For colour work you're better to change the aperture & keep the exposure times constant, reciprocity with colour papers affects the colour balance and contrast.
In the days before minilabs in professional labs the colour roll-head printers were set up and calibrated and the aperture ring locked so that regardless of the lens used on the turret the exposure remained constant - the degree of enlargement was dependant on which lens was used. This was important because it meant that regardless of the size all machine prints had identical colour correction (and exposure).
With B&W this is less important but it's still useful to keep times similar as it also helps with dodging and burning at different sizes.
Last edited by Ian Grant; 04-01-2012 at 11:38 AM. Click to view previous post history.