How to Determine the Magnification Factor?

[silveror0]

Obviously a very old thread, brought back to life. There's a method published in Way Beyond Monochrome for quickly determining the magnification. Cutting two V-notches into the edge of the negative holder, one inch apart, will project at the edge of the image on easel. Measuring the distance between the projected notches instantly gives the mag factor. The idea of defacing my negative carrier does not really appeal to me, so I've been using a transparent ruler on top of the negative carrier, then measuring its projected image to find the mag. That's certainly accurate enough IMO. Changing the mag requires a correction to the exposure time; the new exposure time is calculated by this formula:

T2 = T1 [ (M2+1) / (M1+1) ]^2

EDIT: This formula provides the exposure change but, as conanw noted, there'll likely be a contrast change as well, necessitating a new test strip. For split printing, however, applying the exposure correction to the highlight exposure time may avoid the need for a test strip.

 

[cornflower2]

Split contrast

Would the same calculations aplly to each exposure of a split contrast black and white print?

For example if I make an 8x10 print at f11 with 6 seconds Magenta 200 and 4 seconds Yellow 200; and then I want to convert to 16x20 I should do the arithmetic and apply it to each exposure?

Yes of course. I have produced a video on YouTube which shows the making of an A4 split contrast print on Ilford FB paper using a total exposure time of around 21 minutes! Search