Digital SLR Cameras – What’s New for Military Photography
DSLR cameras are highly valued by military photographers for their resolution, sensitivity, versatility in the field, availability of a wide variety of lenses, and for allowing 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, which are now available with image sensors of the same size as traditional film formats. These large sensors allow for comparable field of view values to film sizes, as well as their comparable sensitivity.
Over 80% of DSLR cameras sold today are Nikon or Canon models. Due to their preponderance, there is a plethora of lenses and accessories available for these two camera bodies, resulting in an excellent selection and remarkable prices. These accessories include: objective lenses that are fast, lightweight, optically stabilized, autofocus, as well as night vision modules and other camera accessories that enhance the overall camera performance. Note that while there are other camera manufacturers that provide excellent products worth considering for certain applications, only Nikon and Canon cameras are mentioned in this article.
Pixels: how many? How big?
The most notable change in the evolution of digital SLR cameras has been the steady increase in the number of pixels that make up the image sensor. Commercial cameras have seen the number of pixels in the camera’s sensor grow twenty-fold in so many years, from 1 MP (1 million pixels) in 1990 to more than 20 MP in 2010.
Are cameras with larger sensor arrays better? Not necessary. It depends on the application. There’s more to understanding the impact of larger array sizes than just the number of pixels. There are two other important considerations to consider 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 sizes (36mm x 24mm). As can be seen in Figure 2, dSLR image sensors are available today in several different 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 DX format). Canon also supplies cameras with a slightly larger size sensor, APS-H of approximately 28x19mm. The largest sensors (known as “full-frame” sensors) are now available in both Canon and Nikon cameras and have the same sensor size as the original film SLR cameras, 36mmx24mm.
For cameras with an image sensor smaller than full frame, a digital crop factor is defined (also known as the focal length multiplier or magnification factor) which can be calculated by taking the ratio of the diagonal size of 35mm film (43.3mm ) to the diagonal size of the camera’s image sensor. As should be apparent from the term focal length multiplier, multiplying the focal length of the lens by the crop factor gives the effective focal length of a lens that would yield the same field of view when used on a full-frame camera. For the military photographer, the concept of a crop factor >1 is normally considered an advantage. For long-range viewing, a narrower field of view is desirable and the crop factor gives photographers a “boost” with a long focal length, making it easier to fill the frame 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 produces the same field of view as a 300mm lens on a full-frame camera. While there are advantages to long-range viewing, the narrowing of the field of view is sometimes a disadvantage for photographers when a wide field of view is desired. For example, a 24mm lens on a Nikon DX-format camera results in the same field of view as a 36mm lens, possibly too narrow for the application that requires the use of expensive ultra-wide angle lenses to deliver the desired FOV (16mm in this example) . Obviously, for cameras with “full-frame” image sensors that are the same size as the 35mm film format, there is no crop factor (crop factor is 1).
Impact of physical pixel size
To better understand the impact of pixel size, compare the performance of two cameras with the same physical array size but different number of pixels (and therefore different pixel sizes). For this example, let’s compare the Nikon D3X (full-frame image sensor with 24MP) and the Nikon D3S (full-frame image sensor with 12MP). For the D3X, the physical pixel size is about 6 microns, while the D3S has pixels that are about 8.5 microns, about 40% larger. In this example, if the military photographer performs remote surveillance, the identification of objects is related to the number of pixels on the target. Thus, if the same lens is used on the two cameras being compared, the distance for the higher resolution model will be greater than for the lower resolution camera. Explained differently,
In terms of resolution, smaller pixels are better and the advantage goes to the high-resolution camera that can use lenses that are lightweight, easy to hold and essentially “faster”. Unfortunately, more smaller pixels aren’t always the best for a surveillance photographer! All things being equal, the smaller the pixels become, the less effective at collecting light, resulting in a lower sensitivity to the camera. Pixel sensitivity is related to the area of the pixel, so in our equation the pixels that are 40% larger are likely to provide twice the sensitivity (1.4×1.4~2). This means that an extra F-stop or a doubling of the exposure time is required to obtain the same light sensitivity. Alternatively, the camera’s processor must amplify the signal. (More on this in the next section). Nikon probably realized that for some photographers, a higher 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 significantly in recent years, making it possible to increase resolution and keep sensitivity roughly constant. So what if you applied current noise reduction strategies to an existing sensor, without adding more pixels? That’s certainly what Nikon meant by designing the D3S, leaving the resolution at 12.1MP allowed them to increase sensitivity for those customers who need it. For photographers who absolutely need more resolution, Nikon has the 24.5MP D3X. Adding pixels means making them smaller, and the smaller the pixel, the worse the light sensitivity. Fortunately, noise reduction techniques have improved significantly in recent years, making it possible to increase resolution and keep sensitivity roughly constant. So what if you applied current noise reduction strategies to an existing sensor, without adding more pixels? That’s certainly what Nikon meant by designing the D3S, leaving the resolution at 12.1MP allowed them to increase sensitivity for those customers who need it. For photographers who absolutely need more resolution, Nikon has the 24.5MP D3X. Adding pixels means making them smaller, and the smaller the pixel, the worse the light sensitivity. Fortunately, noise reduction techniques have improved significantly in recent years, making it possible to increase resolution and keep sensitivity roughly constant. So what if you applied current noise reduction strategies to an existing sensor, without adding more pixels? That’s certainly what Nikon meant by designing the D3S, leaving the resolution at 12.1MP allowed them to increase sensitivity for those customers who need it. For photographers who absolutely need more resolution, Nikon has the 24.5MP D3X. making it possible to increase the resolution while keeping the sensitivity approximately constant. So what if you applied current noise reduction strategies to an existing sensor, without adding more pixels? That’s certainly what Nikon meant by designing the D3S, leaving the resolution at 12.1MP allowed them to increase sensitivity for those customers who need it. For photographers who absolutely need more resolution, Nikon has the 24.5MP D3X. making it possible to increase the resolution while keeping the sensitivity approximately constant. So what if you applied current noise reduction strategies to an existing sensor, without adding more pixels? That’s certainly what Nikon meant by designing the D3S, leaving the resolution at 12.1MP allowed them to increase sensitivity for those customers who 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 ambient light is low or when scenes are distant. Today, dSLR cameras have unique capabilities and accessories are available to help the photographer get the most out of nighttime shooting conditions.
Aperture and exposure time
The main factors that affect low-light imaging are still fundamental to photography: aperture and exposure time. For night photography, since depth of field is not normally an important factor, it makes sense to choose a lens with the widest aperture possible. Selecting an objective lens for a specific application can be just as important as selecting the DSLR camera. In fact, one can opt for the Nikon or Canon body due to the specific lenses available. Similarly, for nighttime imaging, exposure time should be set as long as possible, yet to avoid blurring caused by observing moving objects or camera shake.
Improved ISO – Higher gain with drawbacks
Aperture and exposure time directly affect the amount of light hitting each pixel in the image sensor and affect the sensor’s electrical output. In addition, DSLR cameras allow the photographer to adjust the gain (ie gain) of the sensor’s electrical output signal. (This is distinctly different from the ISO definition for film cameras which had to be adjusted for 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 false color spots. To avoid too much noise, the night photographer sets the camera ‘s ISO to a value high enough to make the image brighter, while maintaining acceptable levels of image noise. Today the maximum value is usually 1600, but some cameras deliver perfectly usable images at ISO 3200, 6400 or even 12800.
In addition to standard ISO settings, digital cameras now perform noise reduction on digital images. This allows some extra gain-up on these cameras, sometimes one step (H1), or actually twice the maximum ISO setting, but also two steps (H2, 4 times) and three steps (H3, 8 times). This improved ISO setting is now available on most DSLRs. For example, both the Canon EOS-1D Mark IV and Nikon D3S allow three levels of enhanced ISO, resulting in an equivalent ISO setting of 102,400! It is important to note that images resized as such will normally have a high degree of noise and a significant degradation in resolution due to the noise reduction techniques. Apparently, as with all high gain electronic circuits, image noise can significantly degrade image quality.
night light modules
Today’s digital SLR cameras give the photographer full control over aperture, shutter speed and ISO settings, capturing excellent usable images in low light that would otherwise not be possible. In many situations, such as shooting scenes at night with very little ambient light or telephoto at night at a distance, even the best digital SLR cameras simply don’t have enough sensitivity to get enough shots. In these situations, night vision modules are the ideal accessory.
The night vision module accessory fits between the objective lens of the SLR 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 integrate seamlessly with the camera and lens combinations, preserving 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 is easily turned on with the launch of the shutter button.
The night vision module has an automatic gain control that ensures that the output is relatively constant. As such, camera settings are relatively easy to configure as follows:
• Exposure time 1/30 sec (no need to shorten it as the amplifier has its own delay)
• Manual aperture (so that the F-stop setting of lens remains)
• F-stop at widest aperture (amplifier adjusts light level automatically)
• ISO setting adjusted to a level where camera noise is not visible
• Autofocus off (image intensifier scintillation can confuse autofocus sensors)
• Image Stabilization On (to to maintain stable images while the camera is moving)
Overview
The evolution of the digital SLR (dSLR) camera continues and impressive features are added at a steady pace. For the military or law enforcement photographer, the new DSLR camera models and new features can be very useful for portable observation and in-the-field news gathering, both at night and during the day.