Wednesday, October 23, 2013

Throwing a (linear) curve

While doing further reading and research on the technical aspects of HDR Imaging I came across a pretty large issue concerning image curves.  Namely how Adobe Lightroom does not apply a neutral linear curve to your images.  Now when you are only working with single images, this is not necessarily a bad thing however, when working with multiple images for HDR's (3, 5 or even 9) this can become a big problem as the colors don't match up from image to image.  

Enter the theoretical linear curve, one that is completely flat from light to dark; which when all the images are stacked and merged keep all values of light, dark and color channels in their correct relationship to each other.

The problem is neither Lightroom or Photoshop has this linear curve profile and even using something like the "camera neutral" profile in Lightroom, there is still a slight curve to the profile which throws in mismatches.  How to modify these profiles or create a new one to fit my needs?

Which brings me to when (yesterday) I was browsing the forums on and found the answer.  Adobe provides the tool to create a custom profile in their DNG Profile Editor.  With it, you can create a custom camera profile that is completely linear and then export it into Lightroom (if like me you use Lightroom as your main workflow tool) and in turn apply to each of your images prior to exporting them to your HDR creation program of choice.

So, now you may be asking yourself, how do I create a custom profile - well here it is in a nutshell:
  1. Create a DNG of one of your photographs.  Again, Adobe has a free tool to do just this which is either standalone or included in Lightroom - Adobe DNG Convertor
  2. Open this DNG file in DNG Profile Editor and in the window that comes up ensure all the adjustments are zero'd and then on the Tone Curve tab, select Linear for the Base Tone Curve.  You want this graph to have the line completely straight from bottom left to top right.
  3. Save the Recipe - name it something useful to you
  4. Export the Profile - this will then add it to Lightroom for you, and can be found in the Develop Module under Camera Calibration
  5. In Lightroom, select the exposure stack, select this new profile and hit the Synchronize button to apply it to all the images in the selected stack.  This is also a good way to insure all develop settings match across the stack prior to creating the HDR Image
You are now the owner of a fully custom camera profile that has a truly linear response curve.  I now have one for my Nikon D2x, which is my current main camera.  It will be a good idea to go back and create on for my Nikon D100 too in case I want to get creative with photographs shot on it, as the profile created in DNG Profile Editor is camera specific, being that it is built off of the curve response of the camera the DNG file was taken on.

After all this you may be thinking "Is this all really worth the effort?", "Will a linear curve really improve my HDR Images?".  In my brief experiments since creating my custom curve I can give you a simple answer to those questions - YES!

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