Part Two of Digital SLR Cameras – What’s New for Surveillance Photography is presented here.
Aperture and Exposure Time
The primary factors that impact imaging in low-light are still fundamental to photography — lens aperture and exposure time. For night-time photography, since depth of field is not normally an important factor, 
it makes sense to select an objective lens that has the largest possible aperture. Selecting an objective lens for a specific application may be as important as selecting the dSLR camera. In fact, one may select either the Nikon or Canon body because of the specific objective lenses that are available (See Figure 5).
Similarly, exposure time for night-time imaging should be set as long as possible but still to avoid blur due to observing objects in motion or from camera jitter.
Figure 5: Nikon and Canon offer fast stabilized telephoto objective lenses that are ideal for low-light applications.
Enhanced ISO – Higher gain with drawbacks
Aperture and exposure time directly impact the amount of light that impinges on each pixel in the image sensor and affect the sensor’s output electrical signal. In addition, dSLR cameras permit the photographer to adjust the gain (i.e. amplification) of the sensor’s output electrical signal. (This is distinctly different from the ISO definition for film cameras which was to be adjusted to match the film sensitivity). For dSLR cameras, increasing the gain will amplify the sensor signal. Like all electrical circuits, image sensors are not perfect, and with the image signal, they also produce some noise in the form of unwanted random errors in the signal. Noise in digital images appears as graininess or specks of false color. To avoid too much noise, the night-time photographer adjusts the camera’s ISO to a value that is high enough to brighten the image while still maintaining acceptable image noise levels. Today, 
the maximum value is typically 1600, but some cameras deliver perfectly usable images at ISO 3200, 6400 or even 12800.
In addition to the standard ISO settings, digital cameras now perform noise reduction on digital images. As a result, some additional gain-up is enabled on these cameras, sometimes one step (H1), or effectively two times the maximum ISO setting, but also two steps (H2, 4 times) and three steps (H3, 8 times). This Enhanced ISO setting is now available on most digital SLR cameras. For example, both the Canon EOS-1D Mark IV and the Nikon D3S permit three levels of enhanced ISO which results in an equivalent ISO setting of 102,400! It is important to note that images adjusted as such normally have a great degree of noise and exhibit significant degradation in resolution because of the noise reduction techniques. Apparently, as with all electronic circuits at high gain, image noise can significantly degrade image quality.
Night Vision Modules
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 are the ideal accessory.
Figure 6: AstroScope Night Vision modules fit between the SLR lens and the SLR body, amplifying the light from the lens and projected an intensified green image onto the image sensor in the dSLR camera. Available for Nikon or Canon dSLR bodies.
The night vision module accessory fits between the SLR objective lens and the camera body (See Figure 6). Simply remove the objective lens, attach the night vision module to the camera body and attach the objective lens. AstroScope night vision modules are designed to seamlessly integrate with the camera and lens combinations, maintaining all the electronic functions of the SLR objective lens (including image stabilization) via the camera’s hot-shoe. The night vision module is powered directly from the camera and conveniently turns on with the initiation of the shutter release button.
Figure 7: Comparison of unintensified vs. intensified dSLR images. Both photos above were taken using the same camera settings. Improvement from night vision module is equivalent to 8-10 f/stops.
The night vision module features an automatic gain control that assures that its output is relatively constant (See Figure 7). As such, camera settings are relatively easy to configure, as follows:
• Exposure time 1/30 sec (no need to make it shorter since the intensifier has its own lag)
• Manual aperture (so that lens F-stop setting stays put)
• F-stop at widest aperture (intensifier automatically adjusts light level)
• ISO setting adjusted to a level where camera noise is not apparent
• Autofocus Off (image intensifier scintillation can confuse autofocus sensors)
• Image stabilization On (to maintain stable images while camera is in motion)
How It Works
AstroScope transforms dark scenes into bright, high resolution images that can be easily photographed without the need for additional lighting or longer exposure times. The objective lens focuses the minute amounts of available light onto the faceplate of its internal central intensification unit that converts the photons to electrons. The internal electron flux is then amplified and the electrons are accelerated so that when they impinge on the output phosphor, a bright green image is created. The image is then focused onto the internal detector of the digital SLR camera. Learn more at www.hownightvisionworks.com.
Photography at a Distance
Sometimes the surveillance photographer needs to capture images of objects and scenes at a great distance from the camera. In these situations, long focal length lenses are often needed in order to produce the magnification required. SLR lens manufacturers offer a variety of telephoto lenses for these applications. Commonly, there are zoom lenses (e.g. 70-300mm or 100-400mm) and fixed focal length options (400mm, 500mm, 600mm). For imaging at extreme distances, photographers will often seek customized solutions such as a 2350mm telescope (see photo) attached to a digital SLR camera with a night vision module to improve the light gathering performance. These solutions can deliver remarkable results for the surveillance photographer.
Figure 8: A 2350mm F10 Celestron Schmidt-Cassegrain Telescope (9.25″ diameter) with a custom AstroScope Night Vision Module permits extreme distance night vision photography and videography. For more information, see Clandestine Photography by Raymond Siljander and Lance Juusola (Charles C Thomas, Publisher, Ltd., Springfield, IL, 2012).
Summary
The evolution of the digital SLR (dSLR) camera continues with impressive features being added at a steady rate. For the military or law enforcement photographer, the new dSLR camera models and new features can be quite useful for portable observation and in-the-field news gathering, both at night and during the day. Some of the benefits are:
• Image sensor arrays continue to grow in physical size, now available with full-frame formats the same as the traditional film SLR cameras.
• Pixels continue to be packed more densely, increasing overall resolution and the ability of dSLR cameras to detect, recognize and identify objects at a distance.
• Sensitivity of cameras is improving, with cameras having un-enhanced ISO settings up to 12,800.
• Night vision modules are available to improve light gathering by 8-10 f/stops while still permitting electronic lens functions to continue to function, including image stabilization.
DSLR cameras continue to be highly valued by military photographers over other camera styles because of their large sensors, resolution, sensitivity, versatility in the field, availability of a wide variety of objective lenses and night vision modules.
This white paper is provided to DV Info Net courtesy of Sofradir EC, Inc., formerly Electrophysics, a leading developer of advanced high performance infrared cameras, portable thermography systems and advanced night vision imaging solutions.
