CCTV Test: Megapixel Lenses
Given the advances made in higher resolution video in recent years, it is astonishing that in many cases little attention is paid to the selection of lenses. Any camera’s performance will be affected by the choice of lens, and HD1080P cameras are an investment made to achieve higher quality. Therefore, surely the lens spec is as – if not more – important than the camera selection?
[dropcap]T[/dropcap]he lens is effectively the eye of the video surveillance system. It focuses light onto the sensing area of the camera: the CCD or CMOS sensor. The sensor then converts the level of light into a video image – the process differs dependent upon the chipset, but the job of the lens remains the same. This very basic explanation serves to underline one fact; focusing the light as best possible onto the sensor is a vital part of creating good quality video images.
There are many different types of lenses, with variations for camera formats, focal lengths, iris requirements, zoom requirements, low light applications, day/night applications and – appropriately for those seeking HD and megapixel solutions – resolution.
Megapixel lenses are essential to allow higher resolution cameras to deliver their full levels of performance. There is a very simple reason for this – lenses used in video surveillance are not perfect. They are only as good as they need to be!
One of the costly parts of making a lens is the precision grinding of the glass. The more precise the grinding, the longer the process takes, and the more care that needs to be applied. As a result, the higher the degree of precision of the grinding, the more expensive the lens will be.
It stands to reason that if a lens is intended for use on a VGA or a 4CIF camera, there is no point in spending time grinding the glass to a higher tolerance than the camera can actually resolve. Therefore, the glass is ground to a degree that eradicates only the aberrations that the sensor will display.
A VGA camera, as an example, contains 640 x 480 pixels. In effect, this means that the chip will contain 307,200 elements that make up an image. If there is a flaw or aberration in the optical surface of the lens, it only matters if it then affects enough pixels to make it visible to those viewing the footage.
The situation changes when looking at high definition video. Rather than 307,200 pixels on the chip, a HD camera has 2,073, 600 pixels on its sensor, but the actual chip is approximately the same size as the VGA unit! This means it has nearly seven times as many pixels in the same area, so any flaw would cover seven times more pixels. Obviously, as the image is magnified, the flaw becomes increasingly obvious.
Using a standard lens might save money – probably little more than a few pounds for each unit – but the reality is that it is a corner-cutting exercise that could end up reducing the effectiveness of the whole system.
A point worth noting is that not all megapixel lenses are the same! Most will be given a rating dependent upon how precise they are. Typical ratings are 1, 3 and 5 megapixels.
It must be remembered that these ratings will be given for a 4:3 image, so an adjustment to the figure will need to be considered if the final image is 16:9, such as with HD video.
Whilst a full HD image is 2 megapixels, HD1080P cameras will require a lens which is rated for a minimum of three megapixels.
For the purpose of this test, the lenses were compared using HD1080p cameras with a 16:9 output, and fine detail across the entire picture area was examined.
Tests were carried out around the clock, with night images being assessed over two sessions, one using white light and the other infrared illuminators. The latter allowed an assessment of focus shift which can occur when infrared light – which has a different wavelength to daylight – is introduced.
The TVAC65501 is a varifocal auto-iris lens with a focal length of 3.1-8mm and an aperture of F1.2. It is rated for 3 megapixel video, making it suitable for HD1080P cameras. The lens features an aspherical profile, which the manufacturer states is ground rather than the more typical glass and polymer hybrid. The lens is a 1/3 inch format unit, and features an IR compatible coating to address focus shift under artificial illumination.
The LVF-5005C-S4109 is a varifocal SR-iris lens with a focal length of 4.1-9mm and an aperture of F1.6–F5.2. It is rated for 5 megapixel video, making it suitable for HD1080P and higher resolution cameras. The lens is a 1/1.8 inch format unit, and has been designed for use with specific Bosch cameras. This means that the lens cannot be used with other devices, as it requires data from the camera’s processing engine to function correctly. It features an IR compatible coating to address focus shift under artificial illumination.
The M13VG288IR is a varifocal auto-iris lens with a focal length of 2.7-8mm and an aperture of F1.2. It is suited for use with Full HD cameras as it is rated for 3 megapixel video. The lens features an aspherical profile, and uses Flat Field technology to ensure sharp coverage, even on the edge and in corners of the image. It is a 1/3 inch format unit, and features an IR compatible coating to address focus shift under artificial illumination.
The VDD31V812IRP-3MP is a varifocal auto-iris lens with a focal length of 3.1-8mm and an aperture of F1.2. It is rated as a 3 megapixel lens, making it suitable for use with HD1080P cameras. The lens is suited for use with 1/2.7 inch format cameras. For day/night use, the lens has an IR compatible coating to eliminate focus shift when deployed under infrared illumination.
The AG3Z3112FCS-MPIR is an auto-iris lens. It is a varifocal unit with a focal length of 3.1-8mm and an aperture of F1.2. The lens is suitable for use with HD1080P cameras as it is rated for 3 megapixel video. The lens is ideal for use with 1/2.7 inch format cameras. It features an IR coreective coating which removes any issues associated with focus shift when deployed in a day/night monitoring application making use of additional infrared illumination.
The five lenses in the test have a large number of similarities, with one exception. All of the lenses are suitable for use with HD1080P cameras, which was the focus of our test. Additionally, all are varifocal units. Whilst all manufacturers will offer a wide variety of focal lengths (and for varifocal lenses, diverse ranges of focal length), our test units were selected based upon achieving a 6mm focal length.
All of the lenses feature IR correctional coatings, and as the use of infrared illumination grows, this will be of importance.
It has to be remembered that due to the increased pixel density of HD camera, low light performance is going to be a challenge. This will see an increase in the requirement for additional illumination, and infrared light’s differing wavelength makes correctional coatings essential to retain sharp focus.
The outsider of this group is the LVF-5005C-S4109 from Bosch Security. Whilst it fits the profile of the other lenses included in the test, it also enjoys additional functionality which has been optimised for use with selected Bosch cameras. This undoubtedly decreases its appeal to the wider market, but does mean that those who are using compatible Bosch devices can enjoy enhanced optical support, especially with regard to low light performance. The unit is well built, albeit a touch on the bulky side, but this is because it features additional electronics. The lens does not function with other cameras.
Generally, build quality of all the lenses is pretty much on a par. The AG3Z3112FCS-MPIR from CBC, the TVAC65501 from Abus-SC and the VDD31V812IRP-3MP from Vista are strikingly similar, to a point that we’d guess they’ve shared a factory! Both are well built, the focus mechanism is smooth, and they give no cause for concern in the field.
Tamron’s M13VG288IR has a solid feel about it, and the build quality is marginally better than the other lenses. Focus motion is smooth, and the focus arms lock easily.
When it comes to any assessment of lenses, the immediate benchmark will always be image quality. The more precise the optics, the better the quality of the final image. With high definition video, the main driver for sales – and the element that an end user’s expectations will hinge upon – will be enhanced image detail. For the end customer, they are not concerned with the differences between resolution, sharpness, detail, etc.. The want to see, and expect to see, an image that is equivalent to those displayed by other HD devices such as TVs, DVD players, etc..
The TVAC65501 from Abus-SC delivers a clean, sharp and detailed image. Abus mentions ‘edge sharpness’, referring to a consistent level of detail up to the edges of the image. Even with the use of charts it is hard to define any real drop in image detail across the entire scene.
Colour accuracy is good in a wide range of lighting. There are no signs of aberrations or loss of quality when the image is zoomed for closer examination.
The LVF-5005C-S4109 from Bosch Security delivers a sharp and clean image, and detail remains high across the entire image area. Colours are accurate, and tonal variances are preserved.
The images show no flaws or aberrations, and the optical quality of the unit does highlight that the lens is a match for the others on the test. Because it only works with compatible Bosch cameras, it is difficult to separate pure optical performance from associated processing within the camera.
The AG3Z3112FCS-MPIR from CBC delivers high detail throughout the image area, even into the edges of the image. CBC refers to this as ‘corner brightness’ which is explained as a larger image circle to ensure consistent coverage for widescreen images.
Colour accuracy is good, and tonal variances remain clear and obvious. Zoomed analysis of images reveals no abberations or flaws within the image, and overall resolution hits the levels expected.
The M13VG288IR from Tamron delivers a high quality image that is sharp and detailed, with consistency to the very edge of the image; test charts clearly illustrate that the ‘flat field’ technology works well. Colour accuracy is high, and tonal variances remain obvious and faithful to the viewed scene.
Careful examination of the final images shows no flaws or aberrations, and image quality is as expected.
Finally, the VDD31V812IRP-3MP from Vista delivers a good quality image, with consistent detail and sharpness across the entire image area. All elements of the viewed scene retain a faithful rendition of the scene, and there are no visible signs of flaws or aberrations.
Colour accuracy is consistent with a good degree of fidelity.
Essentially, all of the lenses delivered the type of image quality that you’d expect, and there was little to separate the differing units in normal conditions.
Low Light Performance
Low light is always an issue for cameras, and especially so for HD and megapixel units. This is predominantly due to the density of pixels on the sensor chip, and maximising the amount of light which falls on these is therefore vital.
The low light tests were carried out with a selection of HD cameras (except the Bosch lens for obvious reasons). Initially the cameras were optimised for clean noiseless images using a low cost unbranded megapixel lens. The test lenses were then used to see what improvements, if any, were realised. Finally, the images were viewed under infrared lighting to see if any focus shift occurred.
The Abus-SC TVAC65501 will allow the capture of sharp and relatively noise-free images with light levels that are approximately 1 lux lower than those we achieved with the unbranded control lens.
The lens also performs well when switching IR lighting either on or off, and there was no visible focus shift. In night mode with IR illumination, the camera captured a high degree of detail.
Judging the LVF-5005C-S4109 from Bosch Security was slightly more complicated, as cross-referencing some of the results was not possible due to the nature of the lens. With a compatible Bosch camera it worked well, and our feeling was that low light performance is good.
It would be wrong to penalise the lens for being operational with specific cameras – it is what it is – but the nature of the device does limit its appeal. When infrared illumination was in place, the image quality was impressive, with a high degree of sharpness.
CBC’s AG3Z3112FCS-MPIR does deliver enhanced low light performance. When compared with the unbranded control lens, the CBC offering delivered an equivalent level of image quality in light levels of around 1 lux less, which is fairly impressive.
There was also no visible focus shift when the test cameras switched from day to night mode, or night to day mode, when infrared illumination was deployed. Image quality under infrared illumination was sharp and detailed.
The Tamron M13VG288IR delivered clean and detailed images at light levels which were approximately 1.7 lux lower than those required when using the unbranded control lens, which is a significant difference.
Under infrared illumination, the M13VG288IR performs well, and there was no visible focus shift when the IR lighting and camera were switched from day to night, or night to day mode. When IR illumination was in use, the detail level was very high.
Finally, the VDD31V812IRP-3MP from Vista again was able to deliver a level of performance akin to that achieved by the control megapixel lens but in light levels that were approximately 1 lux lower.
When switching between day and night modes, with infrared illumination being used, there was no visible focus shift, and the image quality was consistent. Image quality under infrared illumination was high, with a good degree of detail.
The fact that quality lenses enhance video performance is not in doubt, and this is increasingly important when looking at high definition video. If you stick with well manufactured lenses, and assuming that the lens specification is correctly calculated, you won’t go wrong.
The Abus-SC TVAC65501 is a good quality lens, and won’t be out of place on an HD camera. Indeed, the same can be said of CBC’s AG3Z3112FCS-MPIR and Vista’s VDD31V812IRP-3MP. The indications to the test team do point to these three lenses sharing some heritage along the way. The build quality is very similar, as are the specifications, and with regard to performance you wouldn’t really spot any differences between the three.
The LVF-5005C-S4109 from Bosch Security is more limited in its appeal, because it is only of use with specific Bosch cameras. It has been designed to interact with the camera’s circuitry, and as such falls outside of the universal appeal of the other units. It is recommended, but with the proviso that you’ve already specified one or more of the compatible cameras. We did have a debate as to whether this limitation should result in the rating being lowered, but decided against that as Bosch is very clear about its use.
The Tamron M13VG288IR just heads the field. It isn’t significantly better than the other units, but it does have a slight edge with regard to final image quality. It also was rated as the Best Buy.
In reality, none of the lenses in the test will disappoint; if anything, the test results are a reminder to many that the right quality optics will ensure that HD performance is fully realised!