By
adding a night vision module to a digital
SLR camera, night turns into day. Follow
these guidelines to assure the best possible
image quality.
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Today's
digital SLR cameras offer the photographer full control
of aperture, shutter speed and ISO settings enabling the
capture of excellent usable images in low light that were
not otherwise possible. However, in many situations such
as photographing scenes at night with very little ambient
light, or telephoto photography at night at a distance,
even the best digital SLR cameras simply do not have sufficient
sensitivity to capture adequate images. In these situations,
night vision modules such as those described here are the
ideal accessory. This article provides important guidelines
for producing awesome night-time photos when using a night
vision module-enabled digital SLR camera.
Because of the increased availability
of high performance digital SLR cameras, capturing
excellent photographs at night is now easier than ever.
With full control of aperture and shutter speed settings
as well as electronic gain (ISO), the photographer
has the opportunity to capture images in low light
that were not otherwise possible.
However,
in many low-light and night-time situations, digital
SLR cameras simply do not have
sufficient sensitivity to capture adequate images.
For one thing, with the ever-decreasing size of pixels,
it’s remarkable that light sensitivity has not
substantially worsened with each new generation
of camera (since light sensitivity is directly proportional
to detector pixel area). But, fortunately, there’s
been a lot of camera development on noise reduction
so sensitivity has kept pace if not improved slightly
with the decreasing pixel area. Regardless of these
changes, it remains that a sufficiently long exposure
time cannot be used because either there is movement
in the scene or the camera is moving (by being handheld
or on a vehicle in motion) so long exposure times would
result in blurring. For situations such as photographing
scenes at night with very little ambient light, or
telephoto photography at night at a distance, even
the best digital SLR camera will be unable to produce
adequate photographs without blur.
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In
these situations, a
night vision module such as those described here are
the ideal accessory (www.nightvisioncameras.com). Such a device fits between the SLR objective lens and
the
camera body and amplifies the light that is captured
by the objective lens, projecting an amplified (but
entirely green) image onto the digital camera’s
image sensor. The result is up to 10 F-stops of improvement,
a dramatic change enabling many applications that are
otherwise impossible to photograph. The module transforms
moonlit or starlit scenes into bright, high resolution
images that are easily photographed. (See
box below on How
Night Vision Works).
Below are some guidelines to help assure
that the Night Vision digital photos are the best possible.
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Exposure Mode
 Select
Manual exposure mode (“M”). Not “Automatic”, “Aperture
Priority”, “Shutter Priority”, “Program” or
any of the other special modes. In low light,
you’ll want to manually open the lens
iris to permit the most light to fall on the
night vision module’s sensitive photo
cathode.
Shutter Speed Setting
You’ll want to set the camera’s
shutter speed so that it is just long enough
that there will be no blur due to motion, usually
about 1/30th second. Longer exposure times
will usually result in blur (for handheld applications).
Shorter exposure times may provide some improvement
if excessive motion is a concern but could
unnecessarily darken the image due to the reduction
of light being acquired from the night vision
module’s image intensifier. While the
output brightness from an image intensifier
increases with increasing scene illumination,
at higher scene illuminations, the image intensifier’s
output brightness reaches a maximum value and
remains constant as shown in the accompanying
chart. (The limitation on the output brightness
protects troops wearing night vision goggles
from being blinded when viewing a bright light).
Since this maximum brightness is not very bright
(about 2 foot lumens), it is unlikely that
the digital camera’s sensor will be saturated
as a result. So, faster shutter speeds are
not necessary as a method to limit the light
accumulation.
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ISO Setting
 While
back in the days of photographic film, ISO was
a characteristic that described film sensitivity
to light, today, ISO is related to electronic gain
of the digital camera’s sensor. Since increasing
the gain will amplify the low light level images,
even with a night vision module, lower light scenes
will become visible. However, there are disadvantages
to setting the ISO too high. As with all electronic
circuits at high gain, image noise can degrade
image quality. As such, set the camera’s
ISO to a value as high as you can while still acceptable
image noise levels. Usually, the minimum value
would be 800, but some cameras deliver perfectly
usable images at ISO 1600, 3200 or even 6400 (on
some recent Nikons).
Manual Focus
 When
using a night vision module, select Manual Focus
mode (usually a small lever switch on or near the
objective lens mount). Manual focus gives you greater
control over how the focus appears. Since night
vision modules are generally grainy, the image
grain can confuse the SLR camera’s focus
sensors. Also, realize that these focus sensors
will be staring at the output of the night vision
module so the grainy artifacts will not be affected
by the focus position of the objective lens (which
is located on the front of the night vision module).
Use Stabilized Lenses
 A
camera’s image stabilization feature permits
the photographer to use slower shutter speeds
without resulting in blur due to camera shake.
Note that there are two primary techniques for
image stabilization: optical stabilization (where
a small element inside the lens moves in order
to stabilize the image projected on the camera’s
detector) and digital stabilization (which take
advantage of extra rows and columns on the perimeter
of the detector, shift the image an appropriate
amount to stabilize certain types of motion).
Normally, the optical image stabilizers, though
more expensive, are preferable since they better
remove the blur component of the motion. When
using a night vision module, the main disadvantage
of the digital stabilization is that the image
is blurred on the input of the night vision module
resulting in some blur on the intensified output
image. So, optical stabilization is preferred.
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Lens Aperture
The basics still apply for night vision
intensified low-light photography. In low-light situations,
you’ll want to select the fastest objective lens
set to the lowest F-stop setting. (The lowest F-stop
value indicates that the iris is opened up fully and
will gather the most ambient light possible for that
lens).
| Nikon and Canon offer fast stabilized
telephoto objective lenses that are ideal for night
vision applications. |
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Infrared Illuminator
Sometimes, even with a night vision module,
it’s simply too dark to obtain a good quality
photograph at night. A dark and cloudy night with no
ambient light is one example. Since the night vision
module is merely a light amplifier, if there is no light
from the stars and sky or from city lights reflecting
from clouds, the night vision module will not be effective.
In order to photograph without the use of a flash or
visible light source, a near-infrared light source can
be used that emits light that can be seen by the night
vision module but is invisible to the naked eye. Unlike
some digital camera sensors which have some near-infrared
response, night vision modules are most sensitive to
the near infrared wavelength range and consequently
these light sources can significantly enhance the night-time
image and render night photography possible despite
the lack of sufficient visible illumination.
Invisible near infrared
illumination can be used to brighten scenes
that are
otherwise too dark.
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Image Cropping
 Because
image intensifiers have automatic electronic gain features
to maintain a constant light output, the presence of
bright lights in the field-of-view (such as headlights
or street lamps) may have the affect of decreasing
the unit’s overall light gain. This is similar
to back-illuminated scenes for un-intensified photography,
but even more important because of the significant
impact on light gain that could result. This affect
may cause the other regions in the image to darken
to an unacceptable level. If possible, do your best
to exclude non-important bright lights from the field
of view.
Turn-off All Camera Visible/Audible Functions
For covert night vision photography,
don’t forget to disable all camera lighting and
sound functions. (For example: disable flash, any red
eye reduction mechanisms, auto focus assist, LCD preview,
and all audible signals).
Use a Tripod
When handheld use is not required, select
an exposure time long enough that the movement of objects
under observation does not result in image blur. As
with un-intensified photography, long exposures require
that you hold your camera perfectly still to avoid
blurring. A tripod is a perfect accessory. If one is
not available, try bracing your camera against a stationary
object like a tree or wall. In order to avoid the blur
that results from finger pressing the camera shutter
release (which can cause enough movement to blur a
photo), use the camera’s timer.
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