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Adobe Photoshop Tutorial




Getting Started with RAW

The Lens correction Tools

acr lens correction tools This tabbed window provides you with a set of tool designed to correct some lens problems; Chromatic Aberration and Vignetting.

Chromatic Aberration

Chromatic Aberration is caused by diffraction of light waves as they pass through the glass of the lens causing the light waves to fragment and split into their different colors as they would if they passed through a prism. This causes a colored fringe to appear at mainly high contrast edges. Some lenses are worse than others far creating this effect and in general, the more layers of glass the light has to pass through the worse this problem can be.

The image I am going to show you by way of demonstration is a profile shot of one of the four lions that surround Nelson's Column in Trafalgar Square, London. To get it I shot up towards the sky providing a high contrast edge between the black statue and the pale blue sky. I have zoomed in to 200% on the top of the lions head where the problem is most evident. Before making any correction here it is best to be well zoomed in as you won't necessarily see what you are doing unless you are.

acr chromatic aberration

Look carefully and you can make out a greenish edge on the horizontal upward facing surfaces and a magenta edge on the downward 45% sloping edges.

In the lens correction window there are two sliders under the heading chromatic Aberration. One says Fix Red/Cyan fringe and the other says Fix Blue/Yellow Fringe. The fringe here is more Green /Magenta as I have already pointed out meaning that I am going to need to play around with both sliders to fix the problem. If I just had a blue fringe I could simply move the Yellow/Blue slider towards the blue (left) end and that should fix it but with this mix of colors I have to guess a little as it is not that clear which way to move either set of sliders. To fix this image I needed to move the Red/Cyan slider towards the red end to a value of -23 and the Yellow/Blue slider towards the Blue end to a value of +18.

chromatic aberration fixed

It not perfect but at normal size it's no longer visible. You can further increase the effect by clicking on the drop down box marked Defringe and selecting All Edges.


Vignetting is where some images have dark areas at the corners, usually the top corners, and most often when this includes sky. It is going to be hard for me to demonstrate this to you because I have very little vignetting problem with my lenses mainly because I don't use a lens hood unless I actually have a problem with lens flare. For anyone who keeps their lens hood on all the time vignetting is likely to be a big problem. I do get some faint vignetting on wide angel shots but it is so faint you won't see the difference very clearly on the web. As and when i get a good vignetting demonstration shot to show you I will re-write this section. In the mean time I will attempt to explain it with just text.

You will see there are two sliders, one of which by default is greyed out. the top one is the Amount slider and it is set centrally to '0' by default, no correction being applied. To correct vignetting you need to drag the slider to the right. As you do you will see the corners lighten and you will notice the second slider, Midpoint, becomes active. This slider allows you to adjust the radius of the effected corners. Dragging this slider to the right shrinks the area in the corners that is effected by the correction while dragging it left increases the size of the corner correction. If you wanted to add vignetting to your image for artistic reasons you can do this by dragging the first slider left instead of right to give a minus value. Now the corners darken. All quite simple compared to the Chromatic Aberration sliders.


RAW introduction,

RAW - Camera Settings

Importing your RAW files

ACR Tools and Interface Explained

Creating and Applying Presets

ACR Basics & Exposure Tools

ACR Sharpening & Noise Reduction

ACR Curves tools

HSL/Grayscale tools (Hue, Saturation and Luminance)

Split Toning tools

ACR Top Menu Bar (Crop, rotate and blemis removal etc)

Saving your settings and converting your RAW files

Bridge and Converted RAW Files



Recommended Further Reading for Photoshop

Return to introduction & contents page

"A-Z of Digital Editing"


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