Sunday, May 17, 2009

Explaining megapixel cameras and rectilinear lenses

Why megapixel cameras?

Manufacturers have been pushing megapixel cameras for a while now. And, let’s be honest, they look super awesome. But is there an actual business case there? When the bean counters come around, can we justify the added expense past “but, look how pretty it is!”?

Some manufacturers have been telling us that you can use megapixel cameras to replace standard cameras. Use one megapixel camera to do the work of six or eight standard cameras, goes the argument, and save money on the installation. How does that work, exactly?

You gotta use rectilinear lenses, that’s how.

Here's the thing. I don't know how big of an area you are trying to cover, but I'm going to hope for the best and assume it's an indoors area with bright, constant lighting, with the camera mounted about 12-15 feet high looking at an open area (no aisles or displays or desks!) about 35 feet by 35 feet square.

Assuming this is the case, or similar, you may be able to get away with a 2MP camera and a 180 degree rectilinear lens (NOT FISHEYE!!!) for your dPTZ (digital PTZ- remember this term for later) camera. Basically, what you are trying to do is get a VERY wide picture, and then blowing up portions of the picture later, like they do on CSI. Unfortunately, on CSI blowing up a picture actually makes the picture sharper, where in the real world blowing up a picture means a loss in detail. Therefore, you need a very sharp camera, a great lens, ideal 24 hour lighting, and wonderful storage and compression protocols. The actual software is pretty simple and your existing DVR could theoretically do it using the zoom feature (if your DVR could handle such detailed pictures without compressing the hell out of them, which it can't of course).

Now since most CCTV or security people are unaware of the very existence of rectilinear lenses (but YOU aren't, are you, because your good buddy the CameraMan linked to the definition earlier in this rant), so most people wanting to set up a dPTZ camera actually use a fish-eye lens (or buy a camera with a fisheye lens already installed) and use fancy, sophisticated software packages such as the ones made by Avigilon, which actually uses a bunch of very sophisticated programs working together to 1) zoom into a sector of the fisheye picture (ie the easy part) and 2) flattening it out (the hard part). This is very cool, except for the fact that it doesn't quite work the way you need it to work because, well, the picture is still distorted, and a distorted picture JUST may give a jury reasonable doubt.

So just buy a rectilinear lens. The best ones are from Theia. They've got a choice of CS mount, auto iris (the SY125A), a CS mount, manual iris (the SY125M), and a C mount, manual iris (the MY125M). Expensive? Sure. The lens alone is nearly the price of a megapixel, non D/N camera... but worth it, if it replaces six or seven cameras.

Now, the other thing you gotta think about is: storage. This camera is going to eat up storage and eat it up fast, and I assume you need to store video for a goodish while because I assume you are a medium to high risk site because otherwise you wouldn't be using so many expensive cameras and such an expensive NVR package. So. Learn about RAID arrays and buy Seagate Baracuda SV35.3 series drives by the case. Have fun, and kiss your budget goodbye.

At this point, assuming you are still reading, you may be wondering why the hell you should get this setup- wouldn't be easier to just get a freaking PTZ and be done with it? The answer is no. Now, a rectilinear lens plus a GOOD 2 megapixel camera plus a boatload of storage is the same or even a little more than a normal PTZ plus a keyboard plus an encoder BUT dPTZ has an enormous advantage over traditional PTZ, and that is: you never miss any of the action.
Remember, if you point the PTZ north, and someone gets stabbed to the south, you are basically screwed (a technical legal term), but with a dPTZ setup, you are looking at EVERYTHING ALL THE TIME.

We usually only use PTZ cameras to supplement fixed, traditional cameras, but even then it is usually not practical to have full coverage. Proper use of dPTZ cameras could make true, full coverage a reality.

dPTZ is an operational concept and a design philosophy, not a product in a cardboard box. It
requires creativity and true understanding of how cameras work and experience in physical security. There will always be a need for true surveillance video experts to explain and implement solutions such as dPTZ.

More about rectilinear lenses here.

1 comment: