About lenses in CCTV

(by Vlado Damjanovski © 2008)

A system of any kind will perform as good as the weakest link in such a system.

The traffic flow is as fast as the slowest car in the lane.

The water flow in a system will be limited by the narrowest pipe in such a system. 

This is exactly how we can describe the quality of a CCTV video signal. It depends on all the components and it is as good as the weakest link in such a system.

So what are the typical components in a CCTV system?

Well, there are many different types of systems you can have but all of them have a camera with a lens, transmission media (coaxial cable, fibre optics or twisted pair cable), receiver in the form of a digital recorder or matrix switcher and finally a monitor.

As one can imagine, choosing a good quality camera is important but if a lens is of inferior quality then the best picture will be as good as the lens best quality, not as good as the camera can make it.

Similarly, if the monitor is of an inferior quality, lower than the camera/lens combination, then the picture will be as good as the best performance the monitor can give.

If you add to the system a digital compression and a recorder the picture will be limited to the best quality that such a component can produce.

In this article I will concentrate on the lenses because in my CCTV work and training I have seen many CCTV users that take lenses for granted and they do not question their quality.

I will try and explain the lens quality in simpler terms and I hope it will open up your eyes and teach you that you should question lens quality and you should search for better ones in order to make your system perform better.

How are lenses made?

A typical lesson from optics will teach you that a lens is made of polished glass, although lately of a compound that is closer to plastic than glass, by high precision injection molding. The quality of a lens depends on the quality of each separate lens element (as you know each lens is made of a few optical elements), and each lens element quality depends on the material its made of, the technology of how it is made, the polishing quality and mechanical assembly of all elements into a lens.

Consumers always want better for less but this is not always possible. So the actual products are very often compromised between good quality and cost. The fact that lenses in CCTV are now also made out of plastic instead of glass shows this compromise. The end result is that a lens may be good enough for one type of camera but not as good for another camera. This usually refers to various imaging chip sizes, such as ½” 1/3” and lately 1/4”.

Every lens has a limited resolution, usually expressed in “line pairs per millimetre” (lp/mm).

Why pairs? Because in photography and cinema black and white lines which appear next to each other are referred to as “line pairs”.  In CCTV we only use the term “TV lines” and we usually use test charts with black lines on white background where we count only the black lines and express the resolution in TV lines where only black lines are counted.

The simple fact is - the more lines you have the sharper the image is. There are other parameters that define lens quality, such as color and geometric distortions, but for the purpose of this article we will concentrate on resolution only as this is the most important and easiest to spot.

Having a good lens doesn’t necessarily mean you will see sharp images if the imaging device (CCD and CMOS chips in our case) doesn’t have sufficient pixels to show such sharpness.

The opposite is true as well.

A good imaging chip can not show you a sharp image if the lens doesn’t have at least the matching sharpness (reslution).

It is always better to have a lens resolution slightly higher than the resolution capability of the chip (size and pixel count). 

This is one of the most common problems in CCTV.

Camera manufacturers continue to make smaller and smaller chips (they used to be 1/2”, then 1/3” and now 1/4” and even smaller are being made) in order to make the most of their silicon “pancakes” bringing the price per chip lower, thus becoming more attractive to the market.

The problem arises from keeping the same resolution on chips that are getting smaller. This calls for the lens resolution to be increased in order to achieve the same sharpness as with the larger chips.  

Lenses however, as said previously, have a limited resolution defined by the maximum numbers of lines it can reproduce. This limit is imposed by the quality of the optical glass, it’s production quality (polishing or molding), the lens design (defined by the optics of the lens combination) and the mechanical precision of how all this is put together.

A typical good lens in digital megapixel SLR photography (6~12 megapixels) can reproduce around 60~80 lp/mm. This equates to around 1000 line pairs across the height of 16mm (the digital SLR typical chip size dimensions are 24×16mm). Remember this number: 60~80 lp/mm! 

Let’s consider the 1/3” CCTV chips which is very common, especially for fixed cameras. The chip itself  measures 4.8×3.6mm. The number of pixels of such a chip is around 760 x 580 (this is typical for PAL cameras while NTSC has slightly less).

If we divide the number of vertical pixels (580) with the height of the chip (3.6mm) we get around 160 pixels per mm. When reproducing black and white lines we need every second row for the black (or white) lines, hence we divide the 160 by 2 and we get 80, which would be the minimum number of line pairs we would require from a CCTV lens if we want to have a nice and sharp image. This requirement is even tougher than with the digital photographic quality (using 60~70 lp/mm) where lenses cost a few hundred if not thousands of dollars. Needless to say, lenses made for CCTV are of very average quality and inexpensive. You should wonder if your hundred dollar CCTV lens can outperform photographic quality used in megapixel cameras.

The question raised above is going to be even more emphasized if you do the same analysis for 1/4” chips measuring only 3.2×2.4mm. The same 580 pixels when divided by 2.4mm indicates that you will require 240 lines per mm or 120 lp/mm. This is an even higher requirement than the optics needed for such a camera. Not that it isn’t possible but certainly cheap low-end lenses cannot achieve such a quality for the cost that the CCTV market is prepared to pay. This is why your image may not look as sharp as you wanted it. Using the best digital compression cannot improve it but rather further reduce the details due to compression artefacts.

The moral is - having the best camera is not sufficient for you to get best images.

The lens needs to be of equal or better quality too.

So, maybe, we should lift our game and raise the expectations from the CCTV lens manufacturers.

Unfortunately no manufacturer gives their real lens specification so you are left to your own judgement if a lens is good or bad.

There is no other way but to test and compare.

Get a couple of diffent brands (of the same focal length) and put them on your cameras and see for yourself. Look carefully in areas with more detail or for an even more accurate measurement get a test chart and test the same camera with the same test chart and different lenses.

You will most likely see a difference as no two lenses are identical.

The same arguments can be used with pan/tilt/zoom (PTZ) domes. Although manufacturers of the assembled PTZ dome have taken utmost care in their lens choosing for their PTZ dome, nobody can guarantee you that two different PTZ manufacturer using the same 16x zoom lens type will give you the same picture quality. It is possible that the manufacturer that offers you stronger zoom lens (for example 18 times rather than 16) may actually have lower quality lens resolution wise than the one with lower zoom ratio. In my personal opinion this might be more important than actually having a zoom magnification that is slighlty higher.

The conclusion I am trying to get to is this – do not take the lenses for granted. Test them.

 Written by Vlado Damjanovski © 2008.

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