Sky & Telescope - April 2025 - 56

APRIL 2025 OBSERVING
First Exposure
MAGNIFICENT MILKY WAY This untracked,
13-second exposure taken at Mather Point in
Arizona's Grand Canyon National Park shows
some slight trailing, which doesn't really diminish
the appeal of the photo. Some distortion is
visible in the corners due to shooting a Sigma
20-mm f/1.4 Art lens wide open. Still, even with
these minor shortcomings, it's a photo most
people would be happy with.
sor in no way, shape, or form alters
the focal length of your lens - it only
changes the field of view, that is, how
much sky you capture. In fact, it's the
size of the pixels on your camera's sensor
that really matters; the larger the
pixels, the more time it takes for a star
to traverse enough of them to register as
a short streak. In other words, the Rule
of 300 works because of smaller pixels,
not the the overall dimensions of the
sensor. Geometry - it's a thing.
While it's true that typical fullThe
Rule of . . . Whatever
Chances are, when you first learned
to ride a bicycle, you took advantage
of training wheels to ensure that you
didn't tip over when you lost your
balance. Training wheels are great for
building confidence. When attempting
anything new, it's helpful to make
things as easy as possible as your skills
improve. And that's true whether you're
learning to ride a bike or taking photos
of the night sky.
The Rule of 500 is one set of training
wheels often bandied about for
astrophotography. The rule simply
states that by dividing 500 by the focal
length of your lens you get the number
of seconds you can expose with
a stationary camera before star trails
start to appear. This means that for
a 20-mm wide-angle lens fitted to a
full-frame camera, you should be able
expose for up to 25 seconds (500 ÷ 20).
With so-called crop-sensor cameras,
the guideline is often modified to the
AVOIDING STREAKS Exposing too long
yields tiny dashes (top image) instead of dots
for stars. On a camera viewscreen, these little
streaks can create a misleadingly bight and rich
image. It's only when you inspect the photo at
high magnification or on a computer monitor
that you'll discover your error.
56 APRIL 2025 * SKY & TELESCOPE
Rule of 300, ostensibly because of how
a crop factor increases a lens's effective
focal length. The Rule of 300 dictates
shorter maximum exposure times. For
example, the same 20-mm lens would
now yield a maximum exposure time of
only 15 seconds.
Okay, let's pause here for a moment
to address an annoyingly persistent
myth. The size of your camera's senframe
sensors once had larger pixels
than their crop-frame cousins, that's no
longer the case. Today, cameras sporting
the latest full-frame sensors have much
smaller pixels than they did a decade
ago. And so, the Rule of 300 is really the
one to use as a starting point regardless
of which camera model you have.
Training wheels certainly have
their place. I helped a friend who was
teaching a Milky Way photography
class at the Grand Canyon Star Party
a few years ago. We had some students
who just needed a few tips to get up
and running, but others who needed
more help. " How do I set the exposure
time on my camera again? " There's no
shame in this - everyone starts somewhere,
and rules of thumb are a great
aid. But eventually you reach the point
where your equipment is so familiar
that you can operate it with your eyes
closed (or in the dark). So, when you're
ready to take off the training wheels
and move beyond basic guidelines, you
can refine your approach.
Losing the Training Wheels
The Rule of 300 is useful, but applying
it precisely is a bit pointless. That's
because stars don't all move at the
same rate across the sky. How's that? If
you remember what star-trail pictures
Sky & Telescope - April 2025

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