Exploring the Realities of Megapixel Surveillance Technology, Part 3

Comparing Digital PTZ vs. Mechanical PTZ for surveillance operations


This marks the final installment of our 3-part series on megapixel technology. In Part 1, we took a look at the overall cost of megapixel video based on the three types of surveillance, general surveillance, forensic and high detail and their comparative costs on a pixel-per-foot basis. In Part 2, we looked at an apples-to-apples comparison of deploying a megapixel video system instead of a low resolution (4CIF) system and how in many applications, megapixel can be much more affordable.

PTZ Pros and Cons

In Part 3, we tackle the pros and cons of digital pan/tilt/zoom (PTZ) systems vs. mechanical PTZ systems. First, let's talk about some basic of pan, tilt and zoom.

Mechanical PTZ cameras consist of motors, slip rings, gears and/or belts and wheels to drive the camera into position. Traditionally, they were controlled by proprietary keyboards but recent advancements in our industry have allowed these same cameras to be controlled by software interfaces. These devices are available as integrated units, most commonly in the form of domes, or you can purchase the camera separately and mount it on a pan-tilt motor.

The amount of zoom, e.g. 25X, depends on the ratio of the telephoto setting to the wide angle setting. So, a lens that goes from 4mm to 100mm will have a 25x zoom. A lens that goes from 50mm to 150mm will only provide 3X zoom but will be much better at zooming in farther away because of the 150mm telephoto setting. However that same set-up will not have as wide of a field of view. The lower of the numbers (the 50 in a 50mm-150mm lens) is what you use to know how "wide" your field of view is. The lower the number, the wider angle the lens is. So, in this model, the 50-150mm is more of a telephoto lens, and the 4-100mm gives a wide angle view of the scene.

Mechanical PTZ

Today, most mechanical PTZ cameras employ low resolution cameras, typically around 704 x 480 or 1/3 of a megapixel. In order to get high-detail (80 pixels/foot) with that camera, you would need to zoom into an area that is no wider than 8 feet (2.4 meters). If you zoomed out all the way you would now be spreading those pixels out over a very wide area and would lose almost all detail.

If your goal is to give security staff the ability to monitor general activity, and then zoom in over a long distance with good detail, then mechanical PTZ cameras with high zoom telephoto lenses are the way to go but there are a few things you should consider.

First, mechanical PTZ cameras should be thought of as "either/or". Because of the nature of lens options, you either get wide area coverage with low resolution or you get high resolution in a very narrow area. You really cannot get both. This means if you are "zoomed in," you will miss everything else that the camera is not zoomed in on. Alternatively if you are "zoomed out," you won't have the detail you need. If you are recording "zoomed out" mechanical PTZ images you can always "digitally zoom" after the fact, but the real world isn't like "CSI", and the results aren't pretty. This limitation places a great deal of responsibility on the guard controlling the camera, especially if there are simultaneous incidents in which case the guard must decide which is more important..

The second thing to consider is the cost of 24/7 guards dedicated to controlling the cameras-which can be a substantial amount of money. Next, if you are controlling these mechanical cameras over a TCP/IP network, there can be a long delay between when you tell a camera to PTZ and when it actually responds. Finally, mechanical PTZ cameras have a number of moving parts which are susceptible to wear and tear, and will eventually require periodic maintenance and repair.

Digital PTZ

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