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
[QUOTE=Renze de Vries]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[/QUOTE]
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"