Thursday, October 04, 2007

Digital Landscapes and Contrast Masking

This short essay is inspired by one of my flickr friends who goes by the nick Patryaa. He is a trekker and his very nice panoramic photo of Pratapgad made me want to see if I can make its digital form slightly better.

Original Photo

What we observe from this photo is that the dynamic range of the light on this occasion is very large. There are the clouds and even though they dont look that bright to the human eye, they are very bright as compared to the scene below which is in the shadows. This range cannot be captured directly on film or by digital sensor (though digital sensor does a better job at it - it has a lot more shadow detail than film). What one has to do in such a situation is to shoot for the highlight - do not have burned pixels. They are irrecoverable and photo is wasted. Patryaa has done a good job at keeping the highlights. Now e need to flatten the contrast of this image so that the shadows, which are very dense, come out a bit. In traditional photography it would mean dodging the shadows. In digital photo editing, we achieve this (with far greater control than traditional dodging) using a technique called Contrast Masking.

First let us correct the level by applying a gamma to bring out the shadows. (This is by moving the middle marker in a levels control panel)

Photo with Levels correction applied

Then we apply a contrast mask to the shadows.
For this:

  1. Select the shadows using the 'select' tool.
  2. Copy the selection to different layer and name it "Contrast Mask - Shadows" and select it.
  3. Desaturte
  4. Invert
  5. Now change blend mode to "Overlay".
  6. Apply Gaussian blur. Try different strengths from 25-50. You will have different effect depending on your image dimensions. Use your judgment to choose a value which 'looks' the best and least affects surrounding areas.
  7. Change the opacity of the layer to about 70% (or any other sane value you like) so that there is subtleness in the effect

Photo with Contrast Mask applied to the shadows

Now our layers correction had affected the highlights. So apply a contrast mask to the them. Follow the same steps as above except select the highlights instead of shadows.

Photo with additional Contrast Mask applied to the highlights

The thing to remember about digital photo editing and post processing is that we have to be subtle. Manipulations which completely change the 'light' or the 'feel' or the 'truth' of the photo end up creating 'digital art' not photographs.

These kind of techniques become quite important when one uses digital sensors. For example the shadow detail which can be recovered from digital sensor photos is greater than what is possible from film.

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