A User's Guide to Digital Video Surveillance

LENS SELECTION


Iris:

The iris on the lens determines how the camera will adjust to light. A camera lens iris come in a few varieties:
•  Auto-iris - has the ability to adjust automatically to lighting conditions.
•  Manual iris - is one that you can adjust but as it states it is manual.
•  Fixed iris - means just that, it is fixed and cannot be adjusted.

Focal length:

When selecting a lens, you are trying to determine the area you wish to cover; the width of the shot, and where the central focal point will be.

CCD Size

By clicking the following link you can find out what lens you will require in terms of the field of view. Aventura Lens Calculator

The unfortunate problem is; your needs for a shot in a particular camera view might fall into two categories. Let's take, for example, you have a retail shop, and the camera is set up to cover a large area, you may be able to see an incident occur, but not be able to distinguish the facial features because of where it occurred. So, you have one of a few choices – make sure you select a high resolution lens so you have better detail of the image, or select a higher millimeter lens to cover a smaller area, backed up with an additional camera to cover the balance of the target area.

There are also some other tricks you can use to minimize costs. You may not have to use a whole slew of cameras, if you know you are going to get a close-up view of a subject elsewhere. For example, if you had a camera at the front entrance to the store, that the subject has to pass through, then you have a good look at them and what they are wearing. So, even if they move about the location elsewhere, you can associate the face you saw at the front door, with the clothing they are wearing seen from a distance.

Although there are a myriad of lens sizes, there are a small group that comprise 95% of the market. Typically, they are 2.8mm (wide-angle), 3.6mm, 4mm, 6mm, 8mm fixed lens or a 4mm-9mm, 6~15mm or 6~60mm vari-focal lens. The vari-focal lens gives you the ability to dial in manually anything within the focal range. So, when you install the camera, you would make adjustments until it covers that area you desire. Vari-focal lenses cost significantly more money then the fixed 4, 6, and 8mm lens but the quality is also better. There is a measurable difference. The following screen shots give you an idea of what each lens will look like at certain distances.


Lens Comparison

Selecting a lens, as well as the number of cameras, at times can be a difficult task because you have a balancing act of trying to keep cost down at the same time as accomplishing your goal. So, do you go with better quality cameras, but fewer of them, or lesser quality cameras but more of them? Although, when you start to add more cameras, the cost of the DVR increases because you require more available channels (channels are ports that cameras plug into on the digital video recorder).

So it is all dependent on your budget. We like to try to back into a solution predicated on the budget. This way you have a matching system of quality all the way through, and one that allows for change and growth plans.

When a lens is fixed, understand the image is as you see it. Forget what you see on the television show CSI. Even with a high-end digital video recorder will not get any larger than the original. Can the software increase the size of an image? Absolutely, but when it does so, it pixelates, meaning it gets grainier and less crisp. Don't believe claims that say they can read a license plate on a speeding car at 100 feet away with a standard lens or what you see in movies. It's not happening. The only quality you get when enhancing the original is lesser quality.

Lens Quality

What you also need to know about lenses is there are incredible differences in quality. There are plastic lens and glass lens. There is high quality glass and inexpensive glass. There are metal bodies and plastic bodies. There are precision mounts and some not so precise. No different than your still camera there are several thousand dollar Nikon and Leica lens and the ones you have on a disposable camera.

This again is a problem since the only specifications that a supplier provides you with is the iris function and the focal length. So you are relying on reputation. Even if you use a good quality CCD if the lens is not any good the pictures will suffer. There are CCD lens for less than $2.00 from China.

 

   
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