Apparent magnification

[Leif]

Firstly apologies for the high anorak rating of this post, but it concerns an issue that has been puzzling me for a few years.

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. I thought that I was going a bit potty, but others have said the same. 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! This indicates that the illusion is nothing to do with the nature of the prisms. Oddly when considering one glass in isolation, the image through one optical assembly seemed bigger than the image seen through both!

I suspect that this is a psychological illusion created by the brain when presented with depth information. Porro-prism glasses usually have a much greater seperation between the two objectives, and hence a greater 3-D feel to the image. It looks to me as if the greater 3-D feel has the side effect of tricking the brain into 'seeing' a smaller image.

Anyway, if this idea is true, then a corollary is that the image through an inverted porro prism should seem smaller than that through a roof prism binocular, assuming for the sake of argument the same field of view and DOF.

[Art Thorn]

Did you try that with both eyes in turn Lief? Maybe your right eye just sees things differently than your left ;-)

[Art Thorn]

But seriously, I noticed something interesting also, which I can't verify at the moment because it is quite dark outside now. I found that porros present a smaller image close up, but with a distant object the size seems the same as the roofs. That would reinforce Lief's hypothesis, as the lens spacing would have less significance with distant objects. I'll have to check again in the morning.

[Leif]

But seriously, I noticed something interesting also, which I can't verify at the moment because it is quite dark outside now. I found that porros present a smaller image close up, but with a distant object the size seems the same as the roofs. That would reinforce Lief's hypothesis, as the lens spacing would have less significance with distant objects. I'll have to check again in the morning.

Hi Art: I am glad that I am not alone in seeing this illusion. When I mention it most people think I am a bit potty. I heard one other person mention it some time back. I agree that the effect should reduce with distance, though I did not chek this.

BTW A bit off topic, but how does your Nikon 10x42 SE compare to your 8x32 SE in terms of brightness esp. in low light? (I ask more from curiosity than anything else.)

[Art Thorn]

I have spent some time looking through all three pairs as it gets quite dark. I think I am being objective about this as I have no reason to 'want' one pair to be any better than another. I have found that as total darkness approaches (meaning I can't really see anything much without binoculars) I find a slight advantage in the 10x42 SEs compared to the 10x32 HGs. I really can find no difference between the 8x32 SEand 10x42 SE. By the way, I just tried that distance/size thing, and it is true. I look at a distant object (300 ft away) through the porros and the roofs (both 10 power) and I see the same magnification. I look at an object 15 feet away and the porros have a smaller image ( in my brain). I do know that our brains do a great deal to correct the images that our imperfect eyes see, so why not this phenomenon?

[iporali]

Very interesting discussion indeed! "High anorak rating..." LOL By the way, "an anorak" in Finnish means an old-fashioned skiing jacket (with hood).

I was already searching the explanation from the focusing systems (external vs. internal). Both porros and roofs have an objective and an eyepiece with constant focal lengts. With roofs the moving element is objective whereas with porros the focus is achieved by moving the eyepiece (ocular). If the binos are held against a window or another stable support, close focus moves the front lens of roofs closer to the object (magnifies more), whereas porros push the head away from the object. So here is an explanation that fits the observations...

But seriously, I have to agree that it must be just an optical illusion - or maybe it just looks like one :) (according to Alan Rymer).
Eg. the size of the moon looks so unbelievably bigger when it is close to horizon vs higher above.

Ilkka

[alan_rymer]

Very interesting discussion indeed! "High anorak rating..." LOL By the way, "an anorak" in Finnish means an old-fashioned skiing jacket (with hood).IlkkaAnorak= In English has same meaning as Finnish.
But the person who wears an Anorack in England is likely to be strange, compulsive, weird. Like a Birder or Trainspotter!.

[iporali]

Thanks Alan for the clarification - that helped :t:

Ilkka

[gorank]

found this info:
"Quality roof prism binoculars /../. They also produce an apparently larger image."

http://www.birdwatchersdigest.com/optics/optics8.html

you are not alone Leif...