Most colour papers suffer a kind of reciprocity failure for times of longer than 25 seconds. I aim to keep exposures between 5 and 20 seconds when printing RA-4 by adjusting the lens aperture.

If you are plannign a big blow up, or are working from a dense overexposed negative, try balancing at a smaller print size, and then scale the exposure time up by the ratio of the proprtion of the square of any one image dimension to get the new exposure time.

You may then see a colour shift, and then have to dial in a filter and exposure time to compensate.