Re: viewers for real eyes

[P3D John Bercovitz <bercov@xxxxxxxxxxx> To: photo-3d@xxxxxxxxxxxxxxx]

Dave Worrell asks:

>What is a _mild negative trial lens_?

A trial lens is one of those little lenses an optometrist holds up in front of your glasses to see if your glasses need a change. If he holds up a +0.25 diopter in front of you and you see better at infinity that without, he knows you've gotten a little more farsighted than before and so he knows your prescription needs a change. The optometrist usually orders the trial lenses in large sets but he can also order them singly. The reason I suggest a trial lens is that they come in weak powers like -0.5 and such weak lenses can be difficult to get elsewhere. Looking in Edmund, I see, well by golly they do have some pretty weak lenses. #94837 is a -0.5 of about the right diameter (not too big I hope) and #32169 is a -1.0 again of about the right diameter. So I lied. 

The red button has about 4 diopters of travel for nearsighted people. So if your glasses prescription is -4.5 diopters, you need at a minimum an additional -0.5 lens. Now you want to place the thing as close to the viewer's lens as you can. If you place it near the transparency, you've put it in "zero power space" and it won't have much effect. As an example, If you look through the bottom of a bottle at the world, the world looks pretty fuzzy and screwed up. But if you put that same bottle bottom on a page of text, you can read the text just fine. So for the present purpose, put the added lens near the existing lens.

John B

PS for the etymology-3d list:

>And speaking of Americanisms, _goodness of the viewing system_? My goodness, that's quite a usage ;-)

Actually, I think that's a very old usage which is coming back. I think it predates America. But of course if it returns first in America, then it is indeed an Americanism temporally speaking. 8-) 


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