I have been experimenting with Maco infrared film and am finding it v. difficult to estimate exposure. With the if filter in place the film seems to have a max speed of about 6ASA. Is there less infrared radiation earlier in the day than at midday? or later in the day? I understand that there is no substitute for bracketing but is there some rules of thumb that I could apply? Many thanks in advance. Yaakov Asher Sinclair