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.
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.
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.
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.
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.
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.
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!
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
"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.
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
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.
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.
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!