As image quality becomes increasingly important to recorded video for forensic detail and identification, lenses and optics have become a critical component. Optics include the lenses, prisms, mirrors, flats, optical filters and other optical components that help bring the image into focus and to the sensor.
Higher quality images demand higher quality lenses. Megapixel cameras and the move to high-definition (HD) have changed the rules of lens selection. A lens for a megapixel camera is not just an accessory, but rather an essential that optimizes the quality of images captured and as such when specified correctly can help make the most of megapixel imaging.
For security dealers and systems integrators, ensuring that the lens selected is compatible and on a par with the camera enables those applications that realize the full advantages of megapixel video.
Lens capabilities make a big difference, which is obvious in a comparison of a 3 megapixel image using a 1.3 megapixel lens, alongside a 3 megapixel image using a 3 megapixel lens. The latter is much clearer. Lens selection is especially important with regard to face recognition and identification, a common application for megapixel cameras. Progressive output (such as those offered in Panasonic cameras) enables identification of a moving object and the resulting high-resolution image is much clearer. Another common application is license plate recognition, where a high-resolution image is required both at the center and at the edge of a view, made possible only by the correct selection of the megapixel lens.
Megapixel imaging is often used to replace a larger number of standard network cameras with fewer megapixel devices in a large space. To be effective, a camera has to enable image magnification (digital zoom) at any part of the image (including at the edges), and the right lens maximizes those higher-resolution capabilities.
With fundamental design differences in the metal casing, lens mount, glass, etc. that differentiate them from standard or aspherical types, megapixel lenses require more precise mechanical alignment and use different image-sharpening algorithms. The most important variable among lenses is their ability to accurately and precisely transfer information from the object being viewed across the entire image area to each pixel of the CCD imager. The technical term for this is modulation transfer function (MTF). Megapixel cameras that employ lenses that provide edge-to-edge high-resolution viewing coverage and precise mechanical design ensure superior alignment with the megapixel image sensor.
There are certain specifications designed with a megapixel camera lens and the systems integrator needs to know what they mean to properly apply the technology. The commonly accepted metric is line pairs per millimeter (lp/mm). Line pairs are groups of alternating black and white lines and the larger the lp/mm, the thinner the lines and the greater the lenses ability to capture fine detail. The accepted specification is 30 lp/mm for standard lenses and a minimum of 60 lp/mm for megapixel. The megapixel lens needs to capture more detail because of the increased resolution of the megapixel imager. There is some loss of resolution at the edge of the image and specifications are usually based on the center of the image. If a lens is specified as “megapixel,” it usually refers to the number of pixels or dots that can be discerned at the center of the lens (and not necessarily at the edges).
The best way to determine the required lp/mm for a camera is to divide the number of pixels per chip width (or height) by the width (or height) of the chip and then divide by 2 (to account for line pairs). However, since many lens manufacturers do not divulge the lp/mm specification for lenses designed for video use, other methods of matching a lens to a camera are recommended. For example, many video camera manufacturers use lenses that are rated for either a particular megapixel spec (usually 1.3, 3 or 5 megapixels) or standard CCD size (1/2-, 1/3- or 1/4-inch CCD).
A reputable camera manufacturer will match the lens with the CCD and “fine tune” the picture using advanced digital signal processing (DSP) techniques. The easiest way to match a lens to a camera (and the most intuitive) is to use a standard “resolution” chart (commonly used for printers, scanners, copiers, video cameras, etc.) Although the exact resolution derived from using the chart is subjective, the approximation is clear enough to compare with other lenses and cameras and will allow the installer to quantify the precision of quality of the total optical assembly. – By Bob Kramer, product manager, Panasonic System Solutions Co.