[Crushellon (Tim)]

I figured it would be from the base.

I didn't know you had to allow for glass thicknesses. I also wasn't aware of a glass cover on the sensor and can't find any details regarding its thickness in the manuals.

In the manual for the stf-8300m it says the mechanical distance to the focal plane is .7" and the optical distance to the focal plane is .69" do I go from mechanical or optical?

so I'll assume it's the optical distance for now. And I'll assume the glass cover is 1mm, the filters I will be using are 2mm.

So, 55mm for the reducer add 1mm to allow for 3mm of glass = 56mm

56mm minus the optical distance to the focal plane on the 8300 which is .69" (17.526mm) = 38.474mm

So the distance from the face of the camera to the base of the frot off the flattener needs to be 38.474mm is that right?