"CCD Bloom" or Chromatic Lens Aberrations?

Text and photography copyright Thomas L Webster, 2005. All rights reserved.


In the late 1990s I started digitizing my extensive 35mm transparency and negative files using a Nikon CoolScan III LS-10 scanner. I noticed that many of my images exhibited a purplish or greenish color fringing along edges of high contrast. In researching this issue on the Internet I found information that suggested that this color fringing was caused by "ccd bloom". When pixel receptor sites become over-stimulated due to very bright light levels part of the signal from one receptor site "leaks" to pixel receptor sites around it. If this occurred along an edge of high contrast, the "leak" would cause flare in the darker side of the edge. It somewhat made sense to me at the time but did not explain why there was always the same color shift to purple or green and it did not explain why it did not occur on all of my transparencies and negatives.

During this same time period there was an explosion of digital camera models being offered by the major camera manufacturers. Having tired of scanning negatives and transparencies I began researching digital cameras in earnest. Almost every digital camera that would have met my needs exhibited the same purplish or greenish color fringing as I was receiving from my film scanner. Some cameras exhibited lesser amounts than others. I felt that ccd technology was going to have to advance to the point of eliminating the color fringing before I would buy into digital photography. I kept on scanning with my film scanner and kept getting more and more frustrated.

In 2000 Canon introduced the Canon EOS D30 dSLR followed shortly in 2001 with the pro model EOS 1D dSLR. Keep in mind these were $3000 to $5000 digital cameras, respectively. Surely Canon wouldn't install cmos and ccd sensors that were prone to "ccd bloom", would they? I read every review and looked at every sample image I could find on the Internet. Sure enough, the purple color fringing showed up on many images. Even though I could not afford either camera at the time I was still disappointed to see "ccd bloom". In the meantime, I tired of the mediocre scans I was receiving from the Nikon LS-10 scanner and sold the scanner.

I was very active in Nature Photographers Network when Canon released the EOS D10 dSLR. Everybody who rushed out and bought one just raved about the camera. I was a "Macro and Close-up Gallery" moderator at the time and noticed one of our members complaining about the greenish and purplish color fringing around some highlights in a close-up image he made with the EOS D10 and a 100-400mm L Canon Zoom lens. I can't tell you why it suddenly clicked in my head that, for some reason, I didn't recognize this as "ccd bloom" but more as chromatic aberrations produced by the lens. This was early 2003 and I was becoming reacquainted with microscopy and had just completed putting together a video camera to shoot videos through my microscope. I could also shoot single frames with this setup.

I chose to place a transparency under the microscope lens and image high contrast edges at approximately 200x. The results were startling. The image on the right is a full frame scan of a 35mm transparency, using the Nikon LS-10 scanner, that I had much trouble with the greenish and purplish color fringing along the edges of the cat sculpture. Everywhere there was a high contrast edge, there would be a purplish color fringe and that would be echoed in other areas with a greenish color fringe.

The larger photo on the right points out the color fringing that occurs around the front paw of the cat sculpture. I have observed a distinct pattern to the color fringing. Edges with the highest contrast between dark and light display a purple color fringing. You can see the purplish fringing along the cat paw in shadow and the bright, highlighted edge of the stone the sculpture rests upon. Areas with more modest, but still high, contrast edges display the greenish color fringing as can bee seen along the edge of the cat paw and blue sky.




Now, logic tells me that if the purplish or greenish color fringing is due to "ccd bloom", then images captured on film will not show the color fringing. If color fringing occurs in film images then the source must be under-corrected chromatic lens aberrations. To do determine whether or not I was observing "ccd bloom" or chromatic aberrations introduced by the camera lens, I placed the same area of the transparency under my microscope at 200x. This image from the microscope (at right) confirmed that the purplish and greenish color fringing was the result of under-corrected chromatic lens aberrations and not "ccd bloom". If the color fringing had been caused by "ccd bloom" then the colors would not have been present in the photograph through the microscope. This image was made on Kodak Ektachrome 64 with a Nikon F2 camera fitted with a 105mm f. 4 Micro-Nikkor lens! All these years I was blaming the film scanner!

I am not going to go into the optical physics of chromatic aberrations. Suffice it to say that the vast majority camera lenses are of achromatic design. These lenses are designed to bring two wavelengths of light to a common focus point and are generally adequate for most photographer's purposes. To eliminate or reduce chromatic aberrations, lenses of apochromatic design need to be considered. These lenses bring three or more wavelengths of light to a common focal point. However, these lenses will cost hundreds of dollars more than their achromatic counterparts. Do be aware, too, that some manufacturers will list a lens as being "apochromatic" simply because it is constructed with special high-refractive index glass. This is misleading as it is the actual lens design that determines whether a lens is apochromatic or not.

Current "prosumer" digital cameras offer zoom lenses with remarkable ranges of focal lengths. However, these zoom lenses are the ones most prone to under-corrected chromatic lens aberrations and are the most prone to displaying the purplish and greenish color fringing. Owners of dSLRs will find that the common consumer grade lenses will also exhibit under-corrected chromatic lens aberrations to some degree. It is very expensive to correct chromatic lens aberrations, often involving the use of very expensive glass in the construction of the optical elements. Only the very finest of pro lenses will eliminate the chromatic aberrations but you will pay hundreds of more dollars for the pro lenses.

What is a photographer to do? Determine how much color fringing you can live with in your images and choose your prosumer digital camera accordingly. Look at as many reviews and sample images as possible relating to the camera model in which you are interested in purchasing. Of note is the new Panasonic DMC FZ30 prosumer digital camera. This camera incorporates image processing software that is claimed to eliminate color fringing. For those of you using dSLRs and consumer grade lenses, some third party software manufacturers of image editing software and RAW file converters offer chromatic aberration correction in the software. I have not used these programs, myself, but it may be worth your while to investigate these programs if you have considerable purplish or greenish color fringing in your images.

I want to re-emphasize...Just because a lens has "SD", "HLD", SLD", etc. glass does not mean it is truly apochromatic. This image was made with a Tokina SD 400mm AF lens claimed to be apochromatic. As you can see, the images here display the typical green/purple fringing typical of an achromat lens. If at all possible, test lenses under various lighting conditions before you buy a lens.

The ccd and cmos sensors used in today's digital cameras do not suffer from "ccd bloom". The new chips are well designed to eliminate this issue. If your digital camera is producing color fringing blame the lens and not the camera. Remember, photography should be fun! Enjoy!

Copyright Thomas L Webster, 2005. All rights Reserved.
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