Sky & Telescope - August 2022 - 55

VIGNETTING EXAMPLE: RICHARD WRIGHT; LENS APERTURE SERIES: GARY SERONIK
fied nebula can become too faint to
photograph well without a great deal
of exposure time. To compensate, that
200-mm-focal-length lens would need
twice the aperture (100-mm) to produce
as bright an image as the smaller,
shorter-focal-length lens.
To understand why, we have to consider
the focal ratio (f/ratio) - the relationship
between a lens's aperture and
its focal length. To calculate the f/ratio,
simply divide the focal length by the
aperture. For example, a 100-mm aperture
lens with a 400-mm focal length is
an f/4 optic (400 ÷ 100). Camera lenses
have the ability to vary their f/ratio
with an iris that can constrict or open
up in order to let more or less light in.
" Opening up a lens " to let more light
in is a standard operating procedure
in low-light situations. It shortens the
exposure time, which is why lenses with
wide apertures are referred to as " fast, "
while those with smaller apertures are
called " slow. "
Telescopes and astrographs are
built for a specific, fixed f/ratio and
don't have adjustable f-stops. A 200mm
telescope with a 2,000-mm focal
length (like a typical 8-inch SchmidtChanging
the f/ratio of a camera lens is as
simple as opening and closing a diaphragm
to let either more or less light into the camera.
Stopping down the aperture a bit from maximum
usually results in sharper stars, particularly
in the corners of the frame, but at the cost
of less light reaching the camera's sensor.
Cassegrain) has a focal ratio of f/10. By
contrast, a 200-mm astrograph with a
focal length of 400-mm has an f/ratio
of f/2. Both telescopes collect the same
amount of light, yet the latter presents
a much brighter image for your camera
to record.
How is this possible? Very simply,
you are trading the scale of a larger,
faint image for a smaller, bright one in
which the light gathered by the objective
is more tightly concentrated. The
net result is an astrophoto with less
apparent noise.
Devilish Details
All this might strike you as counterintuitive.
After all, if the amount of
light collected is determined solely by
the aperture of the lens, how can one
optic produce a brighter image than
the other? There are two reasons. First,
more light is being delivered per pixel.
In the f/2 system, much
more of that 200 mm of
light-gathering capacity
is concentrated on
your camera's sensor
compared with the f/10
instrument.
Because faster optics
A telescope or camera lens can only produce a fully illuminated
circle of a specific size. A focal reducer will yield a brighter image,
but it may also shrink the image circle so much that your camera's
entire sensor (indicated by the gold box, above) isn't being fully
illuminated, resulting in the dark corners shown here.
deliver a brighter image,
a popular accessory for
astrophotography is
something called a focal
reducer. This is a lens
that goes between your
camera and telescope
(or camera lens) and
concentrates the light
gathered by your optics
into a smaller area -
effectively reducing
the focal length of the
entire optical train and
speeding up the f/ratio.
For a typical SchmidtCassegrain
telescope
f/1.8
f/4
f/8
f/16
sk yand tele scope .o r g * AUGUST 2022 55
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Sky & Telescope - August 2022

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