News and Views from Dave Wilson

HDR Tip #4 – Dealing with Colour Fringes

by on Mar.03, 2011, under Photography

Senate Chamber, Texas State Capitol

Another of my golden rules for processing HDRs is to do as little as possible to the brackets in Lightroom before I export them to Photomatix for merging. The main reason for this is that I want to ensure that I don’t disturb the pixels in each of the bracketed exposures differently and run the risk of messing up Photomatix ability to merge the picture but it’s also because I think of Photomatix as the first step in the process. There are, however, two operations that I typically do before merging the brackets, one of which is cosmetic and the other of which is, in my opinion, absolutely vital.

The cosmetic operation is to correct the image white balance if I didn’t nail it in the camera. The important thing here is to make sure you apply exactly the same change to every one of the pictures in the bracket. Typically, I will correct the center exposure then use the Develop Setting copy/paste operation in Lightroom to apply the same setting to the other images. I do most of my colour correction or tweaking after Photomatix is done with the image but it’s always easier to start with images that are close to where you want to end up.

The more critical operation for me, however, is correcting chromatic aberration. This is a “feature” of lenses (typically cheaper ones, to be honest) caused by the fact that they focus different colour of light at slightly different distances and, hence, give slightly different magnifications to the red and blue channels in the image. This results in colour fringing along high contrast edges near the sides of the image. If you don’t correct this prior to merging, you will find that (a) you end up with an HDR that is less sharp than it could be and (b) the fringing effect will be magnified an enormous amount by Photomatix.

If you are lucky enough to be using very high quality professional lenses, you may never see this (I really only have to deal with it on my Sigma 15mm fisheye since the Nikon pro lenses I have are amazingly clear of CA problems) but most of the time it will be apparent if you zoom your image to 100% and take a look at the corners. If you are using Lightroom, correcting the problem is very simple. In the Develop module, look under “Lens Corrections”. If Lightroom knows about your particular lens (and it knows about a lot of lenses), you can click “Profile” then select “Enable Profile Corrections” and your CA problems will disappear immediately. Note that this also allows you to correct lens distortion and vignetting but I typically leave these set to 0 since I like the fisheye distortion and the vignette doesn’t worry me.

If Lightroom doesn’t know about your particular lens, click on “Manual” and fiddle with the sliders under “Chromatic Aberration” until the fringes you see in the image disappear (or get as small as you can make them). In both the manual and profile cases, make sure you apply exactly the same setting to all the images in your bracket then go ahead and export them to Photomatix.

I’ve added an example of before and after here so that you can see what I am talking about. The top image shows a 200% view of the top right corner of one of my brackets before CA correction and the bottom one shows the same section after automatic CA correction in Lightroom.

Before Chromatic Aberration Correction

After Chromatic Aberration Correction

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9 Comments for this entry

  • Mike Connell

    I do the same thing before sending files to Photomatix. Additionally, I may reduce noise a little on the under exposed images within Lightroom. I find that it does a better job than Photomatix and my tonemapped image is much cleaner. It doesn’t seem to effect the detail if done lightly. For my Canon cameras, I also sometimes set the Camera Calibration to a profile (usually camera standard) as I tend to prefer that to the Adobe default profile. Setting one exposure and then pasting the Development settings to the others is definitely the way to go.

    • Dave Wilson

      I typically don’t do noise reduction before merging but that’s really laziness since I didn’t want to have to round-trip all the brackets to Photoshop to run Noiseware on them. Now that LR3’s noise reduction is so good, I should try that.

      I’ve also heard that some people do some sharpening on the original brackets before merging and I should probably look at that too. Given that sharpening actually moves pixels around a bit, I worry that it may cause problems (fringing?) but I guess if you use the same sharpening on each bracket and this moves the same pixels the same amount (does it?) then you would be OK.

  • Scott

    Gotta love Lightroom. Great tip dude!

    • Dave Wilson

      Thanks, Scott. I may have to republish the one I gave you last month since I’m running short on time for tonight’s tip and it’s one that really needs me to generate a few example images for. I hope you will forgive me!

  • Erik Kerstenbeck

    Its amazing how some subtle corrections can make such a huge difference! Thanks for the pointer!

  • Sergio Valverde

    As it is a great idea, and in fact I use sometimes the profiles correction from Lightroom, I have found that it doesn’t remove the colour fringes at all, and I have to do it in Photoshop at the final stages of postprocessing. I review the pic to avoid purple fringing at 100% view. The tool for purple fringing in photoshop doesn’t work fine. So what I do is duplicate the picture layer, to select Hue/Saturation and desaturate the colours with the fringing and apply a layer mask in the zones affected. I apply a Gaussian blur to this mask and sometimes to the duplicated layer too, in order to have a smooth transition.

  • Sergio Valverde

    On the other hand, the picture is amazing, with a great POV and details !

  • Dave

    A great piece of advice here Dave. Personally I do nothing other than set the camera calibration. Everything else Inc colour fringing is done in LR after, but be it before or after, looking for fringing is something everyone should be keeping an eye out for.

  • Bob Towery

    Ya, the LR info is great, but WHAT a composition! Really a beautiful image. Nice work.

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