Retinal Offset and Field-of-View Effects on Apparent Size Using Binoculars (1 Viewer)

henry link said:

Jonathan,

Thanks for the suggestion. I've done this too, and did it for the binoculars I tested on this tread. I don't know how wide the tolerance is, but the eye seems to be able to successfully merge images of slightly different magnifications. In this case I tried to place the images of the blue bird box from the two binoculars right next to each other. With both images swimming around I find it hard to make really accurate judgements about relative size. I think another problem is that distortion will effect the apparent size of objects differently depending on how much large the objects are and where they are located in the field. A binocular with barrel distortion will have maximum magnification at the center of the field while a binocular with pincushion distortion will have minimum magnification at the center.

A problem for me with using this technique for judging brightness and color cast is that I often seem to see a slightly brighter image from one eye than the other (usually the more rested eye) and I always see a different color cast, warm on the left and cool on the right.

Henry

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Yes, I'm sure this wouldn't work for slight differences in magnification. I can achieve a degree of stability in merging the images by resting the binoculars on a fence rail or table top while viewing, but it is ungainly and requires different size/shape cushions under each.

My eyes also perceive color differently--more red in one eye and more yellow/blue in the other. In comparing color casts I have switched the binoculars from side to side, but it really doesn't tell me much more than looking through each binocular separately.

In any case, a half-power difference is obvious to me when merging images in this manner, and relative brightness is also fairly obvious.

mcdowella said:

First of all thanks for a very interesting series of experiments. Now you can try out different fields of view with the same binocular, do you notice any difference in apparent brightness?

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mcdowella,

No. As it turned out, 5mm is about the smallest hole one might expect to use with a 3.75mm exit pupil, — at least using a faucet washer that's 3mm thick. Assuming this works as a field stop, the brightness (per unit area) should not decrease, and my impression is that it doesn't. If the hole were smaller than 3.75, I guess, then it should limit the light through the exit pupil and get dimmer. But, I haven't had enough experience with this rig yet.

-elk

Jonathan B. said:

Henry and elk,

I hope this doesn't sound off the mark, as it is not a way to measure magnification. However it is a simple way to detect whether there are magnification differences between two binoculars, and I have done it to compare the relative brightness of binoculars in low-light situations, as well as to compare color casts. I hold both binoculars to my eyes simultaneously -- one barrel of each up to an eye. It takes awhile to adjust them, rarely more than thirty seconds, but it is relatively easy to line up the images and even superimpose them. The shapes of some binoculars cause them to interefere with each other and make the comparison almost impossible, but usually if at least one is a roof-prism model, it will work.

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Jonathan,

I've tried the method a time or two, but can't hold it steady. I think Henry's already mentioned that the brain works to arbitrate binocular rivalry, i.e., discrepancies between the left and right eye presentations, which is an adaptive response that would defeat the intended purpose. In this case, if I stop down the FOV on one side of the binocular and view with both eyes, the brain resolves the difference to a large extent. The resolved field seems to correspond with the larger of the two fields.

Thanks for the suggestion.
-elk

elkcub said:

Jonathan,

I've tried the method a time or two, but can't hold it steady. I think Henry's already mentioned that the brain works to arbitrate binocular rivalry, i.e., discrepancies between the left and right eye presentations, which is an adaptive response that would defeat the intended purpose. In this case, if I stop down the FOV on one side of the binocular and view with both eyes, the brain resolves the difference to a large extent. The resolved field seems to correspond with the larger of the two fields.

Thanks for the suggestion.
-elk

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elk,

The brain may compensate to a slight degree, as Henry points out, but for example if you compare a 7x and an 8x, the two images are dramatically different--precisely as much as the respective magnifications would have you expect. In cases where I had an illusion that two binoculars of the same magnification differed, their images proved to appear identical in size when compared this way. However my comparisons of magnification have been few in number; more often I have compared relative brightness.

henry link said:

Elk,

I've continued to play with this, using Kimmo's washers and borrowing a pair of old B&L 8X24 reverse porros to maximize differences in objective spacing. The short result is that in monocular tests I continue to see no effect on apparent image size from FOV if the target object is small (5 degrees of AFOV), and only a small effect if the target object is large.

On the other hand, in binocular tests the illusion of size differences for any size object from parallax can be amazingly large and also extend to unexpectedly long distances. In the extreme case of the B&L 8X24 (37mm objective spacing) vs the Nikon zoom (132mm spacing) the apparent size difference at 15' was so large that equalizing the image sizes required increasing the Nikon's magnification to almost 11X. Back at 130' the Nikon required about 9.5X. I was still able to see a little difference in the size of a small object at a distance I estimate to have been 500-600' requiring an increase in the Nikon to perharps 8.2-8.3X. In monocular tests I could see no difference at all in magnification between the B&L and the Nikon at 8X. The field widths (Nikon 42 degrees, B&L 50 degrees) should have favored a larger apparent image in the Nikon from the influence of FOV. So all the difference I saw would have been from parallax, indicating that when it comes to this illusion the eye/brain is sensitive to very tiny changes in the lines of sight of the eyes.

P.S. I just noticed your addition to an earlier post. When magnification is increased with the Nikon zoom the real field shrinks, but the apparent field widens in partial compensation. I made my size estimates using small objects that I had found were not affected by FOV in monocular tests.

Henry

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Henry,

Unfortunately, it may be very difficult to reconcile differences between our observations, which are made using very different equipment and entirely different methods. My own behavioral science bias (er, religion) is to separate treatment variables using matched equipment, — so I don't know how to make sense of results complicated by devices that change several parameters at once (e.g., zoom binocs) or don't control treatment variables (e.g., introducing reverse porros). Perceptual phenomena are very often influenced by treatment variable interactions, which can't be evaluated without a factorial design.

A Honda car is now in front of the house shown in my 60 yd. viewing scene (earlier post). I guesstimate it occupies 30% of the visual field in either the Bushnell (roof) or Nikon (porro), which is 8.5 deg in all. It may take up perhaps 45-50% of the Swaro's 7.7 deg. field. The Bushnell and Nikon images appear to be the same size to me, but the Swaro seems larger (S>>B~N). I get the same result viewing with one eye or two. In other words, at this distance the field size seems to make the primary difference and not the retinal disparity. (I've recently discovered that the eye relief is shorter on the Bushnell, so I have to be very careful to insert the eyecups into my eye sockets to prevent FOV cutoff. That explains why the Bushnell seemed slightly larger than the Nikon in my original post.) The second dependent variable that I'm taking note of is stereopsis (3-D effect). The Nikon porro has appreciably more depth than the Bushnell. The Bushnell has moderately more depth than the Swaro. (N>>B>S)

When FOV is taken into account even at a shorter viewing distances of 50-75 ft. only the Swaro seems larger. The stereopsis effect of the Nikon is magnificent at this point, and the Bushnell's view is disagreeably flatter, but the images seem very close in size. For size: S>>B>N. For 3-D effect: N>>S>B. Right, the Swaro seems to have more depth than the Bushnell even though they are both roofs. This may be because the optics have greater depth of focus (hic.).

After going through my procedures several times, Henry, I'm somewhat in awe that you are able to use a zoom to measure size differences. I find it a challenge to just get consistent momentary impressions, which fade in a few moments.

On the subject of perceptions with a reverse porro, I use a Bushnell 7x26. Sometimes I wonder why I just don't use that for birding alltogether since the view is magnificent. That's a subject for another study.

-elk

Jonathan B. said:

... where I had an illusion that two binoculars of the same magnification differed, their images proved to appear identical in size when compared this way.

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Or, your brain could have masked a real difference? Not to be argumentative, but that might have been an illusion. I would personally prefer physical measurements as Kimmo suggests when it comes to calibrating equipment.

elkcub said:

Not to be argumentative, but [it] might have been an illusion. I would personally prefer physical measurements as Kimmo suggests when it comes to calibrating equipment.

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Hi, elk

This is not an illusion, and I have just made the comparisons again between 8x20 BCA, 8x SE, and 7x Ultravid. I still concede that a small difference in binoculars said to have the same magnification might be hard to detect, but differences of a half-power are obvious.

The first time I did this was about five years ago, when I had acquired 8x20 BCA and 8x32 SE. The 8x20 gave the illusion that it magnified more (I think it is because of its narrow field of view--purely an optical illusion). The comparison showed that the two have precisely the same magnification. 7x Ultravid provides an image obviously smaller than 8x SE. In other comparisons that I can't replicate today (because I don't own the other bins), 7x FL appeared to provide identical magnification to 7x Ultravid, and 7x Ultravid identical to 7x Zeiss ClassiC; 8.5 EL provided a larger image than 8x SE; 8x LX provided identical magnification as 8x SE.

This is easy to replicate; virtually every contributor to the forum should be able to do it. So could a few of you try it and back me up?.

In a fixed position (i.e. sitting somewhere) I prefocus both bins on the same subject, usually at a distance of about 40 feet. I then put one bin up to each eye (as I said before, if at least one is not a roof-prism model, it is very hard to do). With some fiddling around, it is possible to overcome your dominant eye's tendency to see only its image, and you can see both images and place them next to each other, or to superimpose them. I can usually compare a pair of binoculars this way within one minute, and as I said, I have used it more often to compare relative brightness than magnification.

I tried Jonathan's technique with a Zeiss 8X42 FL, Swarovski 8.5X 42 EL and Nikon 8-16X40 zoom. I found it was easy to see the difference between 8X and 8.5X. I was also able to set the magnification of the zoom between 8X and 8.5X and clearly see that its image of a small target was larger than the Zeiss but smaller than the Swarovski. This result indicates that an accuracy within 3% is possible using this technique. That's better than I would have guessed.

While the size differences were pretty obvious with the target object images side by side I still found it surprisingly easy to merge the 8X and 8.5X images into a convincing binocular image, at least for a few seconds. I suppose the next step is to use Kimmo's method to measure what these three magnifications really are, at least relative to each other.

Jonathan B. said:

Hi, elk

This is not an illusion, and I have just made the comparisons again between 8x20 BCA, 8x SE, and 7x Ultravid. I still concede that a small difference in binoculars said to have the same magnification might be hard to detect, but differences of a half-power are obvious.

The first time I did this was about five years ago, when I had acquired 8x20 BCA and 8x32 SE. The 8x20 gave the illusion that it magnified more (I think it is because of its narrow field of view--purely an optical illusion). The comparison showed that the two have precisely the same magnification. 7x Ultravid provides an image obviously smaller than 8x SE. In other comparisons that I can't replicate today (because I don't own the other bins), 7x FL appeared to provide identical magnification to 7x Ultravid, and 7x Ultravid identical to 7x Zeiss ClassiC; 8.5 EL provided a larger image than 8x SE; 8x LX provided identical magnification as 8x SE.

This is easy to replicate; virtually every contributor to the forum should be able to do it. So could a few of you try it and back me up?.

In a fixed position (i.e. sitting somewhere) I prefocus both bins on the same subject, usually at a distance of about 40 feet. I then put one bin up to each eye (as I said before, if at least one is not a roof-prism model, it is very hard to do). With some fiddling around, it is possible to overcome your dominant eye's tendency to see only its image, and you can see both images and place them next to each other, or to superimpose them. I can usually compare a pair of binoculars this way within one minute, and as I said, I have used it more often to compare relative brightness than magnification.

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Jonathan,

As you and I both know there are a few fundamentals at work:

1. Magnification is magnification and 8X is 8X is 8X. Feather details delivered by an 8X porro are the same as those delivered by an 8X roof (assuming identical optical performance levels). If you alternate an 8X porro and an 8X roof you'll clearly perceive a size difference. However, the level of detail delivered as useful information will be the same! 8X is 8X.

2. Our 7X Ultravid's and 8X SE's are a wonderful example of the porro versus roof illusion. It often appears that the 7X Ultravid image is equal to the SE image when, in fact, it's not. Unique lighting conditions aside, I can say unequivocally that the 8X SE always delivers more detailed information at every distance, due to greater magnification.

I think we mistakenly associate the perceived size of an object with information. If an object looks larger in a roof, even if it’s an illusion, many assume more information is available for processing. In fact, it’s not. The roof image may be more dramatic than the porro, but it’s still the product of simple magnification.

Finally, the following may be interesting food for thought.
http://psylux.psych.tu-dresden.de/i...ww.illusionworks.com/html/size_constancy.html

John

Using Kimmo's technique I measured the relative magnification of 6 "8X" binoculars (Nikon 8-16X40 zoom, Nikon 8X32 SE, B&L 8X24 Discoverer, CZJ 8X50 Octarem, Zeiss/West 8X30B porro, and Zeiss 8X42 FL) and the Swarovski 8.5X42 EL. I found all the 8X binoculars measured within a tight 2% cluster except for the Zeiss 8X42 FL which measured 3% less than the average magnification of the others. If I assume that average is a "true" 8X then the true magnification of the FL is 7.75-7.8X. Using the same assumption the magnification of the Swarovski EL measured about 8.4X.

I did two sets of measurements; one at 25' and one at 100' to see if distance had any effect. There was no significant difference in relative magnifications at the two distances. My telescope is 5X and has about a 40 degree apparent field, so I was sampling about 1 degree of real field in each binocular.

John Traynor said:

Jonathan,

As you and I both know there are a few fundamentals at work:

1. Magnification is magnification and 8X is 8X is 8X. Feather details delivered by an 8X porro are the same as those delivered by an 8X roof (assuming identical optical performance levels). If you alternate an 8X porro and an 8X roof you'll clearly perceive a size difference. However, the level of detail delivered as useful information will be the same! 8X is 8X.

2. Our 7X Ultravid's and 8X SE's are a wonderful example of the porro versus roof illusion. It often appears that the 7X Ultravid image is equal to the SE image when, in fact, it's not. Unique lighting conditions aside, I can say unequivocally that the 8X SE always delivers more detailed information at every distance, due to greater magnification.

I think we mistakenly associate the perceived size of an object with information. If an object looks larger in a roof, even if it’s an illusion, many assume more information is available for processing. In fact, it’s not. The roof image may be more dramatic than the porro, but it’s still the product of simple magnification.John

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John

Agreed that 8x is 8x is 8x, unless of course it varies slightly as Henry showed in his last post. But differences that small might be very difficult to detect while comparing the way I describe, so I will compare more bins as opportunities arise.

I think there are two factors at work in the 8x20 BCA giving the illusion of having greater magnification than the 8x32 SE. There is the illusion that Henry has described a few times on the forum, in which the image in any roof prism bin will appear larger than it is. Henry says he overcomes the illusion by closing one eye, and that does work.

However I also think the confinement of the subject within the narrower field of view gives an added illusion of greater magnification. That illusion is often used in graphic design: two identical subjects of identical size confined within different size boxes or rules will appear to be of different size.

To me the most surprising thing about comparing the 7x roof prism against the 8x porro prism is that the subject in the 7x roof appears as large as that in the 8x porro, even though the field of view in the 7x is greater. So the illusion that Henry has described is pronounced. But it is nonetheless an illusion.

Elk,

Not having a background in behavioral science, I had trouble understanding what your problem was with Henry adding a reverse porro to a setup which already had a standard-type porro and a roof. To my mind, it seems like the eminently sensible thing to do.

Concerning comments on apparent differences in stereo perception between roof-prism binoculars, it might be good to point out that different roof-prism configurations yield differences in objective spacing of as much as 10% for the same inter-ocular setting. This produces a clearly visible difference in depth perception. Swaro EL's will have objective spacing greater than i-o spacing, while Nikon HG (L)'s have it reverse.

Henry,

I have not measured very many binoculars for relative magnification, but I did that for Zeiss FL's and the Nikon SE 10x42. Here, the 7x, 8x and 10x Zeiss's were consistent with one another to an accuracy much better than I would dare to assume for the measuring method, but the Nikon had 2.3% higher magnification than the Zeiss 10x. With scopes, I have only measured Swaro ATS 80 HD and Zeiss Diascope 85 this way, the result being that 60x for Swaro would be 61.3x for Zeiss.

One more method for measuring magnification, this time absolute, would be to measure accurately both the entrance pupil (objective aperture) diameter and the exit pupil diameter. Measuring the former precisely is complicated by black locking rings, O-ring seals etc., which may make it difficult to determine precisely what the clear aperture diameter is, but it should be possible within a margin of error of a millimeter or so. For measuring the exit pupil, you would have to project it on something like a camera viewfinder screen or a piece of wax paper or such mounted in a slide-frame, and measure the diameter of the projected circle with precision vernier calipers or a micrometer. However, my guess is that it would be rather hard to do the measurement well enough to get the margin of error under the 3% which Henry and I have found in the relative measurements. In these relative measurements, the accuracy can be very high, since it is easy to keep the distance from objective lens to the target constant to within some parts per thousand, and likewise for the width of the booster visible field.

Kimmo

Jonathan B. said:

Hi, elk

This is not an illusion, and I have just made the comparisons again between 8x20 BCA, 8x SE, and 7x Ultravid. I still concede that a small difference in binoculars said to have the same magnification might be hard to detect, but differences of a half-power are obvious.

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Johnathan,

Sorry, I may have been too curt. I certainly will try this method again. What I meant to say was that just because one doesn't perceive a difference doesn't mean there isn't one. It depends on how small the difference is. How much of a difference do you believe this method will reliably detect?

Responding to Henry along related lines, I find it curious to use a zoom to calibrate real image size as well as perceived image size. I would think the first purpose might be impacted by the second, and vise versa.

-elk

elkcub said:

Johnathan,

Sorry, I may have been too curt. I certainly will try this method again. What I meant to say was that just because one doesn't perceive a difference doesn't mean there isn't one. It depends on how small the difference is. How much of a difference do you believe this method will reliably detect?

Responding to Henry along related lines, I find it curious to use a zoom to calibrate real image size as well as perceived image size. I would think the first purpose might be impacted by the second, and vise versa.

-elk

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That's okay, I didn't read your comment as abrupt. I don't know how fine a difference one could detect by comparing the way I have, because as you point out, it is hard to hold the separate images steady or to superimpose them, and a difference cannot be measured. Kimmo's method is more absolute and more easily replicated.

Without wanting to sound like a broken record, my comparisons have been very helpful for comparing brightness. In this regard I think comparing two binoculars literally against one another is less subjective than looking through them one at a time, when various other illusions might come into play, caused for example by color cast or contrast. This was how I concluded that 8x32 SE is as bright or nearly as bright as 8.5 EL and 8x42 Ultravid, and that 8x42 FL is no brighter than 8x42 LXL. I have gotten into the habit of making these comparisons whenever possible, and sometimes the result is surprising.

elkcub said:

Responding to Henry along related lines, I find it curious to use a zoom to calibrate real image size as well as perceived image size. I would think the first purpose might be impacted by the second, and vise versa.

-elk

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elk,

Can't say I blame you for your suspisions about the zoom, but I think it has turned out to be a useful tool. It appears to have a pretty accurate 8X, matching the 8X32 SE and a Zeiss 8X30 porro precisely. It also has a uniquely narrow field of 5.2 degrees at 8X for monocular tests of the effect of FOV and a very wide 132mm objective spacing for binocular testing.

As for my efforts to use it to "measure" the illusion of size differences from parallax, I know those are fairly gross approximations but they are reasonably repeatable and I think give a fair ball park estimate of the very large (30-40%) difference in apparent image size between very narrow and very wide spaced objectives at short distances. I would be very surprised if your 7X26 Customs don't produce a larger apparent image than your 8X30 Nikons at close focus, even if you use the washers on the Nikons.

Henry

kabsetz said:

Elk,

Not having a background in behavioral science, I had trouble understanding what your problem was with Henry adding a reverse porro to a setup which already had a standard-type porro and a roof. To my mind, it seems like the eminently sensible thing to do.

Concerning comments on apparent differences in stereo perception between roof-prism binoculars, it might be good to point out that different roof-prism configurations yield differences in objective spacing of as much as 10% for the same inter-ocular setting. This produces a clearly visible difference in depth perception. Swaro EL's will have objective spacing greater than i-o spacing, while Nikon HG (L)'s have it reverse.

...

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Kimmo et al,

The experiment mentioned in the first post was an incomplete 2x2 design. The main effects were binocular offset and field of view. The levels for offset were determined by the roof or porro construction. The levels for FOV were 7.7 deg. and 8.5 deg. The treatment not represented, making the design incomplete, was a porro of the same offset as the first with a 7.7 deg. field. (I'm looking for a suitable porro to fill this gap without going broke.) Looked at this way, the design becomes a three-way 2x2x2 experiment when the dichotomy of near vs. far viewing (i.e., distance "factor") is introduced, and would become a 3x2x2 experiment if the offset factor were expanded to include a reverse porro level.

Within this experimental framework it would be very nice to have two reverse-porros with identical magnification = 8x, and fields to match the 7.7 and 8.5 deg. fields of the other binoculars. Otherwise, the observations taken with a reverse porro are outside the design framework. Henry's observations are very interesting, but hard to reconcile as mentioned in my earlier post.

An aspect not discussed is dependent variable measurement, i.e., the measurement of size and depth perceptions. In my case I simply used pairwise treatment comparisons, leading to ordinal measures. Henry, on the other hand, is using a zoom binocular to obtain interval measures, which are potentially more valuable because in theory one could calculate means and correlations.

Hope this explains what I meant to say.
-elk

A Way to Measure Magnification

It's been of some concern that my subjective observations of size and depth may have been influenced by real differences in magnification between the three binoculars. In particular, Henry suggested that if the Swaro's images were actually bigger it would account for some of the observations (but not all of them).

Having limited resources, I adapted a method discussed by Dr. Henry Paul in his 1964 book Binoculars and All-Purpose Telescopes: How to choose, test, and use them. It worked just fine and is summarized in the next paragraph. All three binoculars were found to be credible 8x magnifiers within the bounds of the method, and I'm satisfied they were sufficiently matched to conduct perception trials. Based on the results and many insightful posts on this thread, my current conclusion is that both FOV and parallax necessarily contribute to a complex, multi-dimensional perceptual response involving object size, 3-space, and distance.

For those interested, Dr. Paul's method measures absolute magnification, and can be set up in various ways with a measurement error of perhaps 1-2%, depending on scaling factors and one's attention to detail. In my case I had a piece of white mat board left over from framing my wife's 50th reunion picture that measures 10"x20". I also cut out a smaller board 1/8th the size in each dimension, i.e., 1.25"x2.5." The two rectangles were placed 20 ft. away, and separated by about 6 in." (smaller rectangle mid-way beside the larger). The binoculars were tripod mounted with a fine control of vertial and horizontal adjustment. When the two rectangles are viewed with one eye looking through the binoculars and the other unaided, an image of the magnified smaller rectangle can be compared with the image of the larger one seen with the naked eye. By adjusting the tripod controls one image can be superimposed on the other or offset slightly to judge size differences. It's pretty neat. Using this method the three binoculars showed no discernible difference from 8x magnification.

Thanks to everyone for a great discussion.

Elkcub