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DSLR cameras are highly valued by military photographers because of their resolution, sensitivity, versatility in the field, availability of a wide variety of objective lenses and because they allow an accurate preview of framing close to the moment of exposure. Many photographers also prefer dSLRs for their larger sensors compared to most compact digital cameras, now available with image sensors the same size as traditional film formats. These large sensors allow for similar field-of-view values to film formats, as well as their comparable sensitivity.
Over 80% of the dSLR cameras sold today are models from Nikon or Canon. As a result of their predominance, an abundance of lenses and accessories are available for these two camera bodies, resulting in an excellent selection and remarkable pricing. These accessories include: objective lenses that are fast, lightweight, optically stabilized, autofocus, as well as night vision modules and other camera accessories that increase overall camera performance. Note that while there are other camera manufacturers that provide excellent products worthy of consideration for certain applications, only Nikon and Canon cameras are mentioned in this article.
Pixels: How many? How Big?
The most noticeable change in the evolution of digital SLR cameras has been the steady increase in the number of pixels that makeup the image sensor. Commercial cameras have seen the number of pixels in the camera's sensor grow twenty times in as many years, from 1MP (1 million pixels) in 1990 to over 20MP in 2010.
Are cameras with larger sensor arrays better? Not necessarily. It depends on the application. There's more to understanding the impact of larger array sizes than merely the number of pixels. There are two other important considerations that are considered here: the physical sensor array size and the physical pixel size.
Impact of Physical Array Size
The first dSLR cameras used image sensors that were significantly smaller than traditional 35mm film formats (36mmx24mm). As shown in Figure 2, today dSLR image sensors are available in several distinct sizes. The smallest image sensors are about 2/3 the size of the 35mm format. They are available from both Canon (known as APS-C format) and Nikon (known as the DX format). Canon also provides cameras with a slightly larger size sensor, APS-H which is approximately 28x19mm. The largest sensors (known as "full-frame" sensors) are now available in cameras from both Canon and Nikon and have the same size sensor as the original film SLR cameras, 36mmx24mm.
For cameras having an image sensor that is smaller than full-frame, a digital crop factor has been defined (also known as the focal length multiplier or magnification factor) which can be calculated by taking the ratio of the diagonal dimension of 35mm film (43.3mm) to the diagonal dimension of the camera's image sensor size. As should be obvious by the term focal length multiplier, multiplying the lens focal length by the crop factor gives the effective focal length of a lens that would yield the same field-of-view if used on a full-frame camera. For the military photographer, the concept of having a crop factor >1 is normally regarded as a benefit. For long distance viewing, a narrower field of view is quite desirable and the crop factor provides photographers a "boost" in long-focal-length enabling them to fill the frame more easily when the subject is far away. For example, the focal length multiplier for a Nikon DX-format camera is 1.5. Using a 200mm lens on a Nikon DX-format camera will deliver the same field-ofview as a 300mm lens on a full-frame camera. However, while there are benefits for long distance viewing, the narrowing of the FOV is sometimes a disadvantage to photographers when a wide FOV is desired. For example, a 24mm lens on a Nikon DXformat camera will result in the same field-of-view as a 36mm lens, possibly too narrow for the application necessitating the use of expensive ultra-wide lenses to deliver the desired FOV (16mm in this example). Obviously, cameras having "full-frame" image sensors that are the same size as the 35mm film size, there is no crop factor (crop factor is 1).
Impact of Physical Pixel Size
In order to better understand the impact of the pixel size, compare the performance of two cameras that have the same physical array size but a different number of pixels (and consequently different pixel sizes). For this example, let's compare the Nikon D3X (fullframe image sensor with 24MP) and the Nikon D3S (full-frame image sensor with 12MP). For the D3X, the physical pixel dimension is about 6 micron whereas the D3S has pixels that are about 8.5 micron, about 40% larger. In this example, if the military photographer is performing surveillance at a distance, identification of objects will be related to the number of pixels on target. Hence, if the same lens is used on the two cameras being compared, the standoff distance for the higher resolution model will be further than for the lower resolution camera. Explained differently, in order for the lower resolution camera to deliver the same resolving capability as the higher resolution camera with a 300mm lens, the lower resolution camera would either need a 420mm lens or the photographer would need to move closer to the target by about 25% of the distance.
As far as resolving capability goes, smaller pixels are better and the advantage goes to the high resolution camera which can use lenses that are lightweight, easy to hold and essentially "faster". Unfortunately, more smaller pixels aren't always the best thing for a surveillance photographer! All things being equal, as pixels get smaller in size, they also become a lot less effective at gathering light resulting in lower sensitivity for the camera. Pixel sensitivity is related to the area of the pixel, so in our comparison, the pixels that are 40% larger will likely deliver twice the sensitivity (1.4x1.4~2). This means that to obtain the same light sensitivity will require an extra F-stop or double the exposure time. Alternatively, the camera's processor will have to gain-up the signal. (More on this in the next section). Nikon probably realized that for some photographers, more resolution isn't always better. Adding pixels means making them smaller and the smaller the pixel the worse the light sensitivity. Fortunately, noise reduction techniques have improved considerably in the past few years, making it possible to increase resolution while keeping sensitivity about constant. So what if you applied current noise reduction strategies to an existing sensor, without adding more pixels? That's certainly what Nikon intended with the design of the D3S, leaving the resolution at 12.1MP allowed them to increase the sensitivity for those customers that need it. For photographers who absolutely need more resolution, Nikon has the 24.5MP D3X.
Low Light Photography
For the military photographer, some of the most demanding situations occur at night when there is little ambient light, or scenes are at a distance. Today, dSLR cameras have unique capabilities and accessories are available that help the photographer make the most out of night-time imaging circumstances.
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. 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.
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.
The night vision module accessory fits between the SLR objective lens and the camera body. 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.
The night vision module features an automatic gain control that assures that its output is relatively constant. 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)
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 unenhanced 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.
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