Joe,
Thank you so much for the detailed information. I just checked out your site and its
I pulled the trigger on a sheet of OD3.8 & 5. My understanding by what you say on your site is that the 3.8 can be used on small aperture/long FL (f15-f40)... So my ED120 & ED80 with an aperture of 50mm will be suited for the 3.8
"ND3.8 (6300 times light reduction) is designed for telescope eyepiece projection photography where effective focal ratios of f15-f40 are typical."
I can use the OD5 on a full aperture setup and on my finder scopes (ST80 and 60mm).