Here's a site I randomly found

[jrod81]

http://gt35pro.blogspot.com/2008/08/gt35pro-35mm-dof-adapter-power-unit.html
Gives decent overviews of each of the adapters, but could someone please clarify what the achromat does and what the lack of having it will cause?

 

[Huy Vu]

The achromat allows the camera to focus in on a ground glass. No achromat, no working adapter.

 

[hoarp001]

Achromat is posh for macro lens....

 

[armisiano]

Achromat is posh for macro lens....

Finally someone says it.