Sky & Telescope - January 2022 - 29

for preventing star trails, freezing the motion of an active
aurora display, or recording a fleeting meteor. (Photographing
meteors is particularly at the mercy of your lens speed.)
Attempting those subjects using typical kit zooms with f/5.6
focal ratios usually yields dark, disappointing results or
excessively trailed and noisy images. And no meteors!
Aberrant Behavior
Of course, there are downsides to photographic speed. The
first is cost. Fast lenses are more expensive, though selecting
a no-frills, manual-focus lens somewhat eases the price pain.
The second downside is aberrations. Stars, which offer the
most demanding test of a lens, can look bloated and distorted
in a fast lens used " wide open " at its maximum aperture.
Spherical aberration enlarges star images across the frame,
turning them into soft blobs. Coma turns stars at the corners
into comets; tangential astigmatism turns stars into radial
streaks, while sagittal astigmatism distorts stars into concentric
arcs. Longitudinal chromatic aberration adds blue or
magenta glows around stars everywhere in the field, while
lateral chromatic aberration spreads stars into tiny rainbows at
the corners. As a general rule, the faster - and cheaper - the
lens the worse the aberrations, with most lenses showing a
mix of imperfections.
An ill effect even the best lenses can exhibit is vignetting.
The corners look dark, sometimes by as much as two to three
f-stops dimmer compared to the center. Like chromatic aberrations
(but not spherical, coma, or astigmatism), vignetting
can be corrected in raw-image processing.
However, if the corners of an image are too dark out of
camera, then boosting their brightness in post-production
often reveals ugly noise, magenta tints, and banding artifacts.
Vignetting is generally worse the faster and wider the lens.
Manual vs. Auto Focus
For nightscapes, lenses with
14-mm, 20-mm, or 24-mm
focal lengths are the most
useful, certainly on full-frame
cameras. When shopping for
one you'll encounter a bewildering
selection of lenses
within that range of focal
lengths. For example, you can
find 14-mm f/2.8 lenses, a
favored size, selling for as little
as $250 to as much as $2,000.
EDGE ABERRATIONS This image
series compares the extreme upper
left corner of a premium Sigma 14mm
Art lens at its maximum aperture
of f/1.8 and stopped down to f/2.8
and f/4. Astigmatism and vignetting
improve markedly at f/2.8, less
so at f/4. Most fast lenses improve
similarly when stopped down.
VIGNETTING CORRECTION Left: The Laowa
Venus Optics 15-mm lens at f/2 displays strong
vignetting. Right: Corrections applied automatically
in Adobe Camera Raw eliminate the vignetting. For
lenses not in the program's database, users have to
dial in such corrections using the Vignette slider.
The most affordable lenses, often costing
less than $600, will be fully manual
models. You have to focus and set the aperture
by hand. That's just fine. While the newest cameras are
capable of autofocusing on stars and dimly lit landscapes, at
night the best method is to manually focus on a star at high
magnification using the Live View feature in your camera to
zoom in on a star.
The lowest-cost manual lenses offer no electrical communication
with the camera. The camera doesn't know what
lens is attached nor its aperture. That's a slight inconvenience
when documenting what gear and settings you used.
Without lens correction
With lens correction
Sigma 14-mm Art
at f/1.8
Sigma 14-mm Art
at f/2.8
Sigma 14-mm Art
at f/4
sk yand tele scope .o r g * JANUARY 2022 29
ALL IMAGES COURTESY OF THE AUTHOR
https://skyandtelescope.org/

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
Sky & Telescope - January 2022 - Contents
Sky & Telescope - January 2022 - 3
Sky & Telescope - January 2022 - 4
Sky & Telescope - January 2022 - 5
Sky & Telescope - January 2022 - 6
Sky & Telescope - January 2022 - 7
Sky & Telescope - January 2022 - 8
Sky & Telescope - January 2022 - 9
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Sky & Telescope - January 2022 - 28
Sky & Telescope - January 2022 - 29
Sky & Telescope - January 2022 - 30
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Sky & Telescope - January 2022 - 84
Sky & Telescope - January 2022 - Cover3
Sky & Telescope - January 2022 - Cover4
Sky & Telescope - January 2022 - SA1
Sky & Telescope - January 2022 - SA2
Sky & Telescope - January 2022 - SA3
Sky & Telescope - January 2022 - SA4
Sky & Telescope - January 2022 - SA5
Sky & Telescope - January 2022 - SA6
Sky & Telescope - January 2022 - SA7
Sky & Telescope - January 2022 - SA8
Sky & Telescope - January 2022 - SA9
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