Posts Tagged ‘Oregon Cascades’

Chromatic Aberration – The Fix

In a previous entry we considered the crop from an image posted on February 6th entry (see below).  Notice the purple fringe around the left side of the rock at the margin between the rock and the snow.  In this entry we’ll examine how the effect can by minimized using Adobe Camera Raw.

Purple Fringe!

We begin by opening the original file in Adobe Camera Raw (ACR).  For Nikon shooters, this means opening the NEF file.  We are using CS4, and the version of of raw conversion software is ACR5.2.  The correction we are interested in is under the Lens Correction section, which is the sixth icon to the right.  A screen shot of the correct panel is shown below:

Lens Correction Panel - Default Paremeters

It is impossible to see the details present on the right hand panel, so here is a zoom:camera_raw_1_zoom

The strategy is to move the slider in the direction of the color of the CA – red in the this case.  To demonstrate the capacity of the software to correct the CA, we’ll move the slider way the wrong direction to +50, then past the proper correction too far towards red to -80, before setting it to the best correction at -28.

Original image: Fix Red/Cyan Fringe slider at 0Original Fringe

Fix Red/Cyan Fringe slider at +50Red Fringe

Fix Red/Cyan Fringe slider at -80Cyan Fringe

Fix Red/Cyan Fringe slider at -28Fringe Eliminated

Notice how the sharpness improves when the CA is properly minimized.  Yes, I know the crops look a little blurry – you’ll have to wait for the sharpening piece, which comes a bit later on.

Chromatic Aberration

Chromatic aberration (CA) consists of two effects.  One manifestation of CA is the residual uncompensated variations in the plane of focus for light impinging on the film plane or sensor. Camera lenses are made up of a series of individual glass elements – a major design consideration is adding compensating elements to cancel out the natural dispersion (spreading out) of light as it passes through the lens elements.  Reducing CA must be balanced against the other design requirements that include brightness, edge-to-edge sharpness, contrast, and bokeh.  There is really no remedy for this sort of lens imperfection.

Red Box Highlight

Consider the image above; which was originally posted in the February 6th entry.  The limited resolution and size of the image prevents you from determining whether there is any CA present.  We’ll expand the image and focus on the region enclosed with the red square as shown below.

Purple Fringe!

Notice the purple fringe around the left side of the rock at the margin between the rock and the snow.  That’s your classic example of CA ladies and gentlemen.  Note that you can also see the effect around a smaller rock near the lower right, and in fact amongst all of the rocks in the lower section of the crop.

Elements of the Workflow – Overview

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I’m always thinking about workflow.  Generating an adaptive and efficient system for processing images.  You’d think with all the books and videos on image processing that this would be easy.  But it’s exactly that there is such a vast variety of images and processing strategies that makes settling on a system the grand challenge.  A constant stream of new tools further complicates matters.

Let’s start with the image above.  Not a world beater by any means, but a pleasant enough shot looking north from Crater Lake towards the southern Oregon cascades on July 29th of last year.  At least we have obeyed the rule of thirds.  The image was made using the Nikon D300 with the AF-S DX Zoom-Nikkor 12-24mm f/4G IF-ED lens at 12mm.  I’m a big fan of both camera and lens.  The exposure was f/16 at 1/250s with an ISO of 200.

I specifically chose this image because it includes many of the elements that challenge my workflow.  Are the colors correct?  The sky is basically blue, but is it really right?  I believe it’s probably too cyan.  Are the trees green?  Maybe a bit too yellow.  Are the snow and rocks neutral?  How can we verify that the colors are correct?  Are there any other problems that might keep this image from being a good as it could be?

Images are first evaluated for artistical potential.  Things that don’t make the grade don’t get processed further.  The initial elements of my current workflow focus on inspecting the image for any defects in the image, which most commonly consist of artifacts due to dust on the sensor (an uncommon occurrence due to the dust reduction system) and color defects of one kind or another.  Color defects include errors in white balance, the influence of reflected light, and chromatic aberration.    The next few entries will cover several workflow issues using this image as an example.