Does the Letus use 4 mirrors?
Considering what you are saying about the PCX and distances, a shorter adapter like the MPIC should have a lot of distortion, but it doesn't. So I’m not sure I understand it.

Yes the letus flip uses 4 mirrors.

Regarding Dan's adapter, he is probably using something different to a PCX lens, hense his high price.

So I would think an adapter without a PCX would be the best. Ben Winter mentioned something about not using a PCX in his Letus with an interscreen didn't he?

Yes, the beattie uses a fresnel lens, apparently.

So the beattie seems to be the bet set up. I wonder why adapters are not using it. If it needs no PCX and probably no achromat (didn’t Ben say he could use it without one as well?) it seems the way to go to me.

Only experiementation can tell.

Lots of opinions about the use of fresnel's. Some say they loose resolution and the 'rings' appear in the image, especially with HD footage. I know Steev used a beattie in his adapter, weather the fresnel was attached or not I have no idea.

Dan is able to use the MPIC on a GS200 without an achromat...but it has a min focus distance of 1.6 inches...so a 24x36mm will fill a 4:3 frame from this cam at that distance.

I believe Ben is using the achromat to address slight vignette on his GL2 with his Beattie mod'd Letus.

Here's my 2c on the adapters based an a whack of reading...and a lot of experimentation (although none personally with a Beattie)

The only downside I can see of a vibrating Beattie is that you have 0 control over the diffusion surface, squirmy grain (Letus has it too), and have a need to tightly control motion. A medium format lense system with a larger Beattie, vibrating, would be the bomb IMO. The spinning GG systems have .5 to 1 f/stop greater light loss, are larger, but will accomodate higher shutter speeds (depending on GG), and will allow "tunable" bokeh.

Pick yer poison.

A fresnel lens just acts as a PCX lens. The Beattie has a fresnel with a focal length around 120-150mm (depending on the screen).

So the Beattie is using a PCX, but in the form of a fresnel (I would assume this is to increase the light transmission since fresnel lenses are very thin).

Regular focusing screens use normal PCX lenses and show no distortion.

Alain,
Any experience with fresnel type lenses? I was just wondering how it effects resolution. I may try it out for fun.

I heard the MPIC uses a fresnel screen and it looks nice in the resolution department. Dan even has some rez chart shots.

The only experience with fresnel lenses I have comes from larger projection setups where the fresnel is just used to condense light from a bulb, so I do not know how it would affect an actual image.

Theoretically, there should be no loss in resolution provided that the fresnel has very thin grooves. But fresnel lenses can have much more distortion than a regular lens.

I really think an aspherical lens should be the best solution with regards to coma and spherical aberration (or perhaps an achromatic condenser?), but I am waiting on some lenses. (plus this isn't the topic of this thread :) )

Fresnel lenses
 

Classical fresnel lenses, as described by Alain have a regular pattern. It is possible to imagine fresnel structures with local random orientation, thus bringing them closer to ground glasses.