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Well, that all makes perfect sense now! Thanks for the info, AussieBob -- that explains why it didn't work.
Maybe so, but I still doubt it would have stopped him trying. When you look through one of those screw on lens fittings you see something similar to what you'd see when looking through an anamorphic lens, and it looks like it might work. What you are seeing in reality is an image formed on your retina through TWO lenses: one is the lens in your hand, the other is the lens in your eye. Many people forget that.
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The formula for a "thin lens" (theoretical, or "paraxial" system) infinite focal length projection lens is as follows:
F1/F2 = Desired expansion,
where:
F1 = focal length of the convex or "positive" lens (entry lens in the case of an anamorphic expander)
F2 = focal length of the concave or "negative" lens (exit lens in the case of an anamorphic expander).
F2 - F1 = D = air gap between the lenses.
For example, a 300mm focal length concave lens (as an exit lens), coupled with a 400mm focal length convex lens (as an entry lens) will give an expansion factor of 400/300 = 1.3333x.
The air gap should be 400-300 = 100mm between the lenses.
I stress this is theoretical only (and somewhat simplified to reduce the need for a longer and more complex explanation) but sufficiently correct for the present purpose. It is based on the "thin lens" forula for calculating the resultant focal length of two lenses combined together and separated by an air gap. This general formula is as follows:
Composite focal length =(F1 x F2)/(F1 + F2 + D)
where the focal length of the concave lens, F2, is expressed as a negative number. Ignoring the value of (F1 x F2) at the moment, it can be seen that:
(F1 + F2 + D) = (300) + (-400) +100 = 0,
Any fraction with 0 as denominator = Infinity, hence this combination, at 100mm separation, will have infinite focal length. (Don't worry that you can't divide by 0, that's only a message computers spit out when things get too hard for them... mathematically speaking, any number divided by 0 is infinity).
The formula takes no account at all of lens shape or aberrations, including geometric, astigmatism or color aberrations. It also takes no account of the two lenses being cylindrically curved, rather than spherically curved, which are the source of many of the familiar aberrations present in anamorphic lenses (if not minimized).
If you flip the order of the lenses, and flip the orientation of the lenses themselves then you will have a 0.75x (300/400) squeeze lens. Rotate it 90 degrees and you will have a
vertical squeeze lens.
Iscos and Schneiders follow the basic F1/F2 infinite focal length form, sort of...
The reason why they have focusing adjustments is because a real life anamorphic lens will be somewhat different (although superficially similar) in form to this, and will need adjustments for astigmatism rectification and color correction.
Making a lens that delivers a good image is a VERY big and complex task, but these are some of the basic principles of anamorphics.
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So now you know why a single element lens won't work as an anamorphic projection lens...
A single element lens has to be taken as part of a multi-element lens comprising the projection lens itself, plus the add-on lens, both of them with discrete focal lengths. The only way you can get this type of combination to work anamorphically is to make the add-on lens of infinite focal length and cylindrically curved. That way it has no (theoretical) influence on the focus or operation of the prime projection lens.
In the uncurved direction it is basically two flat sheets of glass and therefore of theoretically infinite focal length. In the curved direction it is of infinite focal length due to the physical arrangement and focal lengths of the two lenses used.