Essentials of Digital Cameras and Lenses

Welcome to the final installment of Security Sales & Integration’s four-part series — “Digital Video for D.U.M.I.E.S.” (see the March issue for Part 1, “Essentials of Digital Video Compression,” July for Part 2, “Essentials of Digital Video Storage,” and September for Part 3, “Essentials of Digital Video Recording”).

This series has been designed to help educate readers on the fast and ever-changing world of digital video as found in the security industry. “D.U.M.I.E.S.” stands for dealers, users, managers, installers, engineers and salespeople.

This month’s chapter — brought to you by Tamron — explains the relationship of cameras, lenses, digital transmission methods and applications as applied to today’s world of electronic security.

Video Input Matters to DVRs

As proven, a digital recording system relies on video input for success. Without proper video, a digital recording system cannot function correctly. In fact, to produce a highquality playback image, a digital video recorder (DVR) requires many different parameters.

The first feature open for discussion is automatic gain control (AGC). Many people are already familiar with this feature, but what does it have in common with digital video systems? First, for those unfamiliar with the function of AGC, a quick update is in order.

The purpose of AGC is to increase the sensitivity of the camera during low-light level applications. This feature is a compromise. It amplifies the video signal to improve image strength but at the same time amplifies the noise generated by the circuitry within the camera. This amplification of noise causes a grainy image, which randomly moves about the scene. (See image at right.)

With that, how can AGC affect the performance of a digital recorder? One of the features discussed in Part 3 of this series was activity detection. As you might recall, activity detection is a method by which the digital recorder can save storage space. This feature detects video signal movement and triggers the recorder to start recording. Under low-light level conditions, AGC is activated, but to the recorder, the AGC noise generated appears to be motion. The result is large qualities of wasted storage media.

When confronted with this situation, some technicians turn off the AGC switch. This solution may reduce the noise, but the video image usually becomes unusable as well.

A second camera feature is known as signal-to-noise ratio (SNR), or video-signal strength vs. the noisesignal strength. All circuitry generates internal noise, and it becomes a major factor in low-light conditions due to the activation of AGC. In short, the poorer the noise ratio, the greater the noise generation. Because SNR is measured in decibels, the lower the number, the worse the noise generation. It is suggested that when incorporating cameras and digital video recorders in the same system, the

SNR should be at least 50dB to ensure optimum system performance.

This again can cause false activation of motion detection systems used in today’s digital recording systems as well as motion detection in standalone digital multiplexers. This effect on activity or motion detection only occurs if the feature is enabled.  There are a few suggestions for overcoming this noise problem, but no guarantees they will all work to your satisfaction.

Lens Quality Can Fight Noise

The first suggestion to overcome noise requires selecting a camera in which the signal output strength is greater than what is specified by the manufacturer data sheets for lowlight operation. For the most part, camera specifications list the sensitivity of the camera with only about 30 percent of recommended video levels. What this indicates is that AGC must work harder to produce a useable video output signal. By increasing the amount of light to the camera during low-light application, the video strength is better and reduces the need or level of intensity for the AGC circuitry.

Secondly, if the camera performance cannot be improved or the light levels cannot be increased, improving the lens quality may help the situation. The main function of an autoiris lens is to focus the image on the sensor and control the amount of light reaching that sensor.

All lenses are rated by f-stop, indicating how well the lens passes light. (F-stop rating is the ratio of the millimeter of the lens to the millimeter opening of the iris.) The lower the f-stop ratings of an auto-iris lens, the more light reaches the camera sensor during low-light situations. As more light is available, the less need for AGC and the better the noise characteristic of the video signal.

Most auto-iris lenses have an f-stop rating of 1.2 to 1.4, which in most cases is sufficient for most low-light applications. However, with digital video requiring more signal strength to ensure good video imagery, the need to provide more light is a must. Lenses today now offer better optics and designs allowing for improved fstop ratings.

Tamron’s employment of advanced optical designs in all IR lenses and special glass (LD elements) in its 3.0- 8mm IR (see photo above) lens and varifocal series has resulted in the elimination of shifts of focus points in both visible and infrared ranges. Rays of light from both spectrums are focused onto the same focal plane, resulting in sharp images. This type of compensation is necessary because IR lenses are ideal not only for day/night cameras but also for conventional black-and-white cameras sensitive to both visible and near infrared light.

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