Anamorphic convertors - how do they work?

[Greg_E]

Can someone explain to me how the anamorphic convertors work? I'm having a problem understanding why only certain focal length primary lenses sort of work, and why the object distance matters. And then throw in a close focus diopter and see things get suddenly more detailed. I have a "cheap" Century Optics that I'm playing with, and so far none of my lenses look great through it, most don't even work until I hold a +1 diopter in front (don't own any big diameter diopters yet), then my 110mm lens suddenly has a clear(er) image and is just about useable, but only at about 1 meter away from the object.

I'm thinking the the distance between the front and back elements of the convertor have something to do with where it can focus. I see that some of the newer designs have a close and normal focus lever, and I've seen the test done with the Panasonic convertor where the guy cut it in half and changes the spacing for each primary lens so he can focus through the entire range of that lens. I'm not opposed to machining a new housing that lets the anamorphic elements change their spacing (hard as it may be on my machine tools).

Also has anyone ever seen a +0.5 diopter? A +1 is going to be too close, even a +0.5 might be too close. I have cheap +1, +2, +4, +10 set in 77mm shipping soon, so I can play with it a little more, but just trying to understand how the relation of each lens element affects the final throughput to the primary lens. Right now it is all playing to see how deep I want to get into this rabbit hole (how much money I want to ultimately spend).

 

[Greg_E]

I've done a lot of reading about how these different anamorphic devices are set up. I'm guessing that these focus through devices are set to have the adapter at about infinity. But I'm still not sure why you get longer minimum object distances with longer lenses. I did read that changing the distance between the two elements will change the object distance, but will also change the amount of "stretch" which gives your close-up the effect of getting swollen cheeks (mumps). So for close ups, a diopter is still needed, and that there are few and far between diopters under +1.

Also, in reading info about the prototype SLR Magic adapters, it was said that lenses with fewer elements and shorter distances between objective and iris seemed to work better with an anamorphic adapter in front.