Frame rate. Some megapixel cameras provide very high resolution, but only at lower frame rates. In some surveillance applications, three or five frames per second (fps) might be sufficient, while others might require 12 or even a full 30 fps. If a high resolution camera is HDTV-compliant (according to SMPTE standards) it will guarantee 30 fps.
Day/Night capability. If you plan on placing the camera outdoors, or in an indoor environment that has poor or varied lighting at certain times of the day, you need to choose a camera that supports true day/night capability. True day/night capability means that the camera is equipped with a removable infrared-cut filter. Near-infrared light, which spans from 700 nanometers (nm) up to about 1000 nm, is beyond what the human eye can see, but most camera sensors can detect it and make use of it. During the day, a day/night camera uses an IR-cut filter to block IR light so that it doesn't distort the colors of images as the human eye sees them. When the camera is in night mode and the IR-cut filter is removed, the camera's light sensitivity is drastically improved and put into black and white to avoid distortion of colors. Keep in mind that some cameras do not have a removable IR-cut filter, but simply switch to a black and white image at night to give the illusion of night vision by minimally increasing light sensitivity. These not true day/night cameras. Other cameras increase their light sensitivity at night by reducing their resolution. For instance, a 1.3 megapixel camera would combine four pixels into one, thereby increasing the light sensitivity, but at the expense of reducing the resolution by a factor of four. This method brings actual resolution down to VGA quality after dark. If nighttime surveillance is a priority for your installation and you're willing to settle for lower resolution at night in order to get greater light sensitivity, a standard VGA camera could be an even better alternative than megapixel.
Multi-streaming. While megapixel and HDTV cameras do provide a lot of image details, it's rare that you use all that information when doing live monitoring, especially when viewing four or more cameras on the monitor at the same time. In those instances, it would probably be best to stream megapixel images to the storage system for forensics, but use a low-resolution video stream for the live monitoring. With cameras that support multi-streaming, you can customize the system for the optimal performance for both live monitoring and archiving. For instance, you can create different compressions and resolutions for each stream, such as one lower-resolution stream for monitoring and a second full frame rate, high-resolution stream for archiving and forensics. Also, some cameras support an even more advanced multi-streaming technology, allowing the field of view from one camera to be divided into multiple, separate, user-defined streams. This allows the user to use a single megapixel camera to cover a wider field, and segment smaller video streams from that image into their own individual windows (see photo at right). Part of the theory behind this format of multi-streaming is that it allows a user to replace several individual cameras with a single megapixel or HDTV camera.
Lenses. Camera optics makes a big difference. With respect to image quality, the camera is only as good as the lens it uses. Check that the lens is rated for a specific resolution to provide the image clarity and quality you expect, or else your investment in megapixel may be wasted. Also check with the vendor that the lens can handle difficult light scenarios and deliver the depth of field you need. [From Axis: A camera with precise-iris control -- often referred to as P-Iris -- is a special lens with in-camera software used to guarantee depth of field and better image quality. P-Iris automatically adjusts the iris opening to the optimal aperture for image sharpness, depth of field and exposure based on the lighting conditions at any given moment.]
Below: Notice the difference in image clarity in an environment with challenging light conditions. The image on the top was captured with a traditional surveillance camera lens, while the image on the bottom was recorded using a surveillance camera equipped with a P-Iris lens.