My first Magazine Sky & Telescope - 04.2019 | Seite 64

S&T Test Report
Sony α7 III
with Bright Monitoring ON
Nikon D750
with Exposure Preview ON
Canon 6D MkII
with Exposure Simulation ON
p In images of the same scene off their rear LCD screens, Sony’s Bright Monitoring mode provides live views of the Milky Way, far exceeding the image
detail provided by the Nikon D750 or Canon 6D MkII when in Live View, even with their respective Exposure Preview or Simulation modes turned on.
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In addition, the α7 III can magnify
a star up to 11.7× for critical focusing,
making it easy to nail focus with lenses
or when used on a telescope. noisy than what my Canon 6D, 6D
MkII, and Nikon D750 can achieve. An
example video is on my Vimeo channel
at https://is.gd/aurora_test .
Movie Mode Battery Life
One of the plus points of mirrorless
cameras is that they are, by design, also
movie cameras, relaying a live movie
from the sensor to the preview screens
at all times. There’s no need to lock up a
mirror to enter a special “movie mode.”
As with most mirrorless cameras,
the α7 III offers UltraHD video (3,820
x 2,160 pixels, often called 4K), record-
ing internally to a fast SD card, or
to an external recorder via its HDMI
port. Apart from total solar eclipses,
you might think 4K movies would have
little value for astrophotography. But for
me, this capability was one of the main
attractions of the α7 III — for aurorae.
The α7 III can shoot 4K movies at 24
frames per second using a down-
sampled stream from the full area of
the sensor (unlike some cameras that
crop the frame by up to 1.7× when
shooting 4K). That means I can use the
same fast wide-angle lenses I use for
still images and not lose any of their
wide field of view essential for aurorae.
What’s more, the Sony can shoot 4K
movies with “dragged” shutter speeds
as slow as ¼-second, allowing more
exposure time for each movie frame.
This makes it possible to shoot real-time
movies of aurorae at ISO speeds from
6400 to 25,600.
Even at ISO 25,600 videos look
acceptably clean and are much less One drawback of mirrorless cameras
had been their short battery life, due
to their constant use of power-hungry
“live view” at all times. This was needed
to feed a preview image from the sensor
to either the rear LCD screen or EVF.
In contrast to earlier models of mir-
rorless cameras that required several
batteries to get through a night of
shooting, I found the Sony α7 III about
A PR I L 2 019 • SK Y & TELESCOPE
p Sony’s superb Bright Monitoring option is
extremely well hidden: It can be accessed only
by assigning it to one of the camera’s physical
Custom C buttons using the Custom Key then
Display/Auto Review2 menus.
as good on battery life as any of my
DSLRs. One battery proved sufficient for
2 to 4 hours of time-lapse or deep-sky
shooting. However, when a long shoot
is underway, turning off the rear screen
and EVF as well as WiFi and Bluetooth
functions helps minimize power draw.
Deep-Sky Imaging
Capturing deep-sky objects through
a telescope using exposures of several
minutes is the most demanding task
we can throw at any camera, DSLR
or mirrorless. To use the latter, the
first task is to physically attach the
camera body to the telescope. Like all
Sony mirrorless cameras, the α7 III
uses Sony’s E-mount standard. While
traditional T-rings are available for the
E-mount, mating a mirrorless camera
body directly to some telescopes might
require additional spacers.
The field-flattener lenses required by
many telescopes are often designed to
provide the best correction using the
flange distance of DSLR cameras, which
is about 42 mm for Canons. With Sony
mirrorless cameras the flange distance
is only 18 mm.
For my testing I used a Metabones
adapter, one of many available that
allows me to attach my Canon lenses
to the Sony. It also works to attach the
Sony to any of my telescope-to-camera
adapters equipped with Canon T-rings,
placing the Sony’s sensor at the correct
distance from the field flattener.
Control Options
Being popular for time-lapse photogra-