Astronomy

WWW.ASTRONOMY.COM^61

This wide-angle lens will let

you produce high-quality

shots with just a camera and

tripod. by Jonathan Talbot

was extremely easy because the

stars snapped into focus. The

lens does exhibit some chro-

matic aberration. Stars have

bright magenta rings on one

side of focus and slightly green

ones on the other side. This

actually made finding exact

focus relatively easy because

it was the point where the

magenta disappeared.

One of the nice things

about using a wide, short-

focal-length lens is that you

can gather excellent images

just by mounting your camera

on a tripod. No tracking

required! However, you do

need to keep your exposures

relatively short to avoid elon-

gated stars if your goal is a

sharp image (showing the

stars as your eyes see them).

Using the 500 rule (divide

500 by the focal length in

millimeters), gives a maximum

non-tracked time of 25 seconds

using the 5D, and 15 seconds

using the 60Da. Remember,

though, that this rule is a rough

guess, and a test exposure is

good practice to make sure the

star profiles in your exposures

are what you expect.

Even after following these

exposure recommendations, the

stars showed some elongation

when viewed full size, but it

went away and the image

looked sharp when downsized.

These days, with low-noise

CMOS chips and electronics

with extremely high ISO set-

tings, you may be able to get

away with exposures using 400

divided by the focal length.

One issue I dealt with late at

night was dew forming on the

lens. The front of this lens is a

big chunk of glass, and when it

cooled to near the dew point, I

constantly had to use a 12-volt

hair dryer to carefully heat it

up a bit. If you’re going to shoot

in a humid climate, you may

want to invest in a small dew

heater strap to wrap around the

lens cover. (I’ve got several, but

forgot to bring them.) Either

way, a quick shot of heat kept

the lens clear for 10 minutes,

long enough to take some short

untracked exposures.

A bit later the same night,

I decided to take my favorite

shot: a mosaic of the Milky

Way arching across the sky.

The 20mm Sigma did not

disappoint. With hair dryer

in hand, I used the 5D

camera and captured multiple

30-second untracked exposures

at f/3.2 and ISO 1600. With my

geared head and an 85° field of

view, I took six images from

south to north. Then I pointed

the camera high and took six

more in the opposite direction.

Finally, I took six low across

the ground. Six images gave me

about a 20 percent overlap. I

stitched together the 18 images

in Adobe Photoshop using the

“Photomerge” function.

Final thoughts

The 20mm Sigma f/1.4 DG

HSM Art is a well-built and

well-corrected lens. The one

I tested provided the best-

looking stars at f/4, but f/3.2

won’t disappoint if you need

the extra speed. This heavy

lens is built to be solid. If you’re

currently shooting nightscapes

through 14mm or 35mm lens-

es, the 20mm focal length is

one to add to your arsenal. And

if you’re just starting wide-field

imaging, I recommend you

take a close look at Sigma’s

20mm lens.

20mm lens

Sigma 20mm f/1.4 DG HSM

Art Lens

Construction: 15 elements in

11 g r o u p s

Minimum aperture: f/16

Length: 5.1 inches (130 mm)

Diameter: 3.6 inches (91 mm)

Weight: 33.5 ounces (950 grams)

Price: $899

PRODUCT INFORMATION

Contact:

Sigma Corp. of America

15 Fleetwood Court

Ronkonkoma, NY 11779

800.896.6858

http://www.sigmaphoto.com

Jonathan Talbot is an

astroimager and equipment

tester who lives in Ocean Springs,

Mississippi.

The photographer titled this image “Comet Storm.” He used the Sigma 20mm f/1.4 lens at f/2 coupled to a Nikon

D810 camera. He shot fifty-seven 30-second exposures at ISO 1600, then used stacking software and created the

cometlike stars by reducing the opacity for each exposure by 2 percent. DARREN WHITE