Quote Originally Posted by TheFlyingCamera View Post
The inverse square rule applies to enlargement the same way it does to studio lighting - double the distance, lose a stop, halve the distance, gain a stop. So going up to a 16x20 from an 8x10 is roughly 4x the distance, or two stops. If your base exposure is 3 seconds at f5.6, then give it 12 seconds as a starting point on your test strip (still do test strips!). you'll probably want to go up at least one grade in filtration, maybe a grade and a half. So you'll have to add another half to whole stop for the filter grade increase too. Try starting at grade 4, 24 seconds, and give your test strips three exposures at 12, 24, 36 and 48 seconds to see where the truth lies.
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Exposure Height and Exposure Correction Whenever the enlarger head is raised or lowered, and the negative magnification is changed, print exposure must be corrected. in the attached pdf on page 511 you will find a chart to determine the magnification of your enlargement and another to estimate the exposure compensation required to accommodate a change in enlarger height. Strictly speaking, projected print exposures fail to follow the inverse-square law, but they follow the inverse square of the lens-to-paper distance if the paper reciprocity failure is ignored, in which case, a new theoretical exposure time (t2) is given by: