Sky & Telescope - December 2021 - 54

ALL IMAGES COURTESY OF THE AUTHOR
DECEMBER 2021 OBSERVING
First Exposure by Richard S. Wright, Jr.
Understanding ISO
Producing high-quality astrophotos with a digital camera
means understanding a few key parameters.
B
efore digital photography, ISO
signified the sensitivity of chemical
films. A film with a higher ISO rating
has larger photosensitive crystals that
are more sensitive to light. Many useful
best practices evolved around this fact,
incorporating what is called the exposure
triangle: exposure time, lens aperture,
and ISO. These are used to determine
the optimal camera settings to get
a properly exposed image. Increase the
ISO and you can decrease the exposure
time or reduce the aperture.
The whole world of photography
once revolved around the exposure triangle,
and millions of people captured
excellent images under many different
lighting conditions thanks to a thorough
understanding of the relationship
54 DECEMBER 2021 * SKY & TELESCOPE
among ISO, aperture, and exposure
time. Let's set aside the hoops astrophotographers
had to jump through to
gain some additional film sensitivity for
lengthy, low-light exposures and skip to
the part where digital cameras showed
up and changed everything.
Unlike a film camera, for which you
can choose from a variety of films with
different ISO ratings, a digital camera
has an innate, fixed sensitivity, and
there's nothing that can be done to
make its sensor more or less responsive
to the light reaching it. However, you
can still utilize the beloved exposure triangle
to produce appropriately brighter
or darker images, because a digital camera
can simulate ISO changes through
digital scaling - multiplying or dividing
For nightscape images like this one, you can
usually get good results that are easy to process
by shooting with an ISO setting of around
1600 or 3200. If you have an older digital camera,
avoid using the lowest ISO settings, which
often produce noisy low-light images.
the numeric values of the pixels that
make up an image. That's right, in
today's digital cameras, ISO is fake.
If you shoot in RAW mode (as
opposed to JPEG), you can mostly
retain the ability to simulate these ISO
adjustments yourself after you capture
the image - just use your favorite
image-editing software. This capability
might lead you to believe that it doesn't
matter what ISO you shoot. Not quite.
There's a fly in the ointment, and that
fly is called noise.
Know Your Enemy
Let's take a step back to consider how a
digital camera works. Light striking the
sensor is converted into a small electrical
charge, which is then converted
into digital signal and processed into

Sky & Telescope - December 2021

Table of Contents for the Digital Edition of Sky & Telescope - December 2021

Contents
Sky & Telescope - December 2021 - Cover1
Sky & Telescope - December 2021 - Cover2
Sky & Telescope - December 2021 - 1
Sky & Telescope - December 2021 - Contents
Sky & Telescope - December 2021 - 3
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