Chromatic aberration (in short “CA”, sometimes also referred to as “color fringing”) is an effect caused by a lot of lenses, in particular cheaper models and wide-angle lenses. It is caused by the failure of a lens to focus all colors to the same convergence point. The effects are color fringes particularly at high-contrast edges and a generally more blurry image. The fringes tend to get worse towards the edges of the image.
Photomatix is not very good at removing these artefacts and the usual HDR workflow tends to amplify the fringing effect, creating visible strange-looking fringes and blur in the final image. In this recipe, you will learn how to fix this problem before feeding your images into Photomatix (or any other HDR software).
Requirements and Assumptions
I assume that you have produced the source files in RAW format and that you have Adobe Camera RAW (ACR) or a similar RAW converter.
Instead of feeding the RAW files directly to Photomatix, we will develop TIFF files from our RAW source files using ACR. This is the very first step in the entire HDR workflow. One essential part of developing the RAW files in ACR is the CA correction. The resulting TIFF files are then loaded into Photomatix to produce the HDR image and do the tone-mapping.
Illustrating the Effect of CA
Before we take a look at the rather simple procedure of correcting CA, we will first look at an example and show what CA is and what its detrimental effects are on the tone-mapped image. Below, you see a sample image. This is the 0ev JPEG straight out of the camera. I was using a Nikon D90 with a Sigma 10-20mm F3,5 EX DC HSM lens at 10mm focal length. CA is particularly apparent with wide-angle lenses.
Below, you see a series of 200% crops of the area marked in the image above. The first crop shows the original RAW file without any adjustments applied to it in ACR’s default settings. The purple/red and cyan fringes are quite obvious.
The Nikon D90 has a good CA correction mechanism built into the camera. Below, you see the same crop from the corresponding JPEG produced by the D90. You can see that the fringes are greatly reduced. However, there are still some visible blue fringes. Moreover, we would rather not want to use the JPEG output of the camera for our HDR image because using the RAW images leads to a higher quality of the final image. So, the bad news is that the in-camera CA reduction does not help us. The good news is that we can do better than that!
The next image is the same crop from the image developed in ACR. You can see that the CA has disappeared almost completely.
Ok, now we know that we can reduce the CA using Adobe Camera RAW. But why should we bother? The following crop shows the result of creating the HDR image and tone-mapping it using Photomatix Pro 4.0 without any CA correction. One could argue that the color fringes have gotten worse to a point where they are noticeable at normal zoom settings. Moreover, the image looks somewhat blurry.
Of course, we could use the built-in CA correction feature of Photomatix to cure the problem. Below, you see the result of feeding the RAW files directly into Photomatix and turning on the “Reduce chromatic aberrations” option in the Preprocessing Options dialog. Yes, the CA is reduced – kind of. But what becomes apparent is that the image still looks blurry. More importantly though, ugly fringes occur on the edges replacing the former color fringes – not a very satisfying result. Photomatix has never been very good at removing CA and this continues into the newest version of the software.
Finally, below you see the image resulting from the workflow I will sketch in a moment. Notice that the any fringes are almost gone and the image is noticeably sharper than any of the versions above.
It is obvious that this is the way to go in order to obtain the highest output quality of the HDR process.
- Start Photoshop
- Mark the RAW files in your preferred image browser and drag them onto the Photophop window (If you use Adobe Bridge, you can also use the menu to open them in ACR). Photoshop will automatically open the files in ACR.
- Go to the “Lens Correction” Tab on the right side of the window.
At the top, you will see three controls: two sliders for correcting different fringes and a general “Defringe” drop-down list.
- Open the drop-down list and select “All Edges”.
- Zoom into a suspicious part of the image using a zoom factor of 200-300%.
- Use the sliders to remove the visible fringes. The specific setting is highly dependent on the lens you were using. You have to experiment. You can observe how the fringes increase or decrease as you move the sliders. Finding the optimal settings should be a matter of a couple of minutes. Note that the same lens used at the same focal length and aperture produces very similar CA across all photos. Consequently, also your setting will be similar for different photos. In our case, settings of -30 and +10 turned out to by the best.
- When you are satisfied with the result, press the “Select All” button at the upper left corner of the window and then Alt-click the “Synchronize…” button to apply your settings to all loaded images. Note that before doing this, you may want apply any other adjustment in ACR.
- Finally, click the “Save Images…” button at the lower left of the ACR window and save your files to some destination folder. Be sure to select all files before doing this. Save the files in TIFF format.
- Navigate to the destination folder and open your files in Photomatix. You can now continue your normal workflow in Photomatix. However, please uncheck the “Reduce chromatic aberrations” option. We do not need this anymore and we do not want Photomatix to degrade the quality of our image in a desperate attempt to remove any CA that is not there (any more).