I know this is a novice question here but I would like to ask it anyway... If you print an image without any filter at all you have a certain time to create a proper exposure, if you desire more contrast then you can throw say a #3 filter to make it more punchy, but since you filter some light you have to change your exposure to again make a proper exposure. I remember in High School the teacher had a chart that had your exposure (in seconds) and then what your new exposure would be if you used any give filter. I was wondering if anyone had such a chart they would be willing to share with me. Also ilford say thats from 00-3 1/2 that you use 1X the exposure and from 4 and up use 2X the exposure but that seems to yield imperfect results. any help or comments would be appreciated.