I've just had an interesting email exchange with Astronomik to clarify the "Optical Thickness" of their 50mm filters. The text below is copied from this page here "
Available Sizes".
Filters with protective rings for filterwheels (without thread)- round filter with 31mm diameter (optical Thickness 1mm / mechanical Thickness 3mm)
- round filter with 36mm diameter (optical Thickness 1mm / mechanical Thickness 3mm)
- round filter with 50mm diameter (optical Thickness 1mm / mechanical Thickness 3mm)
Unmounted filters:- unmounted square filter with 50mm x 50mm (optical and mechanical Thickness 1mm)
- unmounted round filter with 27mm diameter (optical and mechanical Thickness 1mm)
Now, maybe I take things too literally but I have been blindly following the above with my 50mm filters assuming that the optical thickness meant that the optical path was moved by this amount. I realise that generally it is taken that the optical path is moved by 1/3 of the thickness of the filter glass but I assumed (incorrectly) that I should take the above literally at face value.
My experience (and that of an imaging friend that is also using the 50mm filters) led me to think that the 1mm specification was incorrect. It would also explain some of my confusion in
this post here.
I have written to Astronomik to query this and they confirmed via email that the effect on the optical path from the 50mm filters is in fact 0.37mm or 1/3 of the thickness of the filter glass. I have expressed the view that the information on the web page could be updated to make it clearer. That will be their call as to whether they update the page or not.
It may just be me being too literal but on the chance that others may get tripped up on this I figured that it was worthwhile sharing.
Clear skies,
Rodney
