Sky & Telescope - April 2023 - 55

article: Be careful not to breathe on
your camera's lens while making any
adjustments. The warm, moist air you
exhale might condense as dew on the
front element of the lens, spoiling your
images. (That's the voice of experience
speaking here.)
Long-focal-length lenses often
display mild chromatic aberration (color
fringing) around bright stars. These
halos will change from magenta to teal
as you pass through the point of best
focus. Ultimately, your goal is to minimize
the size of the star and any associated
color fringing. Sometimes the best
setting is a compromise between a tiny
star and one that's completely free of
chromatic aberration.
Turbulence in the atmosphere can
cause stars to appear to " twinkle, "
making it more difficult to evaluate
your progress. In such conditions, avoid
aiming near the horizon, where the
twinkling effect is most extreme, and
instead aim at a star near the zenith.
Open Wide
If you're working with the " kit " zoom
lens your camera came with, consider
upgrading to a fast prime (fixed-focallength)
lens with a wide aperture. Such
lenses usually produce brighter images,
which are easier to evaluate for sharpness.
Keep in mind that each decrease in
f/stop doubles the amount of light reaching
the camera's sensor. For example, an
f/1.4 lens gathers 16 times more light
than one operating at f/5.6. (Turn to page
54 of the October 2022 issue for recommendations
on upgrading your lens.)
Fast primes aren't cheap, but they
often feature better optics and mechanics
that enable more precise focusing.
Inexpensive lenses may exhibit a slight
amount of image-shift as you rotate the
focus ring back and forth, which turns
makes the entire process more challenging.
The focus throw (the amount you
have to turn the focus ring to go from
closest focus to infinity) will determine
how precisely you can adjust the lens.
High-quality models with longer focus
throws are generally easier to focus. Of
course, it's not impossible to achieve
good results with a kit zoom - you just
need to work a little bit harder.
Going Remote
Even with a high-quality lens, the relatively
small size of your camera's rear
display will limit your ability to judge
focus. Worse yet, if your camera lacks
BIG-SCREEN VIEW The screen of an iPad
tablet dwarfs the display of this Canon camera.
Using Live View on a larger display makes
obtaining perfect focus a snap, and since the
connection is wireless the display can be positioned
at a comfortable angle and away from
the camera.
an articulated screen, you'll probably
end up having to contort yourself into
some weird and uncomfortable positions
just to view it. This is especially
true when shooting objects high overhead
near the zenith.
Luckily, many cameras are capable
of wireless control, allowing you to take
advantage of the larger screen on your
mobile phone or tablet. For example,
Canon's Camera Connect app allows Live
View and focus with iOS and Android
MILKY SIGHTS Left: This twilight view of the Milky Way was recorded in a 2-minute exposure made with a Canon 70D DSLR camera set to ISO
1600 and a 15-mm fisheye lens at f/2.8. Wide-angle optics such as this are perfect for beginning astrophotographers because such lenses are more
forgiving of slight focus errors than longer focal lengths are. Right: Poor focus can be masked by low-quality optics, unsteady seeing conditions, or
even passing clouds. That was the situation when this 2-minute exposure of the North America Nebula was made using an 85-mm lens.
sk yand tele scope .o r g * APRIL 2023 55

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Sky & Telescope - April 2023

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