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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
two optical flaws that can no be corrected
are flare and lack of sharpness - it is
important to choose sharp lenses, because
even the powerful sharpening tools of PS
won't turn a super cheap 70-300 f/4-5.-5.6
into a 300 f/2.8 :-)
You
don't need to correct the lens
aberrations on every photo: 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 II and the Canon
24-105 L IS. When CA, distortion and
vignetting are annoying, you can easily
correct them with PS.
Correcting
Chromatic Aberration
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.
18-250), instead, have strong CA, sometimes
even 5-6 pixel wide, that is noticeable even
in small prints.
If your image shows
some CA, this is the first thing that
you have to correct. 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 not as effective. How it
is possible that two similar functions of
the same programs gives such different
results goes beyond my imagination, but so
it is ;-)
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 "Lens Correction" tab: 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.
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 >
Lens Correction (of Filter > Distort > Lens
Correction, if you have Photoshop CS4 or an
older version). 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 may help
to evaluate the amount of 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. 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 not a
big problem, 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). Try different
settings until the vignetting has been
eliminated. It is not always necessary to
eliminate vignetting:
sometimes, I prefer to leave vignetting,
because it can actually add to the mood of
the photo.
Do you have
comments or questions?
If you have comments or questions about
this article, feel free to ask in the
Juza
Nature Photography Discussion Forum!
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