting full 6x7 images.)

Hey, I think I see a pattern here!  ;-)

Most of these numbers are trying real hard to come out to
be 250mm, aren't they??!!

So I now propose the Lewis-Talbot-Glickman rule of stereo WOW
effect:

    The "apparent image size" of an image (measured diagonally)
    is constant (250mm) across film formats if the FL of the
    viewer lenses is "normal" (equal to the diagonal) for the
    masked size of the image.

Practical consequences of the rule:

    - Apparent image size is reduced when the viewing lens
      FL exceeds the diagonal of the masked image--whether
      due to high levels of cropping (e.g., the 50x50 MF
      mounts), or to the viewer lenses being significantly
      longer than the shooting lenses (e.g., Realist).

    - Apparent image size can be increased by either shooting
      with wider than normal lenses (and viewing orthostereo-
      scopically); or by viewing with higher power, but
      "squashy," lenses.  Example: shooting 6x7 with 78-80mm
      lenses instead of 92mm lenses.

The table also sugggests one reason why MF seems to have so
much WOW for most people's first impression:  they are used
to the Realist 5P format, which falls *far* short of the 250mm
"normal" apparent diagonal.

I do still acknowledge that there are other components to the
WOW effect that favor MF.

Paul Talbot