Creating an HDR Image

By Chris Francis

This is a brief run-through of the talk I gave on Monday 13th Feb where I showed the steps taken to produce an HDR image of the sunrise at St Mary’s taken in early 2011. The ‘correct’ exposure out of the camera is shown below.

While technically balanced when looking at the histogram of the image, it doesn’t really convey the mood of the sunrise (a warm light, with quite a lot of colour in the sky). Therefore, I decided to take a bracketed exposure of the scene to capture more of the detail in both the sky and the foreground rocks.

I tend to take a -2 stop to +2 stop exposure range when creating my HDRs, as I feel it gives a little more detail without being over the top in terms of processing resources required (if the scene was very contrasty you might want to go further). This produced the 5 images [below] with exposures of 0.3s, 0.6s, 1.2s, 2.5s, and 5s from left to right, at f/20 and ISO 200.

It’s important when bracketing to just change the shutter speed and not the aperture, as you can introduce focus shifts into the image.

I loaded these images into Photomatix [you can check out this software at this website] , and through the tone mapping tools produced an output image as below.

This is normally a process of trial and error, using the various presets and fiddling with the sliders a fair bit! I felt this was a good image to base my final changes on in Photoshop to create the image I had in my mind.

First, I cleaned up the small points of lens flare in the image, and then introduced a slight ‘S-curve’ using the Curves tool to boost the contrast slightly (you tend not to have to do this very much after creating the HDR image). I then added a warming photo filter with a low opacity to give the image a slightly warmer feel, and then finally sharpened the image to produce this final version.

You can see the tweaks which were applied in Photoshop were very minor, but combined together I think they make the final image that bit stronger.

The above workflow can also be used on a single RAW file if you didn’t have the opportunity to bracket the exposures in camera (there is normally a couple of stops leeway in the RAW file providing you haven’t clipped either the shadows or the highlights). Again, Photomatix can do this (without the need for you to manually create the 5 exposures), and the result is shown below.

This hasn’t had the final Photoshop tweaks applied to it, but you can see it’s done a reasonable job of pulling out some additional detail out of the sky and rocks, but the range isn’t quite as good as the ‘true’ bracketing.


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