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Is "3-D" More Natural in Porros or Roofs? (1 Viewer)

henry link

Well-known member
I understand the allure and entertainment value of increased stereopsis in traditional Porros over inline roofs, but I've wondered whether it is Porros or roofs that come closer to matching the natural stereopsis we experience with no binocular. In other words do we see an exaggerated "3D" with Porros or diminished "3D" with roofs?

To test the effects of a wide baseline vs the eyes' normal separation I recently made a small periscope by combining two telescope star-diagonals. By placing the periscope horizontally in front of one eye I was able to increase the apparent baseline between my eyes from about 60mm to about 140mm, the baseline of a large Porro.

Looking through this viewer gave me the stereopsis of a Porro, but without the added magnification or any of the other optical artifacts of a binocular view. Just like in a Porro binocular the view provided obviously more stereopsis at mid distances and out to around 150 meters, but at close distances inside around 10-15 meters things took an odd turn. Objects looked smaller and smaller through the periscope compared to the normal baseline view as the distance decreased until at 3 meters the size of objects appeared to be reduced by about 20-25%. At even closer distances objects seemed to become toys from a dollhouse. This is the same illusion of lower magnification at close distances in Porros compared to roofs that has has been often reported. The stereopsis at 1-2 meters was wildly distorted through the periscope, with a highly exaggerated longitudinal separation between apparently tiny objects. Clearly the brain is confused by spatial cues that seem to be coming from eyeballs on stalks.

Another effect I noticed is that an unnaturally shrunken object viewed through the periscope also appears to be sharper than the same object viewed normally. Obviously there is no real change in sharpness, but I suspect this is probably the origin of reports of increased sharpness in some Porros, even when their optical quality is nothing special.

We don't normally see the full effect of the up close funhouse spatial distortion in Porros because of another effect that results from the wide baseline. Through traditional Porros the overlap of the right and left fields is much poorer at close distances than through inline roofs (or reverse Porros), so only a very small area of stereopsis is actually seen in the cat's eye shape where the two fields overlap.

This reduced overlap causes another effect, which has long been my main objection to Porros for close in birding or insect watching. While it's impossible to center an object in both fields of any binocular at close distance, reverse Porros have the least off-set at short distances, then inline roofs. Traditional Porros have the largest off-set. Typically, if an object is centered in the right field of an 8-10x Porro at 2 meters the same object will appear near or at the edge of the left field. We usually unconsciously deal with this by simply "centering" the object 15-20º off-axis in both fields, thus compromising the the alignment of the eye's and the binocular's optical axes.

Even though I own many more Porros than roof prism binoculars, over the years I've come to dislike the increased stereopsis of Porros over roofs even though I wasn't sure which spacial presentation was more "accurate". The recent experiences with the periscope have finally offered some good evidence that it's the traditional Porros that have too much stereopsis for a natural view and the inline roofs that have about the right amount.
 
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Binastro

Well-known member
When viewing close up our eyes swivel inwards.
Using a periscope might force ones brain into false information mode.

When using reverse Porroprisms I clearly get a stereo image more pronounced than with unaided eyes.

I have always thought, and still do, that the amount of stereo is determined by the magnification of the binocular multiplied by the objectives centre separation divided by the IPD.

If a reverse Porroprism binocular is 8x and the objective separation half the IPD then the stereo effect is 4x unaided eyes.

However, most of my binocular observing is done at distances of more than 10 metres and often over 100 metres.

The only time I use binoculars close up is to see the title on a book high up on the bookshelf.

I recently talked to a friend who lost the sight in one eye.
He drives but needs to take 6 monthly tests.

The problem he has is when shaking hands with visitors at work.
He often misses the other person's hand.

Regards,
B.
 

Canip

Well-known member
Thank you, Henry!
Great post and food for thought, challenging some common perceptions that tend to be accepted without much debate.

Canip
 

[email protected]

Well-known member
Supporter
When viewing close up our eyes swivel inwards.
Using a periscope might force ones brain into false information mode.

When using reverse Porroprisms I clearly get a stereo image more pronounced than with unaided eyes.

I have always thought, and still do, that the amount of stereo is determined by the magnification of the binocular multiplied by the objectives centre separation divided by the IPD.

If a reverse Porroprism binocular is 8x and the objective separation half the IPD then the stereo effect is 4x unaided eyes.

However, most of my binocular observing is done at distances of more than 10 metres and often over 100 metres.

The only time I use binoculars close up is to see the title on a book high up on the bookshelf.

I recently talked to a friend who lost the sight in one eye.
He drives but needs to take 6 monthly tests.

The problem he has is when shaking hands with visitors at work.
He often misses the other person's hand.

Regards,
B.
"I have always thought, and still do, that the amount of stereo is determined by the magnification of the binocular multiplied by the objective's center separation divided by the IPD. If a reverse Porroprism binocular is 8x and the objective separation half the IPD, then the stereo effect is 4x unaided eyes."

Interesting. I have never seen a mathematical formula applied to quantify the 3D stereopsis of a porro prism binocular. So the higher the magnification and the smaller the IPD, the higher the 3D effect. That is logical. In these Covid times, it might be a good thing to miss their hand!
 

Binastro

Well-known member
I think that stereo vision varies person to person.

It depends on acuity and the sameness or otherwise of each eye.

I know several good observers who have poor stereo vision.

With binoculars the stereo effect may always seem unnatural if one looks carefully.

I just used the Swift 8.5x44. HR/5.
The objective separation is double my IPD.

There are a variety of effects.
Flat planes at different distances.
Subtle changes axially.
Flat trees and depth in trees.
Flat building planes and depth axially.

They are all unnatural compared to unaided eyes.

I can agree with Henry that the same objective separation and IPD may be optimum.

The Canon 10x42L IS is a Porro with little difference.

Small reverse Porros are half the distance.

Minolta Activa up/down Porro about the same.

Roof prisms usually about the same.

The actual stereo effect though is predominately due to magnification.
This is also unnatural.

The strangest effects seem to come at close distance.
I rarely go to less than 9ft.

I looked at a pigeon at 9ft or 10ft and the detail seen was interesting with the Leica 8x32BA.
It also looked unnatural, particularly the eye of the pigeon.

Regards,
B.
 

Binastro

Well-known member
As one reduces IPD with a Porroprism binocular the barrels are rotated and the objective separation reduces, so it may not be accurate to say reduced IPD means more stereo.

However, more magnification means more stereo, but also much reduced depth of field.

Again, very unnatural.

Regards,
B.
 

Binastro

Well-known member
J.J. Gibson's theory regarding stereo vision also has merit, but I am not sure how much it deals with my case.

Regards,
B.
 

Richard D

what was that...
Supporter
United Kingdom
Interesting experiments. My gut feeling is that in the 10-50 metre range the Porro separation helps to compensate for the unnaturalness of magnification - closer than that it has a detrimental effect (whilst reverse Porros don't), and much beyond 50m the difference between, Porro's and roof designs seems to make little difference. It's a gut feeling based on my perception though. I'm not sure how you measure 'natural looking'.
 
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tenex

reality-based
Another effect I noticed is that an unnaturally shrunken object viewed through the periscope also appears to be sharper than the same object viewed normally. Obviously there is no real change in sharpness, but I suspect this is probably the origin of reports of increased sharpness in some Porros, even when their optical quality is nothing special.
Very interesting, Henry. Could you describe this effect further? I'm trying to envision just how relatively nearby objects are going to look "sharper" with the periscope, or how they would look less "sharp" without it. Their edges, or their fine detail, or...?

I will take the 3D view of a porro any day over the "Pie Plate" flat field view of a roof...
Field flatteners have nothing to do with stereopsis.
[Edit: I find it difficult to know exactly what people mean when they talk about "3D" in binoculars, as they often seem to as well. (I don't normally notice such differences between instruments of the same magnification myself.) After I wrote this, Dennis added two long paragraphs to that post from post #8 of a Cloudy Nights thread on "3D", discussing further effects of widely separated objectives which he might have had in mind in saying this... and then deleted them again along with this sentence, why? It's hard to get anywhere that way.]
 
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Ted Y.

Well-known member
Canada
From #9
"The relevance of the so-called 3D effect provided by binocular vision is applicable ONLY to objects observed at close distance."

As I understand, this contradicts my observation: the 3D effect is excellent for a group of 2 geese at 200m distance (from the observation point). The binocular was a 8x42 roof.
The 3D effect is in the human brain, therefore, for me, some calculation is irrelevant without the knowledge of how 3D effect is happening in the association (binocular + human vision).

I would like to ask why using “so-called”, but the text is from another forum.
 
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William Lewis

Wishing birdwatching paid the bills.
United Kingdom
Thanks henry. The close focus disadvantage of porros in the field is noticeable to me too, I generally reduce the ipd to compensate to a degree. I still like them for value for money and brightness reasons though so won't be switching to roofs any time soon.
 

henry link

Well-known member
Very interesting, Henry. Could you describe this effect further? I'm trying to envision just how relatively nearby objects are going to look "sharper" with the periscope, or how they would look less "sharp" without it. Their edges, or their fine detail, or...?

Hi tenex,

You probably already know that close objects appear to be smaller in traditional Porros than in roofs. This oddity has been discussed many times here and I think the consensus has been that the wider objective baseline of Porros leads to the eyeballs swiveling inward more than normal and that is interpreted by the brain to mean that objects are closer than they actually are and therefore smaller than they actually are.

The periscope allowed me to eliminate all the optical variables in that phenomenon except for the wider effective baseline between my eyes. The smaller/sharper illusion appears to result simply from the smallest resolvable objects appearing to be smaller than they really are so that lines look thinner and dots look smaller than normal. I tested whether my eyesight acuity actually improved with the periscope by examining a USAF 1951 chart with and without the periscope. The smallest line pairs I could resolve with the periscope were exactly the same as without it, but they appeared to be at least one or two chart elements smaller than they actually were compared to no periscope, thus giving me the illusion that I was resolving one or two sets of line pairs smaller than I actually was.

Henry
 
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tenex

reality-based
The smaller/sharper illusion appears to result simply from the smallest resolvable objects appearing to be smaller than they really are. Lines look thinner and dots look smaller than normal.
Thanks, that connection makes perfect sense; it just didn't occur to me without further explanation.
 

Tringa45

Well-known member
Europe
I once had the opportunity of looking through a trench (periscope) binocular. The objective spacing is dependent on the IPD setting and, IIRC was in my case around 40 cm. The binocular had a dedicated tripod as hand-holding would be almost impossible. It was of course intended for use at longer distances, but at moderate distances among trees the stereopsis effects were spectacular.

John
 

WJC

Well-known member
I once had the opportunity of looking through a trench (periscope) binocular. The objective spacing is dependent on the IPD setting and, IIRC was in my case around 40 cm. The binocular had a dedicated tripod as hand-holding would be almost impossible. It was of course intended for use at longer distances, but at moderate distances among trees the stereopsis effects were spectacular.

John
The largest rangefinder I ever worked on had an objective spacing of 17 1/2 feet. Many of our biggest battleships had a spacing over 40 feet.
 

Pinewood

New York correspondent
United States
I once had the opportunity of looking through a trench (periscope) binocular. The objective spacing is dependent on the IPD setting and, IIRC was in my case around 40 cm. The binocular had a dedicated tripod as hand-holding would be almost impossible. It was of course intended for use at longer distances, but at moderate distances among trees the stereopsis effects were spectacular.

John
Hello John,

I believe those trench periscopes are known as "donkey ears." Do you recall the minimal focussing distance and how natural was the view at its minimal distance?


Stay safe,
Arthur
 

[email protected]

Well-known member
Supporter
Hello John,

I believe those trench periscopes are known as "donkey ears." Do you recall the minimal focussing distance and how natural was the view at its minimal distance?


Stay safe,
Arthur
Yup, here is a Soviet 10x45 "Donkey Ears" binocular. Similar to a periscope, the idea was to observe the enemy without getting your head shot off!

STUNNING+20thC+SOVIET+10X45+'DONKEY+EARS'+BINOCULARS+ON+TELESCOPIC+STAND+c.1950+-+03.jpg STUNNING+20thC+SOVIET+10X45+'DONKEY+EARS'+BINOCULARS+ON+TELESCOPIC+STAND+c.1950+-+08.jpg
 
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John A Roberts

Well-known member
Australia
In relation to the posts above, some images of devices with significantly altered stereopsis compared to regular Porro prism binoculars:

a) When Zeiss commenced production of Porro prism binoculars in 1894, they also offered a handheld Stereo Telescope, aka a Relief-Fernrohr
(it uses a variation of the Porro Type II prism, with one of the four right angle prisms located at the front of the objective) *

b) While the above never saw much civilian use, by 1904 it had been enlarged and was used in mounted form for military use.
See an image from a catalogue by Cassidian Optronics (the catalogue from 2012 is titled Handheld Targeting Systems,
and on the front page it indicates 'Optics by Carl Zeiss').

c) Enhanced stereopsis was essential to the operation of coincident image rangefinders (aka telemeters).
Depending on the task they went all the way from handheld sizes, as shown in the Cassidian image; to as Bill indicates those for shipboard use.
See an image from 1912 at: USS Nebraska Naval Ship Cover 1912 From Ship Post Master Range Finder RPPC | Topics - Transportation - Boats, Postcard

d) And the Pentax Papilio reverse Porro prism binocular uses significantly altered stereopsis in a different way.
To achieve a minimum focus of only 50 cm/ 20 in, the objective lenses in each barrel move closer together.
See the images in post #4 at: Where are the aspherics?


And finally, there were not only stereo enhancements to vision! See an image from: https://mobile.twitter.com/beidaihebirder


John


* In the early 20th century Zeiss also offered other hand held binoculars with exaggerated stereopsis.
See post #91 at: New Horizons II
 

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