Porro and roof object size (1 Viewer)

This is an effort to pick up an interesting thread which was lost when the site went down recently. Someone posted the observation that porro and roof binoculars of the same magnification deliver the rather puzzling effect of presenting the image (object really) different, i.e. in roof binoculars the same object appears a bit larger than in porro's. Of course the question was asked why and happily someone came up with an explanation. But alas, I dont remember what it was exactly. I would be grateful if the discussion could be revived, as I wondered about it since I swapped my Swift 8.5x44 Audubon porro for a Swarovski 8.5x42 EL roof and was struck by the experience that the Swaro presented the objects not only sharper and brighter but also larger. At the time I saw this as a very special quality, and while I am not so sure about that any more (in the meantime I learned that the size of the object, or magnification, is less important than the sharpness of the image) the issue could be of some significance. For instance, it could explain the popularity of roofs, and the sucess of marketing strategies, from an unexpected angle: roofs are optically (!) simply more spectacular than porro's.

Renze de Vries

It may be caused by the difference in field of view. The Swift's have a 430' field, while the ELs have a 390' field. Obviously, then the eyepieces in the Swifts have a larger apparent field. Objects in the ELs will take up more of the field of view although the magnification is the same.

I believe differences in apparent fields also lead to differences in perceived brightness.

Enjoy the ELs!

Clear skies, Alan

Renze de Vries said:

This is an effort to pick up an interesting thread which was lost when the site went down recently. Someone posted the observation that porro and roof binoculars of the same magnification deliver the rather puzzling effect of presenting the image (object really) different, i.e. in roof binoculars the same object appears a bit larger than in porro's. Of course the question was asked why and happily someone came up with an explanation. But alas, I dont remember what it was exactly. I would be grateful if the discussion could be revived, as I wondered about it since I swapped my Swift 8.5x44 Audubon porro for a Swarovski 8.5x42 EL roof and was struck by the experience that the Swaro presented the objects not only sharper and brighter but also larger. At the time I saw this as a very special quality, and while I am not so sure about that any more (in the meantime I learned that the size of the object, or magnification, is less important than the sharpness of the image) the issue could be of some significance. For instance, it could explain the popularity of roofs, and the sucess of marketing strategies, from an unexpected angle: roofs are optically (!) simply more spectacular than porro's.

Renze de Vries

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Renze,

I believe Henry Link, Ilkka, and others have commented on this illusion. Assuming that the optical magnifications are actually identical, the illusion is said to be broken by closing one eye and looking monocularly. Give it a try. It doesn't work completely for me.

There are several factors influencing image size perception, which fall in the area of "size constancy." In general, known objects tend to appear of a constant size in terms of their perceived distance, and in relation to other known objects. The porro provides a hyperstereo 3-D view due to their objectives being further apart than the observer's eyes. Hence, object size is perceived differently to compensate.

Unfortunately, I'm muddled on their geometric logic here, so Henry/Ilkka needs to chime in. Drat! The greater than normal retinal image offset should provide distance cues that make a known object appear closer, so by that logic it might be perceived as smaller to compensate.

Anyway, I'm sure there is more to apparent size perception, since it is well known from the laboratory that size constancy is also influenced by window effects, which translates to the FOV of the binoculars. That effect may be more subtle but operate at the same time.

Hope this isn't too confusing. It is an interesting subject.

Elkcub
PS. The issue of whether Porros or roofs are more "spectacular" is debatable. Roofs are often perceived as "flatter", i.e., less depth, less sense of space.

PPS. Update. A good discussion of the "illusion" is on the thread "Newbie with 8x32 HG and SE"

I would say that the reason roofs are so popular is that they are waterproof, smaller, rugged and more ergonomic than porros. You saw a huge difference in your Swifts and your Swaros because the Swaros are just that much better. Compare Swaro porros and Swaro roofs and things will be pretty close or they might even be a little in favor of the porros. It is actually much easier and cheaper to get a superior view from porro than it is from roofs. Thus if everything is equal between the two designs, the porros shoul actually come out on top in performance.

ranburr

ranburr said:

I would say that the reason roofs are so popular is that they are waterproof, smaller, rugged and more ergonomic than porros. You saw a huge difference in your Swifts and your Swaros because the Swaros are just that much better. Compare Swaro porros and Swaro roofs and things will be pretty close or they might even be a little in favor of the porros. It is actually much easier and cheaper to get a superior view from porro than it is from roofs. Thus if everything is equal between the two designs, the porros shoul actually come out on top in performance.

ranburr

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If you read BVD, you'll see the Swift was rated as the "Poor man's Swarovski" because of it's superior optical qualities. Roofs, initially, produce an illusion of grandeur that quickly disappears when critically compared to a high quality porro. The Swift is optically very, very good and only suffers from edge softness and too little eye relief. I think it's unquestionably the best optical value in its price range and much better than many more expensive roofs.

If Swift would extend the eye relief a few more millimeters to make it eyeglass friendly, I think they would sell many more of them to older birders who want high quality at reasonable expense.

John

John Traynor said:

If you read BVD, you'll see the Swift was rated as the "Poor man's Swarovski" because of it's superior optical qualities. Roofs, initially, produce an illusion of grandeur that quickly disappears when critically compared to a high quality porro. The Swift is optically very, very good and only suffers from edge softness and too little eye relief. I think it's unquestionably the best optical value in its price range and much better than many more expensive roofs.

If Swift would extend the eye relief a few more millimeters to make it eyeglass friendly, I think they would sell many more of them to older birders who want high quality at reasonable expense.

John

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John,

I agree. They are an excellent value for the money, and it is unfortunate that they don't have a little bit more eye relief. More manufacturers seem to be improving eye relief, and perhaps Swift will go there too (Leica could do better too).

Clear skies, Alan

Hope everyone saw my modification to post #3. Henry Link or Ilkka needs to comment on the Porro/Roof apparent size hypothesis, or correct my interpretation of it.

-elk

Just because BVD rates the Swift as the poor mans Swaro, doesn't make it so. BVD is a good read and I wish it was still being updated. But, it is not the Holy Grail. There are some obvious biases, just like in any other evaluation. The only optics evaluations that I take with more than a grain of salt are those performed by "Gun Test" magazine. Not only do they not take any advertising $$$, they walk into a store and purchase the test optics just as you and I would. Unfortuneately they do not do enough optics testing, but what they do test is pretty informative. And yes, they also have some personal biases, but at least I know that there is no corporate influences. More than one company has tried to shut them down after being given a poor review.

ranburr

ranburr said:

... The only optics evaluations that I take with more than a grain of salt are those performed by "Gun Test" magazine. Not only do they not take any advertising $$$, they walk into a store and purchase the test optics just as you and I would. Unfortuneately they do not do enough optics testing, but what they do test is pretty informative. And yes, they also have some personal biases, but at least I know that there is no corporate influences. More than one company has tried to shut them down after being given a poor review.

ranburr

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Very interesting. I live in California and anything having to do with guns or knives is not PC. Where does one write to subscribe to "Gun Test?" Hope they mail under a plain brown cover, — better with flowers on it.

-elkcub

You can subscribe for a printed or online version. Website is www.gun-tests.com You can even request that they do a specific test. If there is enough subscriber interest, they will do most any test.

ranburr

I get Gun Tests also. Don't have a gun yet, but love to read the articles. I agree with ranburr about the veracity of their findings. They test the hell out of their firearms. Take NO advertising.

http://www.gun-tests.com/

I'm not sure they would be a good source for optics reviews though . They occasionally look at some rifle scopes, maybe once at spotters for viewing targets.

ranburr said:

You can subscribe for a printed or online version. Website is www.gun-tests.com You can even request that they do a specific test. If there is enough subscriber interest, they will do most any test.

ranburr

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Do you recall noteworthy evaluations that could be requested from specific back issues?

-elk

Renze,

I found it. A surviving discussion of the "illusion" is on the thread "Newbie with 8x32 HG and SE"
http://www.birdforum.net/showthread.php?t=27444&highlight=porro+roof

Henry Link (Post #2) explained it in terms of the roof image appearing larger, but it seems just as logical to me that the Porro image would seem smaller due to size constancy distance compensation mentioned above for hyperstereo viewing. Roof binoculars afford normal eye spacing while the Porros are usually wider. One could say the brain is "fooled" when presented with conflicting cues, but it's really quite clever; it develops perceptions to satisfy the cues as rationally as it can — sometimes with amazing results. Sometimes, not being able to find a unique solution, it even alternates between two perceptions — as with ambiguous figures.

Further evidence of the size constancy hypothesis is that reverse Porros (hypostereo), like the Bushnell 7x26, actually appear relatively large for a 7x image; I would guess because of the opposite size constancy compensation. In this case, one would have to be further away to get the same retinal disparity, so the percepual response is to increase the apparent size to make the distance cues more compatible.

Elkcub

See here:

http://www.birdforum.net/showthread.php?t=13021

I noticed the effect but could not find an explanation. I assumed that it was an illusion. Postings from others seem to support that idea. The effect is well known although it does not seem to be often referred to in books and magazines if at all.

My guess is that the brain uses perspective to estimate how big an object really is. Thus increasing the objective separation will both increase perspective, and fool the brain into overestimating the actual size of an object.

It is also possible to view with one optical assembly only and the effect disappears consistent with an illusion associated with perspective.

Regarding the suggestion that it is due to differences in FOV, I tested this by comparing Porro and Schmidt Pechan binoculars with similar fields of view, and one Porro with a much larger FOV, and found that the effect was independent of FOV. The bins used were Zeiss 8x30 BGAT, Nikon 8x32 SE and Nikon 8x40 Egret. The Zeiss and Nikon SE have almost the same FOV, the Egret a much larger one.

Leif

elkcub,

I just noticed your request for information about this. It appears that you and Leif have it pretty well covered now. I agree that it's just as logical to call the porro image smaller. Trying to figure out which one is "correct" would be as futile as trying to decide if people with 55mm IPD's form a "larger" image of close objects in their brains than those with 70mm IPD's. Like Leif, I have tried comparisons with binoculars of different apparent fields and been unable to see any significant effect on perceived magnification from that. I posted my most recent experience with this illusion on the "8X32 FL vs 8X42 FL" thread. In that case I wasn't expecting it and only realized the objective spacing between the two bins was a little different after I had noticed it. That was also a case where the binocular with the wider apparent field actually produced the higher perceived magnification, and yes the illusion disappeared with distance or from closing one eye.

Henry

ranburr said:

Just because BVD rates the Swift as the poor mans Swaro, doesn't make it so. BVD is a good read and I wish it was still being updated. But, it is not the Holy Grail. There are some obvious biases, just like in any other evaluation. The only optics evaluations that I take with more than a grain of salt are those performed by "Gun Test" magazine. Not only do they not take any advertising $$$, they walk into a store and purchase the test optics just as you and I would. Unfortuneately they do not do enough optics testing, but what they do test is pretty informative. And yes, they also have some personal biases, but at least I know that there is no corporate influences. More than one company has tried to shut them down after being given a poor review.

ranburr

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I've personally tested several Swift Audubon's and they are an optically excellent binocular. The public criticisms of this bin relate to: true waterproofing and fog proofing, eye relief, and overall build quality versus the earlier models. The optics have never been in doubt: they are superb!

Published "evaluations" are merely a starting point for personal evaluation and nothing more. Why people get so agitated about them is becoming obvious:

1. Many owners seek affirmation that they bought the right binocular. I guess it's not enough to own and enjoy a first-class bin. Negative criticisms are often explained away by claiming "sample variations" that never apply to the bin they own and use.

2. Prospective owners are looking for definitive advice and "expert" evaluations are often inconclusive. There are no clear winners and losers when it comes to what people like in optics! Purchasing via the Internet further complicates the decision-making process, resulting in additional frustration for new buyers.

3. Owner loyalties, badge envy, and all the other silliness people can dream up rise up in full force every time one of these reviews is mentioned.


Personally, I don't care what anyone thinks of my gear, how I'm dressed, or what bird guide I have in my hand. My life list is in my head, I sometimes forget the names of birds I see every day, and I enjoy watching Turkey Vultures swoop and soar as much as I do Bald Eagles. I've seen $50 bins that impressed me and $1000 bins that bored me. One "expert" told me that lower priced optics are tuned to look great in the store under fluorescent lighting and that the Nikon SE is just another "porro". I believed the first half of the remark and laughed at the second.

The two best birding porro prism bins I've looked at are the 8X32 SE and the Swift Audubon 8.5X44. If someone knows of a better porro prism binocular, suitable for birding, I'm all ears. I've come to the conclusion that I strongly prefer the porro view and I think it's related to my relatively narrow 57 mm IPD measurement. My preference for porros is only true in the 10-100 foot range. Past that point the views are nearly identical. My narrow IPD may also explain why I have no trouble seeing a perfect image at the 10-foot minimum of my SE. I’ve yet to find a roof that is as comfortable on my eyes at that range.

The bottom line is that no review provides a comprehensive list of the “best” bins available to birders. Omitting the Nikon SE is a prime example. I don’t care if Nikon didn’t provide a sample for evaluation; it should have been included as a sterling example of optical prowess, albeit with environmental limitations. The fact that birding “professionals” published a review of binoculars without mentioning the SE is, IMO, a sin of omission. The Swift falls into the same category.

Forget the reviews, listen to the comments of BF members, and always try before you buy!!

John

henry link said:

elkcub,

... I agree that it's just as logical to call the porro image smaller. Trying to figure out which one is "correct" would be as futile as trying to decide if people with 55mm IPD's form a "larger" image of close objects in their brains than those with 70mm IPD's.

Henry

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Henry,

After rereading your post I must respectfully disagree. Unless there is an alternative perceptual mechanism being proposed, the hypothesis of binocular size constancy does a substantially better job than just saying the brain is fooled and letting it go at that. Constancy phenomena have been studied since the 1800s, or earlier, and there is a large body of literature to form a foundation. Since the perceived relative size betwen Porro and roof is the question, it is clearly not equivalent to the problem of comparing absolute perceptions between individulals, which admittedly is "futile." In all candor I'm making this point because I sense a dismissive attitude. In my opinion I simply put some meat on the bones of your otherwise well-founded observations.

Regards,
Elkcub

John Traynor said:

I've personally tested several Swift Audubon's and they are an optically excellent binocular. The public criticisms of this bin relate to: true waterproofing and fog proofing, eye relief, and overall build quality versus the earlier models. The optics have never been in doubt: they are superb!

[snip]

The two best birding porro prism bins I've looked at are the 8X32 SE and the Swift Audubon 8.5X44. If someone knows of a better porro prism binocular, suitable for birding, I'm all ears.

[snip]

John

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John: The Swift Audubon 8.5x44 has notable off-axis sharpness, so people who are more picky than you would not like it, though you are obviously not bothered by less than perfect edge to edge sharpness.

Leif

BTW The Swift Kestrel 10x50 is also said to be first rate with a huge field. I am tempted to buy one for the night sky, but they are not sold in the UK.

Leif said:

John: The Swift Audubon 8.5x44 has notable off-axis sharpness, so people who are more picky than you would not like it, though you are obviously not bothered by less than perfect edge to edge sharpness.

Leif

BTW The Swift Kestrel 10x50 is also said to be first rate with a huge field. I am tempted to buy one for the night sky, but they are not sold in the UK.

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Leif,

I agree that the very edges are marginally useful. However, the 430' FOV of the Swift Audubon is so large for an 8X that the sweet spot of this bin is more than adequate.

A good review is at:
http://www.betterviewdesired.com/audubon/audubon.html

An older review is at:
http://www.betterviewdesired.com/SwiftED.html
and this one contains commentary on the ED question. Ten years ago Steve said ED glass made a difference!

This bin shows what can be done for less than $300. If Swift would only extend the eye relief and make the thing truly rugged it might be more popular among older birders. As it is, I've yet to see one of the newer versions in the field.

John

I'll probably get flamed, but the 8.5x44 is one big ole nasty porroprism. Sure, it may be fine optically, but the handling (for me anyway) is a complete nightmare compared to other bins available today.

I do think it has the best cover material though.

I know of one (the newer porro) for sale if somebody is interested.