Quote Originally Posted by Helinophoto View Post
According to this article on wikipedia, 25A has a factor of 5, which is slightly over 2 stops http://en.m.wikipedia.org/wiki/Filter_factor , i have hoya and opening up roughly 2 stops worked fine for me.
And according to this one (http://en.wikipedia.org/wiki/Wratten_number) 25A has a filter factor of 8. Thus my comments to check the recommendations of each filter manufacturer.