P3D Re: VM and Kids, VM interaxial/interocular

[abram klooswyk <abram.klooswyk@xxxxxx>]

>From P3D Digests 2879, 2880:

Bruce Springsteen wrote:
>Does anyone have a guess as to the usable diameter of lenses 
>in the custom VM viewers currently offered, such as the one 
>from Hugo de Wijs?

Paul Talbot wrote:
>What do you consider a "usable" lens diameter.

Grant C. Campos wrote:
>I'm wondering if people are making the mistake of thinking that if
>the viewers lenses are each 15mm wide, and they are 65mm apart, 
>then they >would accomodate someone with an interocular of 50mm. 
>(...)
>It would be interesting to see a sketch of the eyeball in front of
>a lens looking at an image, or an equation showing the relationship
>between the size of someones pupil, the distance from the eye to 
>the lens, the focal length of the lens, and the size of the image 
>that can be seen.

-------- 
I must admit that I was intentionally vage in speaking of "usable
diameter (...) not including the edges". As Grant Campos points out
more in detail, you cannot use the edges of a viewer lens because part 
of the field of view is cut off.
I'm afraid the issue is not simple because it involves so many factors.
First of course the seize of the pictures and the lens focal distance,
which settles the viewing angle. Standard VM viewers have a rather 
small viewing angle, small pictures with relatively long focal distance
of viewer lenses. 
In the latter case the non-usable rim in smaller of course, which led 
me to my optimistic estimate of interpupillaries which can use VM's 
(my guess is not better than anyone's, I did no research). Custom VM 
viewers mostly have a shorter focal distance, which seems better in
viewing but gives a larger non-usable rim. 

The issue is complicated by the fact that you cannot take the simplified
lens scheme for granted, you have to measure focal distances from the
front and back nodal points, especially when lenses (rather: oculars) 
like Hugo de Wijs' ones are considered.

Then indeed eye movements are next, and here eye seize plays a role.
The front nodal point of larger eyes move more mm for a given angle
than that of smaller eyes. This is significant because the eyes are
close to the oculars. Then, people vary in how close they hold the 
viewer to the eyes. Children often wipe the lenses with their eye
lashes, other people have Neandertal stirns or wear glasses. Indeed,
with some viewers nose seize can be important. 
I don't think pupil seize has much influence, compare the diafragm 
setting of a camera, which has no influence on the angle of view.

This issue should be addressed by an optician or optometrist.

Abram Klooswyk


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