Split grade printing is meant to boost contrast by eliminating the muddying of midtones. When you print with a low contrast filter, you have to expose longer to get darker black. In the process, the midtones can get close to black. Very high contrast filtration makes the shadow areas appear first and the midtones can be too bright, and highlights non-existent. You expose the same piece of paper with a high contrast filter for half or three-quarters (I'm being arbitrary) of the time then use a low contrast filter for the remaining time. Or whatever works.First of all thank you very much for your advices. All of them counts.
So in resume, lets imagine I'm using fractions of 5 seconds, so in the end I like the exposure achieved at 20 seconds with a filter at grade 2. If I decide I still need to add a bit more contrast I'll just need to add a grade 3 filter, for example? Does this mess with exposure time or only from 4 and 5 filters?
I also read some posts mentioning about split grade printing. It sounds to me something like processing the same file twice in camera raw, one for shadows and other for highlights and then merging them in PS. Does this make sense?
So if my exposure is 20 seconds at grade 2, should I then apply a low grade filter first and then a high one at 10 seconds each? I confess Im a bit confuse with this concept. Im sure it must be something simple, but Im just curious.
Changing contrast filter from 2 to 3, with Ilford filters, should require no additional time. However, what you were seeing at filtre #2 may have actually not only been too low contrast but also not enough exposure. It's something you need practice to determine. The #5+ Ilford filter requires and additional stop of exposure (or the #4 requires 1 and the #5+ requires 2? You should look it up).
I say you should just jump in and get started.