[MF3D.FORUM:42] Re: Apparent image size, MF verus

[Paul Talbot <ptww@xxxxxxxxx>]

Alan Lewis wrote:

> It is very easy to see how these formats appear compared to each other.
> You put a slide (or just an empty mount) in each viewer you want to
> compare.  Hold the right lens of one to your left eye, and the left lens
> of the other viewer to your right eye.  You can then toe the viewers to
> get the images to superimpose.  It is very apparent the magnification
> differences.

Great suggestion, thanks!

> Some subjective observations:
> 1) A Realist image (in a Revere viewer) is MUCH smaller than a MF image
> in the SaturnSlide viewer.  Why?  One reason is the lens f.l.  The
> Revere (and Realist) viewers do not use a lens that is the same f.l. as
> the camera.

I'd call that more than a subjective observation!  :-)  It
agrees with this part of the chart I posted:

| Format   Size     Viewer FL    Mag Factor        Apparent size
| ------   ----     ---------    ----------        -------------
| MF       50 x 50  80 mm        3.125 (250/80)    156.25 x 156.25
| Realist  23 x 21  44 mm        5.68  (250/44)    130.68 x 119.32

> The SaturnSlide does.  MF wins in the wow category.
> Perhaps if the Realist viewer used a 35mm f.l. lens the image size
> would  have less difference.

I could add another line to show what the Realist image would
look like in an ortho viewer:

 Realist  23 x 21  35 mm        7.14  (250/35)    164.29 x 150.00

So the use of an overly long (FL) viewing lens does cut
down the apparent image size of the Realist image by
quite a bit, as you indicate.

> 2) A full frame 35mm slide in a 50mm lens viewer (orhto)

I think you are assuming the full frame 35mm camera has a
50mm shooting lens when you call the 50mm viewing lens
"ortho."  My chart used the FL of the Red Button viewer,
which is probably what most people use to view full frame
stereo images.  These images might be shot with twinned
point and shoots with 35mm lenses, or an RBT with 35-70mm
zoom lenses, or a variety of options in between.

> will be approx.
> the same width as a MF image in the SaturnSlide (or any MF viewer with
> 80mm f.l. lenses).  But the height is not as great as a MF image.  So
> the MF wins in the WOW category.

With the Red Button lenses the extra width of full frame seems
roughly equivalent to the extra height of MF.

> As you go to larger film formats you should also match your viewing lens
> to the camera lens.  So merely moving to a larger format does not
> guarantee a larger image, it depends on what lenses you are using.  As
> you move to larger formats than 6x6 (6x9 would be a larger format for
> this discussion) you need a different viewer design, and should use a
> longer f.l. lens.

But if you have an 80mm lens on your 6x7 or 6x9 camera, then
the roughly 80mm FL of the SaturnSlide is "ortho" for those
images.
 
> One thing I feel is true:  6x6 MF will give you the most WOW factor
> using a standard parallel viewing design viewer with a monolith mount.
> This is probably the most convenient design for stereo.
> 
> If you move up to a larger format then you will need to use a Wheatstone
> or other base spreading viewer design with individual single mounts.
> 
> Of course if you don't care about having an orthoscopic setup then to
> get the biggest WOW factor you can merely use much shorter lenses to
> increase the magnification.

What about simultaneously decreasing the FL of the shooting
lenses and the viewing lenses?  The image will be more "wide
angle" and objects will look smaller.  But it will still be
"ortho" and the images will fill more of your field of view.

> I've done this with a standard MF viewer so
> I end up with 60mm f.l. viewer lenses.  There is no way you will be able
> to cover the entire 6x6 slide area with this f.l., but you do get an
> immersive feel to the image.

I've been chatting with someone who seem to have an immersion
obsession!  ;-)
 
[then later]

> I've been doing more calculations and thinking on apparent image size
> vs. format size.
> 
> I did some calculations similar to Pauls', then tested some of them out
> with real viewers (I have lots of viewers).
> 
> I assumed for fair comparison's sake that all calculations would be
> based upon the full format size (not mount mask size)

That makes a big difference!  The standard MF mounts crop out
a lot of image area!

> and the viewer
> having the same f.l. lenses as the camera.  This is just a starting
> point.
> 
> After all was said and done it appears that no format has a clear
> advantage on WOW factor based upon magnification alone. If all viewers
> used the same lenses as the camera (the "normal" f.l. lens that is) then
> the magnification differences would be hardly noticeable (IMHO).

I think that is a very good conclusion.

> So what gives the WOW effect?  Well, certainly apparent image size can;
> as can image clarity.  (Along with good composition, good subject
> matter, etc.) Also, I feel that using a square format will maximize the
> coverage area of a round lens.  You will probably feel more immersion
> with a square format vs. a rect. format.

What about the apparent "landscape" orientation of our
two-eyed view of the world?  Might not there be some
advantage to favoring a landscape format in stereo
slides?

> I would venture to say that to get the best WOW effect you should choose
> a format size that will give a minimum of grain showing, then use the
> shortest f.l. viewer lenses that you can manage to find. But remember
> that you will most likely not see the entire image.  It will be almost
> impossible to find short f.l. lenses that will cover the entire image.
> It is hard enough to find "ortho" lenses that will.
> 
> Usually lenses are easier to find for this purpose as you go up in
> format size.  In the full frame 35mm and Realist sizes it is difficult
> to find good short f.l. lenses.  In MF and larger formats it is easier
> to find lenses that will work.

Very true!

> It will help too to maximize the final size of the stereo mount mask so
> you are viewing as much of the frame as possible.  But setting the
> proper stereo window limits this size.

Agreed.  Keeping the design within the limits of normal
(non-prismatic) viewers is also a significant limitation.

Paul Talbot