Understanding Contrast Masking

About three years ago I wrote atutorial on contrast masking. It was published before I had started to usedigital cameras, and was intended for use with files from scanned film. Belowis an updated tutorial designed for users of digital cameras.

___________________________________________________________

Too Hot is Not Cool

A digital file is much like a film positive (transparency) in that the worstsin is to blow out the highlights. In fact it‘s even worse with digital thanwith film. Highlights on film tend to trail off gradually on the shoulder ofthe curve, while with digital it‘s more like hitting a brick wall. Overexpose— expose too far to the right of the histogram — and you end upwith nothing, nada — empty white pixels with no data whatsoever.

So the strategy for digital photographers is to expose as far to the rightside of the histogram as possible. (Please see my article Exposeto the Right for additional reasons why this is the correct approach todigital exposure, and If you are unfamiliar with how to readand use the histogram, read my tutorial UnderstandingHistograms).

Given that digital cameras have a dynamic range of 5-7 stops (aboutthe same or slightly greater than transparency film), one often finds thatthe shadow areas of a frame are rendered darker than one would wish. What todo?

There are three solutions. The first is the traditional approachtaken by photographers for the past several decades — the use of splitneutral density filters. (This is covered in detail in a tutorial in Issue#6 of The Video Journal).I used these for many years, but they are slow to use well and not appropriatefor anything other than landscape work.

The second approach, and a viable alternative to the use of splitNDs, is digital blending. But,it isn‘t always possible to shoot more than one frame of a given scene.

___________________________________________________________

Photographers Have Been Using Contrast Masks for Almost 100 Years

The solution is to use what is called a contrast mask.This is now done digitally, but it is nothing new. It‘s been around since theearly years of the 20th Century. This used to involve the production of a B&Wnegative copy of the original which was then placed in superimposition withthe positive while a print was made. This caused a flattening of the contrastas shadows received extra exposure while highlights were held back.

Here‘s how we do it today in Photoshop.

Figure 1

In Figure 1 above we see a frame taken just afterdawn at Moraine Lake in Banff National Park. I exposed sothat the sky wouldn‘t burn out, but which caused the dimly lit foreground tobecome quite dark. To the unaided eye the scene didn‘t appear this dark, andso it‘s desirable to try and reproduce what they eye wants to see, rather thanwhat the camera actually recorded.

Here are the simple steps needed to create a contrast mask, andopen up the shadow areas of this image.

1. Create a Duplicate Layer ( Layer / Duplicate Layer) and name it Mask
   
2. Desaturate the Mask Layer. (Image / Adjustments / Desaturate)
   
3. Invert the Mask layer (Image / Adjustments / Invert)
   
4. Click on the Mask layer thumbnail and select Overlay then Blending Mode
   
5. Enlarge the image to 100% and look at the horizon line or other high contrast area. This is important for the next step.
   
6. Use Gaussian Blur to reduce resolution degradation of the image by the mask, yet at the same time avoid halos on sharp contrast boundaries such as skylines. (Filter / Blur / Gaussian Blur). Low amounts are usually best.
   
7. Adjust Opacity as desired. (Click on the Mask Layer thumbnail and use the Opacity slider).

Figure 2

Figure 2 shows the intermediate result afterexecuting steps 1 — 7 above. The shadows have been opened up while thebright areas have not been compromised.

The next steps are to "season to taste" usingthe usual tools, such as Levels and Curves. Figure 3 belowshows what the frame looks like after a few quick adjustments. A larger versioncan be seen by clicking on it.

Figure 3

___________________________________________________________

Other Approaches

Every time I publish an article like this one I am inundated withe-mails detailing various alternative approaches. There are indeed many otherways of doing this. For example, one alternative is to usea mask created from one of the colour channels. Here is anotherapproach that allows individual adjustments for both highlights and shadows.

Yet another alternative is a new Photoshop plug-in called SRPro from Fred Miranda. Selling for $19.95 this is a welldesigned product for both PCs and Macsthat automates the entire masking process and which also adds some unique customizationfeatures