I am not sure of what you are saying. Is this the old argument about ortho vs non-ortho? In my experience a 5mm difference in viewer fl makes no noticeable difference to the viewer. I would expect that I have to rack out the lenses that much just to focus the slide for my aging eyes. This makes the image smaller for me. I prefer a larger image and would prefer a shorter lens to achieve it.

Am I missing something?

Timo

 

I am in full agreement with Timo.  My favorite MF viewers use 68mm and 45mm fl lenses.  I am not bothered by the theoretical assertion of flattening of the stereo image. I try to deal with the real World, not the theoretical , "ortho",musings from the past.

Cheers,

DON

> Hello all,I wanted to ask if any of you has modified a 3D World,
> or a 3D World compliant stereoscope like the Heydascope
> to fit 80 mm lenses
> instead of the regular 75 mm lenses ?

> First of all, the 80x140 mm standard (thank you 3D world)
> for the MF stereo frame seems a very good invention to me :
> the former 6x13 (cm) standard, inherited from glass photo plates
> of the nineteenth century, was simply not relevant for 120 format film rolls.

Posted by: John Thurston <juneau3d@thurstons.us>

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>
> On 21-Aug-14, at 2:12 PM, John Thurston
> juneau3d@thurstons.us [MF3D-group] wrote:
>> If you can find a "very old", first generation 3D World
>> focusing viewer, the lenses are very easy to swap and moving
>> them away from the film plan can be done without hacking up
>> the body of the viewer.
>>
> How old are we talking? My viewer looks like the lenses are
> held in from the back. The final version has extensions to
> bring the focus a little further out. I wasn't aware there
> was a 3rd version.

Sure Timo, you’ve got it, this is the old argument “ ortho. / non ortho. ”.

When you start playing with focals (keeping the same base, to make it simple)

you rapidly find out that immersion vs depth is a trade-off.

Increasing the perceived depth (in other words, stretching the axial dimension in the view), either by selecting a shorter taking lens for the camera, or using a longer viewing lens in the stereoscope,

will be immediately penalizing in terms of immersion. This is part of “the tunnel effect”.

Conversely, a higher immersion, obtained with a longer focal of the camera or a shorter focal of the viewer will automatically produce flat images.

This is what I don’t like with the 3D World system.

By chance, this  is the kind of trade-off where the central (equilibrium) point is perfectly known to us : it is called ortho-stereoscopy. Just as much immersion as in reality ; just as much depth as in reality.

                               ___________________

But I think you raise a very interesting question : if we cannot achieve pure orthostereoscopic conditions, when do we enter the domain of visual illusions and funny deformations ?

Will a 5 mm difference in the focal distance be noticeable ? and if it’s noticeable, will it be such a hassle ?

First I’d like to point out that the difference between 75 and 80 mm is almost 7 %.

When you consider the extreme sensitivity of human eye, and even more, human viewprocessor (our brain) you cannot directly put this apart.

But this is still theoretical debate, and this is not sufficient ;

you need to look at images, to make up your mind.

I did, and I compared a lot of pictures taken with my TL120 (80 mm taking lenses)

with those of the Spoutnik (75 mm taking lenses). But not from the same scene, unfirtunately.

My conclusion : apart from other differences (TL 120 images have no vignetting, they are sharper, etc …) the views from TL120 are somewhat disappointing. The presence (immersion) is incredible, sometimes in excess : you cannot perceive the edges, where the stereoscopic window lies.

But the depth is a bit poor : not much difference between 2 eyes opened and one eye closed, except for very close objects. The lack of vision of the stereoscopic windows reinforces this  disappointing impression, unfortunately, since you lose a milestone in depth.

The views from TL120 in a 3D World viewer are good -no doubt- and they are pretty  comfortable (apart from the immersion in excess). But making so much effort for 3D pictures, and getting flattened images, insn’t it a pity ?

Of course, in the highly subjective domain of visual impression, someone else may draw a different conclusion from the same raw material, and I will respect it … but if you are sensitive enough to feel a better immersion with a 5 mm difference in focals, you should be sensitive enough to feel the corresponding loss in image depth.

                                 _________________________

Regarding the comfort of vision provided by a shorter focal length, I do not believe it makes a difference (and I have aging eyes, too J). Let me explain.

Switching the stereoscope lenses to 80 mm will undoubtedly make the image look smaller

(but you cannot say “too small”, because this will be pure ortho-stereoscopic conditions).

And this -I agree- can be described as a “loss in immersion”.

But the visual comfort mainly depends on the accomodation distance of your eyes to the (virtual) image, not on the focal length nor the viewing angle.

This accomodation distance, you tune it by changing the distance between lenses and slides, making it a little inferior or very close to the focal length. This way you set the virtual image at a distance of 2m (recommended) or more (aged person).

Of course this is not possible on basic, cheap viewers like the 3D world STL viewer or cardboard viewers.

But on a viewer with a rack which carries the lenses, the tuning, resulting in the proper visual comfort whatever your visual performance, will be just as easy with 80 mm lenses as with 75 mm lenses.

 

I think that this historical point of view is quite interesting ...but not to demonstrate "the weakness of going ortho" (quote from your message)

In my opinion it simply demonstrates that only Medium Format is adequate
for ortho-stereoscopic viewing.

I may be rude to state it like this (but normally many readers in this forum
should agree :-)  ) 24x36 format is only good for projection
(and medium format stereoscopes will be used long after the last projection
of double 5x5 views with polarizing filters, don't you think ?

Hello Jean Paul

     I am in agreement with your views concerning MF, when viewed in a high quality MF viewer.  I do not consider the 3D World focusing viewer to be a high quality viewer.  I consider projected 3D images to be significantly inferior to what can be seen in a high quality MF viewer. I recall reading that at an ISU Conference, several years ago, Dr Werners' MF slides were projected on a large screen, which amazed the audience, as the camera used to take them was an old Heidoscop, owned by Dr Werner. I am certain that if viewed in a quality MF viewer, the audience would have been amazed even more, as projected 3D does not ring my bell.

IMO, regarding "Ortho Viewing", digital 3D, more or less eliminates it being seen, except by accident, as the fl of the camera taking lenses is often unknown, when being presented, via the internet.

Cheers,

DON