Home System Design Lenses: the Key to 4K Image Capture

Lenses: the Key to 4K Image Capture

by Benchmark

In any video surveillance system, the lens has a critical role to play, and the selection of the right lens can significantly change the performance of the camera. However, the impact of a lens is even more important as higher resolution cameras are deployed in surveillance systems, as the increased detail makes any aberrations more obvious. Lens choice can also affect other aspects of the system, such as low light performance. As a result, budget lenses could be a false economy.

The lens is effectively the ‘eye’ of the video surveillance system. With a poor quality lens, the video image will always be compromised. Unless the optics are specified correctly and are built to a sufficient quality, the captured video will always be compromised. When high performance cameras are specified, it is nonsensical to save a few pounds by selecting a mediocre lens. The reality is a more significant saving could be made by selecting a lower resolution camera. One thing is certain: any quality lost due to the wrong specification lens will stay lost. All the processing in the world won’t bring it back.

Selecting an appropriate lens for a camera is a skill that is less in demand than it used to be. This is because manufacturers are increasingly offering static dome or bullet-type cameras which are factory fitted with a lens. The problem with this approach is that the integrator or installer has less choice as to which optics are deployed, and often has to accept the quality level which the manufacturer has set. In some cases, the integral lenses are high quality, but in others the economies of scale are a more significant driver.

Even the manufacturers of high quality body cameras will often include a lens from a selected manufacturer, as the firmware will include specific support features for a given range of optics. While all of this seems to point towards the end of integrators and installers specifying lenses for themselves, such a move would be detrimental in many video surveillance applications.

Why lenses matter

The video surveillance camera lens is a device which focuses light onto the sensing chip of the camera. The sensor then converts the light into an electrical charge, dependent upon its intensity, which is used to determine the pixel value and intensity when creating a video image. Of course, there is much more to the creation of a video image, but this very basic explanation serves to underline one fact; focusing the light as accurately as possible onto the sensor is vital in creating good quality video images.

There are many different types of lenses, with variations for camera formats, focal lengths, iris requirements, low light applications, day/night applications and – appropriately for those seeking high resolution, HD or 4K UHD-based solutions – megapixel lenses.

One of the most basic specifications for integrators and installers using megapixel, HD or 4K UHD cameras is that an appropriately-rated lens, in terms of maximum resolution, should always be used.
There are some who will claim that certain scenes won’t require a true 4K UHD specification lens, for example, and that the money saved by using an HD quality lens can offset the cost of the camera. If integrators or installers follow such advice, it raises the question why they don’t simply save more money by using an HD camera in the first place?

An underspecified lens will rob the camera of its full resolution from the start, and so using a lower specified lens is a pointless option.

Benchmark has carried out tests with 4K UHD cameras using appropriately specified lenses, and considered the achieved performance against lower specification optics to see how significant the differences were in performance. In every case, when back-to-back comparisons were carried out between under-specified and correctly specified lenses, the cameras offered higher quality images in regard of fine detail, and enhanced low light sensitivity, when used with an appropriately rated lens.

There is a very simple reason for this: lenses used in video surveillance applications are not perfect; it would not be cost-effective for them to be so. Lenses are manufactured to finite tolerances, based on the specified resolution. In simple terms, they are only as good as they need to be.

Grinding out performance

One of the more expensive tasks involved in manufacturing a lens is the precision grinding of the glass and setting up the lens combinations. The more precise the grinding and accurate the placement, the longer the process takes, and the more care that needs to be applied to the process. Therefore, it stands to reason that the more accurate the lens, the more expensive it will be.

If a video surveillance lens is intended for use on a standard definition camera, there is no point in investing time and effort in grinding the glass or building the lens to a higher tolerance than the camera can resolve. It is, frankly, not going to make one jot of difference to the final image quality. However, putting an SD lens on an HD camera (or, for that matter, an HD lens on a 4K UHD camera) will immediately deliver an image of lower quality than the camera can potentially resolve.

The idea behind specifying a resolution for lenses is that the figure indicates how accurate the lens will be. It will only be manufactured to a quality level which eradicates the visual aberrations that a specific resolution sensor might potentially display. Anything more is a waste of time and energy.

Take an HD720p camera, as an example. This will utilise a chip that contains 1280 x 720 pixels. In effect, this means that the chip will contain 921,600 picture elements that make up an image.

If there is a flaw in the glass of an HD720p lens, it only matters if it affects enough pixels to make it visible to those viewing the footage. In reality, a flaw could cover a few pixels, and wouldn’t be obvious.

If the image was magnified, the digital pixelation would make the image unusable before the flaw was obvious. Therefore, it makes no sense for a manufacturer to remove that flaw through more careful grinding, because no one will see it. In truth, the cost difference of doing so would mean the lens would not be competitively priced.

However, if that lens was then to be placed on a 4K UHD camera, things change dramatically. Rather than the 921,600 pixels on the chip, the 4K UHD camera has 8.3 million pixels, often on a similar sized chip. This means the image sensor has nearly nine times as many pixels in the same area, so the flaw would cover significantly more pixels.

Obviously, as the video image is zoomed, the ratio is far higher than with a HD720p chip, so the flaw becomes increasingly obvious when viewed.

Good quality lenses should deliver the same quality at the very edge of the image as in the centre. Again, this is an area where under-specified lenses may struggle, especially as higher resolution cameras often employ marginally larger image sensors.

One of the most significant features of 4K UHD devices is that one camera can stream resolution equal to four Full HD video streams. Therefore it is essential that the lens can resolve this higher resolution, even in all four corners of the image.

There are many lenses in the video surveillance market which claim to offer 4K quality (it’s a subtle difference between specifying 4K UHD resolution). Some of these will only deliver true 4K resolutions in the image centre but not at the very edges. As a result, it’s not possible to deliver video streams in genuine 4K UHD quality.

However, lenses from the quality security manufacturers which are appropriately specified will be able to keep the high resolution from the image centrer to the four corners of the image.

Design considerations

Lenses for video surveillance are complex. A single element lens has limited capabilities, meaning it is difficult to get a viewed scene in focus. Adding more elements to the lens increases the number of variables that can be used to control the light, resulting in a better and tighter focused spot. For the smaller pixels of a 4K UHD sensor, the light needs to be focused on a very small spot and so it requires many elements in the lens.

A lens of lower image quality may be able to transform a linear light-to-dark edge, but the transition on the sensor will not be as sharp as is required. This transform is called the lens MTF (modulation transfer function).

The transition may have a width of 4µm to go from white to black. If the pixels of the HD sensor are 3.1µm there is a pretty good match between the lens transfer function (MTF) and the pixel size. It’s okay for the MTF to be a greater than the pixel size because inter-pixel interpolation can use surrounding pixels to determine exactly where a line is in the image.

However if the same lens is used for a 4K UHD sensor with pixels of 1.55µm, there is too great a mismatch between the MTF of the lens and the pixel size (4µm versus 1.55µm). Small lines in the image such as the letters on a number plate will become fuzzy and gray.

A lens with higher image quality will be able to focus the light to dark edge to a width of only 2µm. The extra sharpness allows the image sensor to better determine the location of the edge, resulting in a sharper image with higher contrast.

There will always be a fine balance in image capture. A high quality lens is needed to create sharp, high contrast edges at the image plane. A high resolution sensor can see these edges clearly, and the image processing chip will not degrade the image more than necessary. The goal is to match the image quality of each camera element – lens and sensor – to avoid paying for a specified image quality which then cannot be realised.

The reality is 4K UHD cameras need 4K UHD lenses in order to take advantage of the available resolution on the sensor chip.

In summary

Using an under-specified lens might save a small amount of money, but it is a corner-cutting exercise which could end up reducing the effectiveness of the whole system. If an end user has invested in 4K UHD cameras, then fitting inferior lenses makes such an investment pointless.

As the use of 4K UHD or higher megapixel video increases, so the specification of suitable megapixel lenses becomes more critical.

One note of caution is that not all megapixel lens specifications are the same! The rating can be defined in two different ways. While a 4K UHD video stream is 8.3 megapixels, it is still common for megapixel lenses to be rated for 4:3 images, so a 4K UHD camera would require a lens rated for 12 megapixels if that is how the manufacturer rates their lenses.

A high quality lens will also aid low light performance, and given that most 4K UHD cameras may not have the best sensitivity levels, any boost obviously helps.

There is no substitute for an appropriately specified quality lens. As improved image quality is one of the main drivers behind 4K UHD deployments, any other option seems a foolhardy path to follow!

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