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If your camera can shoot in RAW, you should shoot in RAW because Adobe's RAW processing engine has dozens of powerful tools for fine tuning your digital images. I'll try and show you how to remove Chromatic Aberrations using Adobe Camera RAW processor.
But first, what are Chromatic Aberrations?
CA or Chromatic Aberration is a purplish, bluish or greenish fringeing around the edges of Digital pictures caused by poor lens design, or good lenses used under certain lighting conditions.
Teleconverters or Extenders are notorious for inducing CA. So are low cost P&S Digital Cameras. You can see this purplish fringeing at the edge of Pharaoh Ramses’s face in the illustration below, which is a crop from a NEF (Nikon RAW) file.
In the diagram, I have tried to illustrate how Chromatic Aberrations are caused by poor quality, or poorly designed lens systems. The good lenses are expensive because they incorporate several lens elements and exotic coatings to reduce or remove Chromatic Aberrations.
Light is made up of several wavelengths of different colours. When light passes through a lens, the light is refracted, or bent depending on its wavelength.
A good lens design will focus all light wavelengths precisely on one plane without any CA, but poor lenses often cause the red or green or blue wavelengths to focus in front or behind the plane of intended focus causing the familiar purplish or green fringeing.
Its quite easy to remove the fringeing. If you want to follow these steps on your own PC using my NEF image of the Temple of Abu Simbel in Egypt, you can download the 8MB Nikon RAW file Abu-Simbel.NEF from HERE.
To be sure that your Adobe Bridge can open the file, please visit the Photoshop website and update your copy of Adobe Bridge.
This is a magnified crop of what the Chromatic Abberations in the file should look like:
Step 1 - Normally, clicking on a RAW image from within Adobe Bridge will automatically launch Adobe's Camera RAW processor.To launch Adobe Bridge from Window's Start menu, click on the Start/All Programs/Adobe Bridge icon. From within Adobe Bridge, locate the RAW image Abu-Simbel.NEF that you've downloaded (or your own file with pronounced Chromatic Aberrations), and double click it to open the image in Adobe's Camera RAW (ACR) dialog box. Then click on the Lens Tab and you should see the same ACR window as in the illustration below.
Step 2 - Before doing anything, click the Detail Tab and ensure that sharpening and colour noise reduction values are reset to zero. The default values are usually pre-set at 25% by Adobe Camera RAW, and sometimes automatic sharpening and noise reduction algorithms can induce unintended Chromatic aberrations in digital images. You just need to make sure that the fringeing in your picture were not caused by these default settings. Sharpening should be the last step in your image processing workflow. The amount of sharpening you apply will depend on whether the image is intended for printing or for web display. Images destined for printing can normally tolerate slightly higher Sharpening settings.
Step 3 - The CA may not be very obvious or visible at low magnification. After resetting Sharpening values to zero, click on the Lens tab, and click the magnifying glass symbol at the top left corner on the menu bar. Alternatively, you can also click the short-cut key Z to get the Zoom Tool. Click several times to zoom into an area in the image until you can clearly see any fringeing in your picture. While the zoom tool is active, you can move to other non visible areas of the image, simply by holding down the space bar, which will transform the Zoom tool into a Hand tool, and then left-clicking to grab and drag the picture around the preview window. In the illustration below, I have zoomed into the area of the Pharaoh's head on the left-most statue, where the purple-red fringeing can be seen very clearly.
Once you are zoomed in to 50% or greater, you can make the fringeing appear even more visible by holding down the Alt key, and clicking and holding onto the Fix Red/Cyan Fringe slider button. Doing this forces the preview window to display only 2 colours, which will make any fringeing even more highly visible. If you have my image above opened in your RAW processor, you can depress the Alt key, click and hold the Red/Cyan Slider button, and simultaneously move the slider leftwards to about minus 56%. As you move leftwards, the Red fringe will gradually disappear .If you move the slider too far to the left, not only will the RED fringe be removed, but it will be replaced by a Greenish fringe. So you need to hunt for the sweetest spot where the fringe removal is optimized.
Use the upper slider to fix fringeing on the Green/Red axis, and use the lower slider to fix any Bluish fringeing on the Blue to Yellow continuum. If you use a Mac, the corresponding keys to change the preview display to just two colours are Option+Click. Note that you can also remove or add vignetting in your pictures by moving the Amount and Midpoint sliders located in the Vignetting box. The illustration below shows the Red-Purplish fringe completely removed by sliding leftwards to -56%.
Once the fringeing has been successfully removed, you can continue with the other RAW image tweaking steps. You can do most of your image colour, contrast, sharpening, white balance adjustments, batch processing, and cropping, directly from within the Adobe Camera RAW Dialog box without ever opening Photoshop proper. You do these simply by clicking on each of the five tabs in the ACR dialog box, and moving the appropriate sliders while visually observing the changes being made in real time in the preview window. Clicking the Adjust Tab will let you adjust White Balance, Exposure, Brightness, Shadows, Contrast and Colour Saturation settings. The Detail Tab will let you adjust Sharpening and Luminance, and also reduce Colour Noise. The Curve Tab is a very powerful tool for adjusting the colour, contrast, shadows and highlights in your images. The Calibrate Tab has sliders for adjusting the Tint, Hue and Saturation for all the 3 primary colours of Red, Green and Blue. And I have described in detail how the Lens Tab can be used to remove Chromatic Aberrations and Vignetting. The Tab settings I used are shown in the image below:
I shot this picture of the Temple of Abu Simbel in the summer of 2003, using a Nikon Coolpix 5700 without a lens hood, and when the sun was about 3 o'clock on the horizon - ideal lighting conditions to induce Chromatic aberrations. This magnificent ancient Temple is located on the west bank of Lake Nasser, near the border between Sudan and Egypt. Lake Nasser is the biggest artificial lake in the world, created when the Aswan high dam was constructed across the mighty River Nile. The temple was originally built more than 3000 years ago by Pharaoh Ramses the Second, for his God Amun. It would have been submerged under Lake Nasser if not for the Herculean effort of UNESCO, the international community and the Egyptians, who meticulously cut, removed and relocated the entire hillock from which the temple was carved, block by block to higher ground, even as the water level in Lake Nasser rose. I remember the best view of Abu Simbel was at night from the open deck of a vehicle ferry I was on, while travelling from Khartoum to Aswan in a 4x4 in 2004 to begin my trek across the Sahara Desert.
Here is the picture of Abu-Simbel after I've removed the Chromatic Aberration as described above. I also tweaked, cropped and converted the NEF file to JPG, using only those tools found in Adobe Camera RAW Dialog boxes. Do note however, that image tweaking is a personal exercise. There is no right or wrong level of tweaking. You adjust an image according to how you want it to be and how you personally saw it when you composed the image in your camera's viewfinder.
Go and see Egypt. Its one helluva nice place to visit. Now that you know how to get rid of Chromatic Aberrations, go and shoot great pictures and post them here in POTY......
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