7

CHAPTER
NIGHT AND LOW-LIGHT PHOTOGRAPHY / The Nighttime Sky

So to get the proper exposure and the effects of

the light trails, the shutter needs to stay open for

a very long period of time and this can cause digi-

tal noise due to the length of the exposure. The

longer the exposure, the more digital noise there

will be. There is another method that you can use

that doesn’t create as much digital noise since

there is no single long exposure, but it does take

more work later in postproduction. This method

is called stacking.

What is stacking?

Stacking is a technique where, instead of one

long exposure, you split the exposure up into

smaller pieces and then recombine them later

using software. This helps to greatly reduce the

digital noise in the image because the sensor is

not subjected to the extremely long exposure

time when the star trails are done as a single

exposure. The idea is simple, but to do it cor-

rectly, you need a programmable interval timer

(intervalometer) set up to take a series of images.

Each of the images will be combined to make a

final single image, and the exposure settings are

worked out the same way as the single image in

the previous section; however, instead of a very

slow shutter speed, the exposure time should be

in the 3- to 5-minute range.

This takes a little math. Each time you halve the
ISO, you let in half as much light, and each time
you double the shutter speed, you let in twice as
much light. Based on starting with an ISO of
1600, moving to an ISO of 100 is four full stops of
light. This means you can take the shutter speed
that gave you the right exposure and double it
four times.

For example, if the shutter speed that gave the
right exposure was 45 seconds, then the equiva-
lent would now be 720 seconds (12 minutes).

Here’s how it works: The ISO goes from 1600 to
800 to 400 to 200 to 100, which halves the
amount of light each time; then you take the
shutter speed of 45 seconds, and you double it
four times: double 45 to get 90 seconds, double 90
to get 180 seconds, double 180 to get 360 sec-
onds, and double 360 to get 720 seconds.

So at this point, you have a shutter speed of 12
minutes (720 seconds) and an ISO of 100.
However, the slower the shutter speed, the easier
it is to get the motion of the stars in the frame, so
it is time to adjust the aperture. Adjusting the
aperture works in the same way as adjusting the
ISO and shutter speed: You take the original
aperture setting and make it smaller, letting in
less light, and decrease the shutter speed to end
up with an equivalent exposure. In this example,
the aperture started at f/5.6. You can halve it to
f/8, then halve it again to f/11, and then halve it
again to f/16. This is another three full stops of
light that you can make up by increasing the
shutter speed. To get the equivalent shutter speed
with the new aperture, you will need to decrease
the shutter speed by three stops. A shutter speed
of 720 seconds doubles to 1,440, then again to
2,880, and again to 5,760 (1 hour and 40 minutes).

The key to stacking is in how the images are pro-

cessed after the shoot. The images need to be

loaded into a software application and combined

Turn off the camera’s long exposure

noise reduction; otherwise, it will

cause gaps in the image because it causes a lag

between exposures.

note