minimal HDR processing

HDR (high dynamic range) editing is our attempt to increase dynamic range of an image taken with any camera which is many stops inferior than our eyes. Cameras, however expensive/ advanced may be the model, are no match for our eyes when it comes to dynamic range. But using software, we may give a modest try to “increase” the dynamic range a bit. As the name suggests, HDR is about having a wider range of exposures in an image, not about making post-apocalyptic images, as we mostly see around.

HDR requires unique modifications for each image. Even if I create a great HDR with one image, I may fumble with the next, because this one may not support the same level of modification. Hence my subject’s exposure at the time of taking the shot will determine how much I must modify it. I should not try to bring all images in same range – that may spoil originality of a scene. To experiment with HDR, we need a scene with different exposure levels in different parts of the frame. In the example below, my frame sees sunset from a roof, so to accommodate the higher-exposed sun, the camera will make the rooftop darker (Image 1). Now if I want a brighter roof, I must over-expose the shot, so the sky becomes whitened (Image 2). HDR will be the fine balance between these extremes (Image 3).

Image 1 – Exposure for sky

Image 2 – Exposure for roof

Place the camera on a tripod and take two exposures of the same frame back to back – in this example one exposure is for the sky and other for the roof. Any decent camera will have “bracket shots” feature, but manually setting the exposure will also do. The two shots now need to be merged into one using a software. No setback due to expensive software; we’ll use good but free products – 1. The GIMP and 2. Picasa. Both are available for Windows/ Linux. In GIMP, open the lighter of the images and on top of that, open the other as a layer. Now  select the darker layer and merge the two layers using “Normal” or “Overlay” mode, sliding the percentage of merge to the scene’s demand. Sliding from the darker layer is better because we want to minimize blown-out (over-exposed) areas as much possible. When you have sufficient details in low and high exposure areas, flatten and save the image.

Image 3 – Increased DR

Now bring all three images to Picasa and study the histograms (you can get histograms in GIMP also, but Picasa provides the simpler one and doubles up as file explorer). You’ll know that you’ve not damaged the frame when the histograms of one of the images and your modified image have similar pattern.

As we can see, the first and last histograms follow similar patterns, hence there is no loss of image data due to this processing. For this reason the processed image doesn’t seem to be out of place.


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