With a little vacation time, I finally got around to making a handy reference chart to keep next to the enlarger, helping determine how exposure time needs to change going from one contrast filter to another. I use a cold light head without a yellow filter, just using the Ilford under-the-lens contrast filters. As a result, my times donít match Ilfordís ďconstant timeĒ design. Although Ilford makes their filters to provide a constant time for some middle tone (except for the extreme filters, which require twice the time), the color of your light source may differ from their standard, or, you may wish to choose a significantly different tone as your constant.

How It Works
Explaining takes much longer than doing. Let me know if it's not clear.

You need to spend an hour or more making test strips with no negative in the enlarger, to determine how much exposure time is required with each contrast filter to match your chosen, single tone. These tests give you a specific relative time for each grade. I have no densitometer and matched a chosen Zone VIII (very light gray) value by eye. My first pass gets me very close; I can refine it later. Since my subjects vary a great deal, no tone will provide exact matching for very light textured areas anyway. Some, such as Alan Ross, have two charts, one for a low value, one for high. Itís up to you.

You will leave your enlarger head in one position and keep the same aperture for all test strips. To save time, I made my test strips just one stop closed down, in increments of 1 second and some decimal, often around 1.3 seconds. Thatís why the exposure times are so short. Longer total times will tend to increase precision, especially with the highest contrast grades, as increments between strip steps can be more closely spaced.
I chose to use percentage of change. In other words, going from Grade 2 to 1Ĺ requires 122% exposure increase in my system; 1Ĺ to 2 requires a reduction to 82% of the original time. Itís an easy calculation with a pocket calculator: total main exposure time multiplied by the percentage number and a press of the % button. I make my exposures in multiples, so, for example, a print might get 8 exposures of 3.4 seconds each, for a total of 27.2 seconds. Iíll take my new total time and also divide it by 8, to maintain any dodging and/or burning routine.

Using the Calculation Section
After you have made your test strips and have a tone-matching time for each contrast grade, enter the time in the column to the right of the one labelled To across from its contrast grade. (For readability I used decimals for the half-grades here.) In mine, Grade 00 is 4.8 seconds, 0 is 5.2., etc. You will replace these with your times. Once you have these entered and saved, enter the time for Grade 00 in the cell under From, where I have a 6 (taken from Grade 3 Ĺ ). All the percentages will automatically calculate, as follows:
If your From time is more than your To time, chose the percentage in the If Decrease column. If the From time is less than the To time, chose the percentage in the If Increase column. With 6 in the From column as I have it, going from Grade 3.5 to anything from Grade 00 through Grade 3 requires a reduction in time, so you use the If Decrease column. For going to Grades 4 through 5, however, the time increases, so use the If Increase column.

Filling and Using the Exposure Chart
You will fill in your appropriate percentages in the chart, starting with the time for Graded 00 in the From cell of the Calculation Section, then doing the same for each grade. I have left a few values in the chart, in red as examples (see below).

The chartís light blue left column under From gives the contrast grade you from which you are starting. To find the time percentage increase or decrease for changing to another contrast grade, read across the row to find the cell under the To grade to which you are switching.

From the example given above, for instance, to match my chosen tone going from Grade 3.5 to Grade 00 requires 80% of the exposure time. From Grade 3.5 to Grade 2 requires 75%. If I were using Grade 1.5 and wanted to try 3.5, I would multiply my time by 109%.

I hope some may find this helpful.

Paper Grade Change Exposure Chart for LF Info.zip