I am new to Infrared film photography. I plan on using Rollei IR 400 film and the Hoya R72 filter. My question is that I have read several threads regarding exposure and have gleemed the following: shoot at ISO 25, at approximately 1/2 sec @ f16. My confusion is how do you meter? Do you make your TTL measurement and adjustments before putting the filter on the lens and adjusting 6 - 8 stops or do you measure TTL with filter on?