Why! (1 Viewer)

[Stanbo]

I have been looking at a bunch of orange mountain ash berries through different binoculars and the SIZE of the image is significantly different between each one. Why!

Without naming each binocular, but suffice to say that they are all in the top echelon of their respective designs.

They are an 8x32 porro 7.5 degrees, a 8x32 roof 7.8 degrees and a 7x26 reverse porro 6.9 degrees. I don't have an 8x reverse porro, but I don't think that it makes any difference to what I see, as it appears to be the design of binocular that counts.

The size of the image is, as I have said, significantly different with the 8x32 porro being the smallest, the 8x32 in the middle and the 7x26 the largest. I would have thought that the latter one with lowest magnification would have had the smallest image, but it is exactly the opposite, so I don't think that it's related to magnification. In case the angle of the view was the cause I looked through another 8x32 with only 6.4 degrees and size of the object was the same as the 8x32 with 7.8 degrees, I don't think that it is the angle of the view either.

The objective lenses are respectively 133 mm, 73 mm and 42 mm apart and the 3D effect increases with increased objective width which I can understand and can see clearly. However, what I can't understand is WHY the size phenomenon happens at all.

Scientific explanations accepted, but please try and also include a layman's explanation for people like me, if possible.

Stan

 

[Chosun Juan]

Stan, the simple layman's stab is that the roof prism has the objective centrelines in-line with the eyepiece centrelines - think of that as giving a "par" image for the magnification of the bin.

When the objective centreline spacing is wider than the eyepiece centreline spacing (as in a normal porro, or to a much lesser extent an ABK prism roof like the bigger Zeiss FL's), the image size is reduced for the same magnification.

When the objective centreline spacing is narrower than the eyepiece centreline spacing (as in a reverse porro), the image size is increased for the same magnification.

All these effects reduce with greater distance from bin to the viewed object, and are most pronounced under ~200ft or so.

Minor complications arise since the objective size, and magnification also have small +/- tolerances, so for example your 7x bin, may in fact have a magnification of 7.334156!, etc



Chosun :gh:

 

[Binastro]

. The orange coloured berries probably introduce chromatic aberration of the eyes.
This in itself produces a 3-D image if viewed against a different colour background.

It would be interesting to try to view different coloured berries and also similar coloured berries to the background to see if the situation changes.

I think much of the phenomenon might be because of the brains interpretation of what you are seeing.

The Moon illusion comes to mind where a full moon near the horizon looks to be twice the diameter of the moon when it is high in the sky.

Our eyes do play tricks on us.

You can easily check the magnification by viewing a brick wall with each binocular at the distance of the berries using alternate eyes and taking multiple readings and averaging them out.
you use one barrel of each of two binoculars swapping them round as one's eyes are usually not matched.

 

[Stanbo]

. The orange coloured berries probably introduce chromatic aberration of the eyes.
This in itself produces a 3-D image if viewed against a different colour background.

It would be interesting to try to view different coloured berries and also similar coloured berries to the background to see if the situation changes.

I think much of the phenomenon might be because of the brains interpretation of what you are seeing.

The Moon illusion comes to mind where a full moon near the horizon looks to be twice the diameter of the moon when it is high in the sky.

Our eyes do play tricks on us.

You can easily check the magnification by viewing a brick wall with each binocular at the distance of the berries using alternate eyes and taking multiple readings and averaging them out.
you use one barrel of each of two binoculars swapping them round as one's eyes are usually not matched.

Binastro,

Thanks for the explanation about 3D. Unfortunately I missed a word out in the second to last paragraph which should have read size phenomenon. I will modify the original post

Stan

 

[Stanbo]

Stan, the simple layman's stab is that the roof prism has the objective centrelines in-line with the eyepiece centrelines - think of that as giving a "par" image for the magnification of the bin.

When the objective centreline spacing is wider than the eyepiece centreline spacing (as in a normal porro, or to a much lesser extent an ABK prism roof like the bigger Zeiss FL's), the image size is reduced for the same magnification.

When the objective centreline spacing is narrower than the eyepiece centreline spacing (as in a reverse porro), the image size is increased for the same magnification.

All these effects reduce with greater distance from bin to the viewed object, and are most pronounced under ~200ft or so.

Minor complications arise since the objective size, and magnification also have small +/- tolerances, so for example your 7x bin, may in fact have a magnification of 7.334156!, etc



Chosun :gh:

Chosun,

Thanks for that, but does that mean that the 8x32 porro has less magnification than the roof. If so should they both be called 8x32's if the magnification of one is greater than the other

Stan

 

[cycleguy]

I'll guess that you are judging the size relative to how much of the field of view is occupied by the object you are looking at.... and the result is the smaller field of view yields the larger object.

The actual magnification and field of view could be off for the 8x porro and may actually have the larger FOV.

My .02 guess,

CG

 

[Stanbo]

I'll guess that you are judging the size relative to how much of the field of view is occupied by the object you are looking at.... and the result is the smaller field of view yields the larger object.

The actual magnification and field of view could be off for the 8x porro and may actually have the larger FOV.

My .02 guess,

CG

CG,

Thanks for your reply.

Unfortunately, the field of view angles I quoted don't support your suggestion and I will set all of them out together with the field of view width at 1000m and approximate relative image size

Bino Degree angle FOV width Image size

8x32 porro 7.5 131/1000 small
8x32 roof 7.8 136/1000 medium
7x26 reverse porro 6.9 121/1000 large

2nd 8x32 roof 6.4 111/1000 medium
(same as other
8x32 roof)

When viewing, there is significant difference in image size between the the 8x32 porro which has a smaller image and the 8x32 roof, both of which have a similar FOV, yet the second 8x32 with a FOV of only 111m has the same apparent object size as the other 8x32 roof with a greater FOV of 136m.

Stan

Sorry, all the spaces disappeared from the table when saved to the thread, but I think that it is still understandable.

 

[henry link]

Stanbo,

This illusion has been discussed here before, but maybe not for a few years. Chosun is on the right track. Our brains make size judgements partly based on parallax. An object that requires the eyes to toe in more is interpreted by the brain as closer and therefore an inherently smaller object. The wide spacing of the objectives in conventional Porros increases the parallax so the brain interprets an object as closer and therefore smaller than the same object viewed through binoculars with narrower spacing. Try looking at the berries with one eye only and I think these size illusions should mostly disappear.

Henry

 

[Mark9473]

Henry, isn't it so that the magnifications are valid at infinity? As closest focus is approached I would expect the actual magnification to depend on the design of the binocular, and I would especially expect a difference between internal-focusing and regular binoculars.

 

[looksharp65]

I can but confirm the convergence explanation. Based on the three types of binoculars involved, this is very obvious.

The viewing distance has great influence on this illusion. If the three binoculars are used for star-watching, it's obvious how the illusion is gone. And conversely, the reverse-porro will seem to make the image significantly larger when used at short distance. Its objective lenses are close to each other and this provides much of the convergence needed for that short distance.
As a result, the eyeballs will converge much less or nothing at all, which the brain interprets as if the object is farther away and larger, (an important clarification!)

Remember this is not real magnification. The 6x reverse-porro will not provide the same resolution as an 8x porro even if the images look similarly large at shorter distance.

Anyone who has any doubts regarding this should visit an optometrist and ask to look through a trial lens with 5 prism diopters power towards any object at a distance of about one meter.

When the base (the thicker part of the prism) is placed temporally ( a k a base out), they will experience a brief moment of double vision until the eyes have converged to neutralise the effect of the prism, and the object will seem slightly smaller.

When the base is placed with the base nasally ( a k a base in), the eyes needn't converge as much as before the prism was introduced. The actual retinal image's sizes aren't changed but the 5 prism diopters allow one of the eyes to look towards a point which is placed 5 centimeters wider apart at one meter without getting double vision.

The brain calculates the relation between accommodation effort, convergence effort and retinal image size but the prisms (well, in fact any spectacle lenses but the prisms in particular) cause a miscalculation.

An even easier way (but with probably less predictable results) is to put the edge of a pencil a few centimeters/2 inches in front of one eye.
Then try to make a maximum convergence effort as if trying to find it with the other eye (you will not be able to see it because the nose will be in the way unless it's very tiny).
You will se, though, that the pencil seems to shrink while you make the convergence effort for the reasons mentioned above.

//L