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Our eyes are spherical, our retina is circular, but still our eyes can see more in the horizontal direction than in vertical direction. Why is it so, why is the preferred aspect ratio not square?

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This is just a hypothesis: but there is probably not one "preferred" aspect ratio, even for one individual. It probably has to do with the following points: 1. the eyes are positioned on a horizontal line; 2. the width between the irises; 3. how far you are looking. And let's not call it a "preferred" aspect ratio. It's more like an "effective" aspect ratio. My hypothesis is that the closer you look, the wider the effective aspect ratio is, and the further you look, the more circular it is. And if you cover one of your eyes, you'll probably get a circular FoV regardless of distance. –  Kal Nov 5 '12 at 9:28
    
I have to agree with Kal, but mainly about #1. Essentially, the eyes are next to each other, and this allows for stereoscopic vision. If you cover one eye, your field of view is more or less circular (due to the retina and macula), but you'll lose depth perception. Plus, your eyelids slightly limit the amount you can see in the up-down directions. –  jello Nov 5 '12 at 11:19
    
@jello Surely the eyelids can't be having an effect as you describe because they don't normally cover any part of the pupils. –  Alan Boyd Nov 5 '12 at 19:08
    
@AlanBoyd I think you're right. For some reason, I was probably thinking about ptosis, when a drooping eyelid does limit your visual field. –  jello Nov 7 '12 at 8:35
    
I will add that in other animals, having a wide horizontal angle helps in hunting and also in defending against hunters. The vertical angle is also important, for example for hunting birds, but not so much for terrestrial animals. –  149781-32509185 Nov 9 '12 at 13:45
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up vote 7 down vote accepted

I found the explanation given below here (my emphasis). It may not be authoritative, but it makes sense to me.

You can determine the approximate shape of your visual field by looking straight ahead, and holding a hand out to each side and noticing where you can no longer see them. Do the same for above and below, and you'll see that your peripheral vision probably extends to about 180 degrees (or a little less) left and right, and about 100 degrees up and down. Your brow, nose, and cheeks narrow the vertical angle.

So your visual field has a ratio of about 1 to 1.8.

Interestingly, and not entirely coincidentally, ordinary films and wide-screen TVs have a ratio of 1.78 (16:9).

Another way to see this effect is to look at someone's head in profile and from directly above. In the former case you have a clear view of their eyes, in the latter you don't. (Tested on a small sample - one 3 year old male, playing with an iPad).

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The reasons your field of view is not circular are:

  • Even when looking directly forward your nose, brows, and cheeks obstruct your view.

  • You have two eyes which are aligned horizontally.

The field of view is measured by a method called perimetry, and looks approximately as depicted in this graphic. The guide here provides further comments on the graphic.

As you can see in the image I linked to, the field of view (were it to be approximated with a rectangle) spans 180° horizontally and 130° vertically. Yielding a width-to-height ratio of 0.72. Which is better approximated by the traditional 4:3 screen size and NOT by modern 16:9 displays. Having said that, 16:9 may not necessarily be a worse format, as researchers found that horizontal saccades are faster than vertical ones.

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