60 ASTRONOMY • JULY 2018
This lens’ maximum aper-
ture comes at a focal ratio of
f/1.4, so it’s extremely fast and
lets in lots of light. This fast
ratio is perfect for capturing
objects in the dark night sky.
Other specs include autofo-
cus utilizing an ultrasonic
motor, nine rounded aperture
blades, lens mounts for various
camera manufacturers, and
super multicoatings to reduce
lens f lare and ghosting. The lens
also has a built-in hood with a
lens cap and a carrying case. But
it’s not light: This lens weighs a
hefty 33.5 ounces (950 grams).
Short focal lengths make
shooting nightscapes and Milky
Way panoramas easy. They also
minimize the need for a track-
ing mount. I have previously
shot nightscapes with 14mm
f/2.8 and 35mm f/1.4 lenses
attached to my Canon 60Da and
Canon 5D cameras, respectively.
The 20mm lens, falling in
between the focal lengths of
those other two lenses, offers a
slightly different perspective.
I used both of my cameras to
evaluate the Sigma lens. My
Canon 5D, which uses a full-
frame sensor, achieved a field of
view of nearly 83.5° and a reso-
lution of 85" per pixel. The
Canon 60Da, with its APS-C-
size sensor, had a field of view of
58° and a resolution of 44" per
pixel. With a 1.6x crop factor,
the field of view through the20mm lens was similar to using
a 32mm lens on the 5D.Performance
The 20mm feels well built.
It is definitely heavier than
other short-focal-length lenses
I’ve used, but in my hand the
camera and lens didn’t feel out
of balance or unwieldy. The
manual focus ring is buttery
smooth. I prefer one that is a
tad stiffer, but it worked excep-
tionally well. The lens came to
focus very close to the infinity
mark on the lens distance scale.
The 20mm Sigma Art lens
has all the hallmarks of well-
built optics. It excels when it
comes to sharpness, even with a
wide-open aperture. As you’d
expect, stars in the extreme cor-
ners are not well corrected when
the lens is wide open at f/1.4,
and they exhibit significant
coma. Near the center of the
lens, however, it’s a different
story. The 20mm Sigma is a bit
sharper than anything I’ve used.
It’s normal practice when
using a camera lens for astroim-
aging to stop it down to get well-
corrected stars in the corners of
the image. The 20mm Sigma
offers superb correction at f/4
and is easily usable at f/3.2. At
f/2.8, the corner stars start to
show elongation, and at f/1.4
coma is evident.
Focusing using the “Live
View” capability of the 60DaI recently tested the astroimaging capability of the
20mm Sigma f1.4 DG HSM Art lens with my two
DSLR cameras. Released in fall 2015, this Sigma lens
has 15 elements in 11 groups. The optics contain
two F low-dispersion (FLD) elements and five
special low-dispersion (SLD) elements to specificallyIt takes a tackle the pesky problem of chromatic aberration.
lot of high-
quality glass
to create an
optical layout free
of aberrations. For the
20mm, Sigma arranged
15 elements in 11 groups.
COURTESY OF SIGMA CORP.
Sigma’s 20mm f/1.4 DG
HSM Art lens is 5.1 inches
(130 mm) long and
weighs 33.5 ounces
(950 grams). COURTESY OF
SIGMA CORP.