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Correcting
CA, Distortion and Vignetting
Distortion, vignetting, chromatic aberration - all
these aberrations were a serious problem with film. With digital
cameras and Photoshop, they are no longer an issue: you can correct
almost every aberration easily and effectively. The only optical
flaw that can no be corrected is softness - it is important to
choose sharp lenses, because even the powerful sharpening tools of
PS work well only with very slightly soft images, while the
blur caused by bad optics can't be corrected.
That said, you don't need always to correct the lens
aberrations. The majority of professional lenses - e.g. 180 Macro,
600 f/4 - are so good that there isn't any appreciable aberration.
Nevertheless, some optical designs - in particular wide angles - are
particularly complex, and there are often minor aberrations even in
hi-end lenses as the Canon 16-35 f/2.8 L and the Canon 24-105 L IS.
When CA, distortion and vignetting are objectionable, you can easily
resolve the issue with the powerful tool of PS CS2.
Correct
Chromatic Aberration
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The CA is truly annoying: it creates very visible
and "artificial-looking" artifacts. A perfect lens should focus all the wavelengths (i.e. colors)
into the same plane and with the same magnification; if,
instead, the lens focuses the various wavelengths in different
planes of focus, or with different magnifications, it creates
the chromatic aberration. The visible effects of CA are green
and magenta halos around the borders of the objects.
The Chromatic Aberration is the plague of super zoom
and wide angle lenses. Even the best wide angle zooms have a bit of
CA - the Canon 24-105 L is an example of an excellent lens that
shows some CA, at the shortest focal lengths. If the CA is just 1-2
pixels wide (as in the case of 24-105), it is very difficult to
notice it in prints, unless you print really large. The super zooms
(e.g. 28-300), instead, have an huge CA, sometimes even 5-7 pixel
wide, that is very noticeable even in small prints.
If your image shows a little of CA, this is the
first thing that you have to correct. The reason is simple, while paradoxical
: both Adobe Camera RAW (the RAW conversion plug-in of Photoshop)
and Adobe Photoshop itself offer CA correction tools, but the CA
correction of Camera RAW works really well, while the CA correction
of PS is really bad. How it is possible that two similar functions
of the same programs gives such different results goes beyond my
imagination, but so it is ;-)
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| The lens correction tab
of Adobe Camera RAW |
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Double click on your RAW file to open it in the
Camera RAW window. Set all the sliders on neutral values, as
explained in the "Introduction
to Adobe Photoshop" article. To correct CA, click on the
third setting tab ("Lens") : there are two sliders to fix
red/cyan fringe and and blue/yellow fringe. Zoom on the image to
view the detail at 100%, and move the sliders until the CA
disappear. Now, you just have to click on "Open" to open
the CA-free image with Photoshop and to continue with your usual
workflow.
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| This is a detail of the intro
photo, taken with the Canon 17-85 IS. The is a strong
chromatic aberration... |
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| After the correction with
Adobe Camera RAW, the CA is completely gone. |
Distortion
Correction
The majority of wide angle and zoom lenses have some
distortion (straight lines are rendered as curves); in some cases
the distortion can be noticeable, but don't worry: it is
easily fixed.
There are two types of distortion,
"barrel" (typical of wide angles) and
"pincushion" (typical of tele lenses). You can
easily correct even the strongest distortions - it is actually
possible to correct even fish-eye lenses. The price to pay is
a slight loss of sharpness but - except for the most extreme
corrections - this loss is barely noticeable.
Click on the menu Filter >
Distort > Lens Correction. The "Lens
Correction" window has the usual navigation tools, a
preview of the image and many controls. It is possible to
display a grid, that is very useful to find if there is tilt
or distortion.
| The Remove Distortion
tool is very intuitive and easy to use. If the image has
barrel distortion, you just have
to move the slider to the right until the lines are
straight; if it has pincushion, move it to the left. |
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As already said,
this tool doesn't lower visibly the image quality, the
corrected image is almost as sharp as the original one.
The only real issue is that it crops slightly
the photo, usually you lose 10-40 pixels from the borders
(the amount of cropping depends from the intensity of
correction). The cropping is done automatically for barrel
correction, while you have to crop "manually" when you apply pincushion correction. |
Vignetting
Correction
The vignetting consist in a light
fall off towards the edges of the frame.
| If the lens has a
strong vignetting, the corners looks quite darker than the
center of the image. Usually, the vignetting is more visible
in wide-angle lenses and full-frame sensors: that said, the
vignetting is really a non-issue, since it can be fixed very quickly
and easily.
The Vignette tab has
two controls: Amount and Midpoint. Amount
defines how much to lighten the corners of the image; Midpoint
defines the width of the corners (i.e. the width of the
area that will be lightened). |
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| The
image has some vignetting |
The
vignetting is completely disappeared |
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