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Anonymous
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I have often noticed that the view through porro-prism binoculars looks smaller than that through roof-prism binoculars of the same nominal magnification. I have tried several makes of binocular and always seen the same effect. Anyway, today I compared the view through one optical assembly of an 8x porro prism glass with that through one optical assembly of an 8x roof prism glass. Each had almost the same field of view. Well each image appeared to be of the same size i.e. the psychological impression was one of equality of size. Then I tried looking through each instrument normally. Lo and behold the size difference re-appeared!
I suggest that this is a psychological illusion created by the increased separation of the objectives in the porros and the consequent increase in perspective or depth impression. In other words, the greater 3-D effect of the porros has the side effect of tricking the brain into thinking the image is smaller.
Anyway, if this idea is true, then a corollary is that the image through an inverted porro prism will seem smaller than that through a roof prism binocular, assuming for the sake of argument the same field of view and DOF.
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Anonymous
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That's very interesting and it makes sense. How far away do you need to look in order for this effect to become invisible to you?
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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This topic was brought up last year on Astromart.
I think the simple test that proved the phenomena to be illusionary was suggested by Ed Zarenski -- using a tape measure against a wall just beyond close -focus distance.
I had to prove to myself last year that 10x magnifcation REALLY WAS 10x -- because a similar pyschological effect fools my brain into thinking that the moon for example , only appears "about 3 or 4 time BIGGER" through my 10 x 50 Porro prism binocular than it does with naked eye.
This is of course is nothing more or nothing less than an optical illusion.
That said , there are sometimes minor discrepencies between stated and actual magnification , usually of no more than 10% though from what I've read.
I might have mentioned another thing before on this forum , but maybe it was another forum , but when you consider than a 10x binocular makes a square appear to have sides 10 times longer AND 10 times wider than the naked eye image, which it most certainly does, -- then it must make the AREA of a viewed object appear 100 TIMES larger !
So when you consider that I "think" the moon only look about 3 or 4 times bigger -- it must ACTUALLY look 100 times bigger -- now that is some serious shortfall to be caused by an optical illusion isn't it ?
Magnification and it's effect truly fascinate me.
Regards -- Kenny.
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
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