Split-filter printing allows you to fine tune your prints.
This procedure works because variable contrast papers have 2 separate layers, each layer sensitive to a different color filter. Filter #00 affects only the low contrast layer, filter #5 affects only the high contract layer. All the other filters affect both layers but to different degrees. By using the #00 & the # 5 filters according to the following method, it is possible to achieve both very good blacks and a full range of gray tonality, even using fairly high contrast negatives (those exceeding a 7 stop range.)
- Make a test strip with a number zero filter. Choose the best exposure for the highlights.
- Make a second test strip with a number 5 filter. Choose the best exposure for the shadows.
- Make a full print first exposing with the number 5 filter in place for the best exposure time determined from step 2.
- Expose the print again, this time replace the number 5 filter with the zero filter for the best exposure time determined from step 1.
Note: The number 5 exposure must be made first or else the highlights will be too dark.
reference: page 131 in London text.