I have been to several sites where they have attempted a tradeoff of sorts, reducing the resolution so they could increase the frames per second. This will lead to more issues if the field of view calculations were done with resolution for identification in mind. Reducing resolution from 4CIF to CIF on standard definition cameras reduces your ability to identify targets at a distance.
Standard Definition Cameras
This industry staple has come a long way over the last decade. A good standard definition camera today comes with H.264 compression, a 20-bit dual exposure CCD, and day/night capabilities for deployment with infrared illumination, as well as back light compensation, white balance, and auto black features. If deployed correctly, standard definition cameras can do a terrific job.
What's the catch? With the advent of large HD monitors deployed in operations centers, customers are disheartened with the image that they once were satisfied with, saying the images are pixilated. It is the same 4:3/704x480/345,000 pixel image they once loved, but it has been stretched across a 60 inch/16:9/1920x1080/2,073,600 pixel canvas. You would look a little pixilated too.
Over the last few years, megapixel cameras have been all the rage. The lure is that you can buy fewer cameras and cover more area while still being ready for that CSI moment. But as always, there is a catch. Megapixel cameras are not subject to any standards except for the number of pixels involved. There is no imager compliance, no specific compression compliance, aspect ratios can vary (4:3, 5:4, 16:9), and megapixel cameras have often been limited to lower frame rates (1 to 15 frames per second).
Lenses play a much larger than advertised role in megapixel cameras. Modular transfer function is the measurement of light before and after the optics of a camera. This measurement tells you how well a lens can resolve a single spot (pixel) on the camera's imager. It is represented in line pairs per millimeter (LP/mm). Typical lenses for standard definition cameras have a 30 LP/mm rating, while megapixel cameras must have a 60 LP/mm rating or better.
Choosing the correct lens for a megapixel camera can be difficult, as most lens manufacturers do not disclose the LP/mm rating of their lenses, and the LP/mm needed can vary depending on the size of the sensor in the camera and the size of the pixels on the sensor. Here is the calculation:
0.5 x 1/pixel size (mm) = LP/mm to resolve
Let's look at an example of how this works: You have two 2.1-megapixel cameras with the same resolution of 1920x1080. One camera has a 1/2-inch sensor and the other has a 2/3-inch sensor. The 1/2-inch sensor has a pixel size of 4.5um and the 2/3-inch sensor has a 6.5um pixel. We would need two different lenses to get the most out of the cameras, the 6.5um pixel needs a 77 LP/mm lens, while the 4.5 um pixel needs a 111 LP/mm lens.
Finally, the bandwidth/storage consumption of a megapixel camera is very large, especially if the camera's full capacities are being used. As an example, let's take the "Brand X" 3-megapixel camera that I have in my lab that records in MJPEG format. If I record 10fps for 15 days with an average bit rate between 40Mb/s and 56Mb/s, I would need about 9TB of storage. If this was the only camera I deployed to monitor a parking lot, I would also have a single point of failure. Using this same storage limitation and standard definition cameras, I could deploy roughly 40 cameras at higher frame rates.
High Definition Cameras
HD cameras are megapixel cameras (2.1MP max) with rules. In other words, while all HD cameras are megapixel, not all megapixel cameras are HD. High definition is a global video standard defined by the Federal Communications Commission, Advanced Television Systems Committee and the Digital Video Broadcasting Project.
Where megapixel cameras only adhere to a pixel count, HD cameras are progressive scan devices and have defined resolutions of 1280x720 and 1920x1080. They have a defined aspect ratio of 16:9 and frame rate standards of 30 and 60 frames per second. HD cameras fill the void between standard definition and megapixel cameras; this is the same void created by the discrepancy between reality and the futuristic world of CSI.