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Originally Posted by
riskpeep 
Mark,
Thank you for your reply, this is helpful. Is the distance between the lenses particularly relevant, or just the alignment between them?
Spacing is absolutely critical and why I now have a massive threaded focus ring on the MK5.
Lets see if this gets me another suspension
. At the distance it is set in this photo (recessed), the image is not in focus at my TR and projection distance. To be in focus, the threaded centre needs to protrude about 12mm extending the depth of the lens to about 140mm.
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I've been doing some additional research on this and what I know so far is that eyeglass blanks are measured in diopters. At the risk of being pedantic, a diopter (
http://en.wikipedia.org/wiki/Dioptre) is a measure of the optical power of a lens and is the reciprocal of the focal length, so a 3 diopter lens would have a focal length of 1/3 of a meter. Lenses can be found in 0.25 increments of diopters.
Eye glass blanks are typically CR39 plastic these days and you should be able to specify a cylindrical cut at a specific diopter. As you see from the first image I attached, you need both a concave and convex lens pair. They can be "round" or "square". I've drawn then "square" because I could not get Sketch Up to allow me to cut the profile round after cutting the basic shape. Something about not being able to shift curved surfaces?
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The interesting bit is that for an eyeglass prescription (and thus for readily available lenses), you have a spherical measure (which is a general magnification), and a cylindrical measure. We only care about cylindrical power, so the appropriate lenses with the correct cylinder rating is what we'd need.
A blank should be a small cylinder - R=40mm H=10~15mm. They cut them to be spherical (both x and y axis) where a cylindrical lens is just one axis.
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My understanding is that for an Anamorphic lens, we want a 1.33x expansion lens which should be achievable with two lenses having the right ratio between their Diopter ratings. I found one person on the web who'd had success with experimenting at an anamorphic stretch using a -6 cylinder lens paired with a +4.5 cylinder lens, but he was using opthalmic test lenses with a diameter of ~1" and thus couldn't pass the whole beam of his projector.
I read that too and where 6/4.5=1.33. There are many other combinations that can give 1.33x
You should be able to use any two lenses with the correct ratio between them, so if I have this right, you could use a -6 & a +4.5 cylinder lens, or a -3 and a +2.25 lens.
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If we can find someone with access to eyeglass lens blanks and a projector, it would seem a simple test to put two lenses in front of a projector to see if we're approaching this problem right. Obviously you'd need to get the orientation right (being cylindrical lenses, they do have a directionality, and between the two lenses the directionality would need to be exactly lined up.
You might have to go to an optometrist and place an order for these lenses. If they can cut the precise multi-aspheric curves of corrective eye wear, I am sure that they can cut the simple single radii you need - especially because it is in just one direction.
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Yeah, I've noticed that about the largest lens blanks you can find are in the 100mm range, most are 70-80mm range.
One of the reasons I started looking at eyeglass lenses, is that outside of those, for high quality optics, most parts are in the 25mm or smaller sizes. The prices get ridiculous if you get up much past that.
Welcome to my world. I still have the very first plano convex lens I bought back in 2006. 25mm, BK7 singlet and cost me just $15USD. I do believe it is the 6D as described in that thread at DIY Audio, but I could not find a 4.5D concave to match for an anamorphic pair.
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I still haven't given up on the idea of using lens blanks, but reading aussie bob's posts have given me pause. I had heard that lens blanks aren't ground to the same precision as your average camera or projector lens, but to have it confirmed pushes me back a step.
Whist his posts are very informative, he tends to do that. I wonder how different thing would have turned out if I actually took his "don't bother" advise back in 2005?
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My current like of research (if you can call google searching research) is to see if I can quantify the parameter we care about (surface quality?, surface deviation?), and then compare various kinds of lens materials to that to see how close I can find. Many firms now tout 'High-Definition' lenses that I think might be market speak for better surface quality or lower surface deviation. I'm still trying to figure out the market speak from the engineer speak (which is what I want).
I have no allusions that I'm going to create a $50 ISCO lens. But I'd like to think that we should be able to outdo your typical prism lens.
Commercial optical surface finish is scratch and dig 60/40. That means the largest scratch on the surface of the glass should be no greater than 60 x 40 microns. There is not even a published spec for those trophies. All I ever got from the manufacture was something about a 4 step polishing process.
There is nothing wrong with SD 60/40 and in fact it is the spec used by at least 2 anamorphic lenses available today. Sure you ask for a better surface finish, but your lens set just cost you triple.
