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CAPTURING THE UNIVERSE
tools And rules For
getting the right exposure
Watch your histogram display to get better deep-sky photos text and photography by tony puerzer
W
hen it comes to making detaiL-rich photos of the night
sky, one of the most basic—and important—questions is, how long should
the exposure be? you might expect there to be a single correct answer, but
that looks about right. although “chimping” (as it’s known) gives you a ballpark sense of
which exposures work, your dark-adapted eyes can be tricked by the camera’s bright display.
as a result, you’ll likely end up choosing an image that’s badly underexposed. a much
better way to evaluate the overall exposure of a digital image is to use the camera’s histogram
display. (check your camera’s manual for instructions on how to enable this feature.)
the histogram graphically shows the number of pixels at each brightness level—
because sky conditions and the types of equipment used all play a role, the best way to
answer this question is with some experimenting. a good place to start is by making a
series of test shots with your camera set to iso 1600 and your lens wide open. start with
a 30-second exposure, and work your way up to several minutes. but how do you decide
which of your test exposures is the best? the key is something called a histogram.
it might be tempting to simply view each image on the camera screen and pick the one
16 SKYNEWS • JANUARY/FEBRUARY 2017
NORTH AMERICA NEBULA the author took
this image using an astro-modified canon eos
60d dslr camera at iso 1600 with a canon
300mm telephoto lens at f/4 and an Astronomik
cls light-pollution filter. the 5-minute exposure
provided a great starting point for some light
processing in Adobe photoshop lightroom. the
histogram on the back of the camera, above,
shows what a well-exposed, unprocessed deepsky
image should look like. note that the hump,
indicating the tone of the image, is positioned
away from the left edge of the graph.
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