Split Filtering
The idea of split filtering is to use multiple (usually 2) contrast filters to achieve a greater range of tones than is possible using just one contrast filter.


The rule of thumb is to have a difference of at least two between filters (ie. 2-4)
Example: Here is a negative that was over-exposed (left). The information is present, but the tones between the brightest point and the darkest are very close to each other resulting in a low contrast (flat) image (right).
 
The first thing to try to fix this, is to raise the contrast filter. Unfortunately, this usually fills in too many of the darker grays and blows out the detail in the highlights (left). We could try printing the lower filter at less time to keep our highlights, but it would just result in a light gray print with no "true" black (right).
 
The only way to get both detail in your highlights and have a true black in your shadow areas is to divide the time between two filters; split filter.

Once you decide that a single filter isn't going to give you the results you want, then it's time to split filter.

How To Split Filter
1.) Using a low filter (00-2) make a test strip to determine your print time for your highlights (left, 3 second increments). Basically, you are only trying to identify at what point does your highlights area start to fill in gray. This image is going gray pretty fast so I would probably use the second one (6 seconds, right). Make a print, but don't develop it yet.
 
2.) Now that we have our highlights, with detail, we will try to bring back our shadow areas. Now take out the lower filter and insert a filter at least 2 greater than what you were using (above was a #1filter, so I need at least a #3 filter) I want a little more contrast so I am going to use a #5 filter. Go ahead and make a test strip right over the latent image you made in step one (left). At what Point does your black go black? That's your time for the second filter. Here it is somewhere between the second to last and the last one; I chose 14 seconds. Now make a full print using both filters and times (right).
 
The final print times ended up being: #1 filter for 6 seconds and a #5 filter for 14 seconds.

Normally using a high filter such as 5 for the entire time would produce filled in shadows and blown out highlights, but because you are only using the high filter for a percentage of the time, it will make your shadows black without filling them in entirely. It would usually blow out your highlights too, but we printed those in in step one!.

Adjust your filters to fine-tune your image

It doesn't matter whether you start with a high or low filter, but most people start with the lower one.
 
 

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