My first Magazine Sky & Telescope - 04.2019 | Page 63

can also connect DLSR lenses from
just about any brand, preserving your
investment in existing lenses.
For all types of photography, a major
mirrorless advantage is that the preview
image you see on the rear LCD screen
or in the eye-level electronic viewfi nder
(EVF) closely matches the image you’ll
shoot because it is the image you’re about
to shoot. That’s because the preview
comes directly from the sensor that will
take the image. As described below, the
Sony I tested can also provide a live sim-
ulation of what a long exposure will look
like, making it possible to compose night
scenes more easily than with a DSLR.
Mirrorless cameras also offer benefi ts
in improved auto focus, in-body image
stabilization, high frame rates, and
silent shooting — all great for sports,
weddings, and other normal fi elds of
photography. But are they better than
DSLRs for astrophotography?
To explore the question, I purchased
Sony’s new and highly acclaimed α7 III,
the third generation of their 24-mega-
pixel, entry-level mirrorless camera.
With a sensor the size of 35-mm fi lm,
the α7 III is a “full frame” mirrorless
camera. I tested many key traits, includ-
ing a few only astrophotographers will
ever notice.
Live View
p The camera accepts E-mount lenses and
requires less back-focus on most telescopes,
permitting the use of additional accessories in
the optical path, such as an off-axis guider.
smaller 5.7-micron pixels. The same was
true when comparing long exposures
taken through a telescope.
The Sony offers a back-illuminated
(BI) sensor which, in theory, should
reduce noise even more. However, at the
usual ISO settings we use for astropho-
tography, I found it did not signifi cantly
outperform the four-year-old Nikon
D750, which also uses a Sony (though
non-BI) sensor.
That said, the α7 III provides noise
levels as low as you’ll fi nd on today’s
consumer cameras, with the exception
of Sony’s own α7s Mark I and II models
with their detectors’ larger 8.4-micron
pixels, which are optimized for low-
light videography.
In astrophotography, focusing is usually
done manually, using the camera in Live
View mode. You zoom in on a star and
focus to make it as small as possible.
With DSLRs, framing is usually done by
looking through an optical viewfi nder.
Seeing anything at night can be tough,
requiring trial-and-error shots to com-
pose a scene.
Mirrorless cameras don’t have opti-
cal viewfi nders. Instead, the Sony α7
III offers a “Bright Monitoring” mode
that electronically boosts the image in
its EVF or on the rear screen. This works
so well with fast, wide-angle lenses that
you can see the Milky Way “live.” On
a fast telescope, you can even see the
brightest deep-sky objects without hav-
ing to take framing shots!
To access this option, go under
Camera Settings 2 > Custom Operation
1 > Custom Key, and select one of the
Custom “C” buttons (I put it on C2).
Scroll to Display/Auto Review2 > Bright
Monitoring, a well-hidden choice that
appears only here under the Custom
button assignments, not in the main
menus. It’s worth the hunt, as Sony’s
Bright Monitoring is far more effective
at emulating a long exposure than the
“exposure simulation” modes of my
Canon and Nikon DSLRs.
Noise
While noise performance usually
improves with each new generation of
cameras, it is the size of the sensor’s
photosites, or pixels, that determines
overall noise levels — the larger the
pixels, the lower the noise. The 6,000
× 4,000-pixel sensor of the α7 III has
6-micron-square pixels, comparable
to other full-frame cameras in the
24-megapixel class.
As such, I would expect the Sony to
exhibit noise levels comparable to its
competitors, and that’s what I found.
I shot nightscape images with 10- to
30-second exposures over a range of
typical ISO settings from 800 to 6400,
pitting the Sony against a 24-mega-
pixel Nikon D750 and a 26-megapixel
Canon 6D MkII. Images from the three
cameras looked similar, with the Canon
showing slightly more noise due to its
Sony α7 III
Nikon D750
Canon EOS 6D MkII
p This comparison shows the noise levels in a typical moonlit nightscape, all recorded at ISO
3200 with three competitive cameras: the Sony α7 III, Nikon D750, and Canon EOS 6D MkII. The
24-megapixel Sony and Nikon are similar for noise, while the 26-megapixel Canon appears slightly
worse due to its sensor’s smaller pixels.
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