What can ruin an image?

Of all the things that can ruin your image are camera shake, barrel distortion and chromatic aberration to name a few. In this article chromatic aberration is discussed from a practical point of view. If you want know the technical definition and nitty gritty of chromatic aberration, please "google" it and you have more heavy literature on the subject that you care to read. This article is attempts to basically explain it what to do about it when an enlargement shows a certain fuzziness or loss of sharpness.

 

What is Chromatic Aberration?

.In optics, chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens) causing longitudinal and lateral chromatic aberration of a lens seen as "fringes" of color around contrasting parts in an image.  Each color in the optical spectrum cannot be focused at a single common point on the optical axis.

Since the focal length of a lens is dependent on the refractive index, different wavelengths of light will be focused on different positions. Chromatic aberration can be both longitudinal, in that different wavelengths are focused at a different distance from the lens transverse or lateral, in that different wavelengths are focused at different positions in the focal plane (because the magnification of the lens also varies with wavelength).

The use of hybridized glass (fluorite) in the manufacture of lens glass has reduced this phenomena a great deal. Lenses with a shorter focal length are more prone to exhibit this problem than longer focal lenses.

So, in the nut shell, I am saying it is a lens problem. Not a camera or user problem. It is however a "what you photograph" problem complicated by light and contrast. Don't bother looking for it the view finder because the problem is simply elusive.

Where or when does it show?

f you are a serious digital amateur photographer and do large image printing, you may have noticed the presence of a colored edge between two contrasting colors. This narrow band can have different colors like red, cyan, blue or yellow  and is referred to as red, cyan, blue or yellow chromatic or color fringing. This fringing is not noticeable in the smaller prints like the 4 x 6 prints but becomes noticeable when enlarging to 8 x 10 and larger. It sure becomes a problem when getting into the 12 x 18 and up.

 

When to expect color fringing or chromatic aberration

Images taken with a light background and a dark foreground or visa versa with clean line of separation between the two zones are susceptible to this phenomena and occurs more frequently around the edges of the image frame in wide angle shots. Examine the image example 1 and notice the white area on top of the stairway to the left of the door way and the stairway baluster. That section of the archway separating the dark from the light is an area where chromatic aberration or color fringing can be expected. .

	The image to the left is a RAW image as it came of the camera. Notice the over-exposed area in the center causing loss of detail bordered by the much darker arch. A classic invitation for color fringe caused by lens chromatic aberration.
Example 1 - Click to enlarge

The enlargement in example 2, cropped @ 400%, shows the red fringe at the edge of the arch which is a definite problem when enlarging the image. This phenomena is often found where light meets dark. The color of the fringe normally referred to as chromatic aberration, can be either red, cyan, or purple and sometimes blue and yellow. In black and white images it usually show as a grayish/white band. It rarely is visible in

Example 2 - Click to enlarge

snapshots but sure can be seen in an enlargement.

Since it is invisible at the moment the image is shot it can be anticipated and therefore reduced or eliminated with photo editors like Photoshop or DxO to name a few. Chromatic aberration can soften (reduce detail and sharp edges) color as well as black and white images and editing the image for this problem only improves the image. Editing color fringing is usually done on an image by image basis and most photo editors use Photoshop where the correction is available via Filter -- Distort -- Lens Corrections -- Chromatic Aberration. Plus/minus sliders control the amount of correction applied. See below, top left where two slider control Red/Cyan and blue/yellow fringe amounts. Color fringing is best visible at 400% enlargement or at actual pixels. depending on what color the fringing is, use the appropriate editor adjustment (slider adjustment) to remove or reduce the fringing to a minimum.

DxO chromatic aberration correction depends which DxO version one has. below is a screen print of DxO version 5.00. The chromatic aberration adjustment in located to the bottom right in the Preset Editor. Clicking on the chromatic aberration label or button creates a drop-down box showing the types of adjustment available. The selection box shows Purple Fringing and Lateral chromatic Aberration. Slider, controls adjustment intensity and pixel width adjustment. Why DxO emphasizes purple fringing and tries to control all other color fringing with the lateral adjustment I can not explain however, it seems to work. The DxO editor/processor has the benefit of processing any number of suspected images for only this problem or a number of corrections.

 

The example images below where edited with DxO however, editing the same images with Photoshop netted the same results. The image to the right (#3) is the same as #1 but after color fringe removal. Click on the image and notice the absence of the red fringe in the arch area where light meets dark.  For close examination, click on example #4 and compare it to example #2.
	Image 3 - Click to enlarge
Image 4 - Click to enlarge	Image 5 - Click to enlarge

 At 400% enlargement, the absence of the red fringe line is very noticeable. After additional editing with Photoshop CS2 to correct some Shadow/Highlight adjustments we now have an image worth printing. Click on mage #5 to show the final product.