News and Views from Dave Wilson

Archive for March 3rd, 2011

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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