Sky & Telescope - June 2022 - 54

ALL IMAGES COURTESY OF THE AUTHOR
JUNE 2022 OBSERVING
First Exposure by Ade Ashford
An Astrophotography Jargon Buster
Don't get tangled in the pursuit's techno-lingo.
A
stronomy and photography are
both topics laden with complex
concepts and technical terms, so it's
not surprising that combining the two
can lead you into a swamp of jargon.
Fortunately, however, many readers
have backgrounds that afford a certain
level of familiarity with optics and cameras.
Perhaps you're a budding nature
photographer equipped with an interchangeable
lens camera and now wish
to capture starry vistas over terrestrial
landscapes. Or maybe you're an experienced
amateur astronomer wanting to
record what you see in your telescope.
But whichever route led you to consider
astrophotography, you're going encounter
some basic terms, many of which
may be unfamiliar. Let's have a look at
some of the more common ones you'll
run across as you begin to capture the
night sky.
Lenses by the Numbers
Look closely at the barrel of a camera
lens and you'll see several numbers,
such as 50 mm, 1:1.4, or perhaps
200 mm, 1:2.8. These are also often
engraved 1.4/50 mm or 2.8/200 mm.
The larger number indicates the lens's
focal length in millimeters. This is the
distance from the lens to the image
it forms. So-called prime lenses have a
single, fixed focal length - 50 mm or
200 mm in the examples given here.
Lenses are often described as wide angle
(with focal lengths of 35 mm or less),
standard (35 mm to 85 mm), short telephoto
(85 mm to 135 mm) and telephoto
(135 mm and greater).
You'll also encounter zoom lenses of
variable focal length, say, 28-300 mm.
Chances are your camera came with a
zoom lens, but for shooting the night
sky a prime lens is generally preferred
because the optical quality is usually
better. Regardless of design, a lens's
focal length governs the amount of sky
you can record with your camera - the
field of view. The longer the focal length,
the smaller the field of view.
Unlike your telescope, most photographic
lenses possess an internal
diaphragm or iris that can open or close,
much like the pupil in your eye. This
mechanism regulates the amount of
light entering the camera by changing
the effective aperture of the lens. And
if you divide a lens's focal length by its
aperture, you'll get what's known as
the focal ratio, or f/ratio - just like you
would with a telescope. However, with
a camera lens you don't have to do any
math since you can choose the focal
ratio by turning the aperture ring.
Looking at your lens, you'll notice a
The author captured this view of Cygnus and Lyra rising with a single, 60-second exposure
made with a cropped-sensor Canon EOS 550D camera and 24-mm lens riding on a Sky-Watcher
star tracker mount. Vega is the bright star on the right side of the frame, while Deneb and the pale
red glow of the North America Nebula are visible just above the treeline.
54 JUNE 2022 * SKY & TELESCOPE
sequence of numbers around the lens
barrel, often ranging from 1.8 to 16 or
higher. These are your aperture, or f-stop
settings, as photographers like to call
them. Each successive number represents
a decrease in aperture (and image
brightness) of half that of the preceding

Sky & Telescope - June 2022

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