One important, but often neglected consideration in video surveillance systems design is the trade-off between image resolution and field of view. Amid such trade-off, how to ensure image quality becomes critical. Lens selection also plays a key role in image quality optimization. In the previous article we dealt with some issues and now we are to discuss the rest of issues.
If you decide to increase your image resolution by increasing the camera resolution, you must also consider lens selection. The lens needed for an IP/megapixel camera is much different than the lens needed for a traditional analog camera. These higher resolution cameras demand higher performance lenses in order to take full advantage of the increased resolution.
For instance, in a megapixel camera, the focal plane spot size of the lens must be comparable or smaller than the pixel size on the sensor (Figures A and B below), otherwise the light falls off the pixel resulting in a fuzzy image.
Using a standard resolution lens with a 5 megapixel camera will produce a blurry image. Most often if you want to take advantage of all the pixels a multi megapixel camera provides, a high quality lens with matching resolution is needed otherwise the money spent on a high resolution camera is wasted.
One would assume, then, that all someone specifying a megapixel camera needs to do is specify a megapixel lens. However, it's not that simple and in many cases it can be quite difficult to ensure that the lens needed is the lens being offered. Many manufacturers refer to their products as a "megapixel lens" but they seldom define what the adjective "megapixel" means. Is that "MP" lens to be used with a 1.3 megapixel camera or a 3 megapixel camera? Can it be used with all megapixel cameras?
In megapixel cameras the pixel size and the required lens spot size varies depending on the size of sensor and quantity of pixels it contains. You can have 2 different cameras, both 1.3 megapixel, but if the sensor size is not the same the pixel size will be different.
In selecting a lens it is important to choose one that is designed for use with both the sensor size and the resolution of the camera. Over-specifying the lens to have a higher megapixel rating than the camera is acceptable. This ensures the lens is not the resolution limiting factor in the system.
By referring back to Figures A and B, you would probably expect that different levels of megapixel cameras would also need different lenses. Higher megapixel cameras with their smaller pixels need lenses with more internal glass elements and higher precision elements than lower megapixel models. These superior lenses should deliver the greater performance required for higher megapixel cameras.
Rectilinear vs. fisheye
Ultra-Wide lenses can be classified into two groups: Fishe eye and Rectilinear lenses.
Most wide-angle lenses have barrel distortion (also known as fisheye distortion) that causes the image to look curved and bulged out in the center. Rectilinear lenses like those made for the security and machine vision industries keep lines that appear straight in the real world straight on the image sensor. This has the benefit of increasing the resolution of the image at the edges (i.e., an object will cover more pixels in the image when the object is at the edge of the image) whereas lenses with barrel distortion cause the image to be compressed at the edges and resolution is reduced. With typical distorted wide-angle lenses, potentially valuable information is lost in the lens and no software, de-warping or otherwise, can recapture or reconstruct this lost information in the image. Any de-warping will create an image that looks like that from a rectilinear lens but at lower resolution. With a rectilinear lens, the image is spread over a greater number of pixels at the edges, increasing the probability of detection and identification.
There are also times where fisheye lenses can perform the job far better than rectilinear lenses! A fisheye lens enables you to focus very close to your subject. When shooting wide angle, you benefit from a large depth of field. You can achieve great colour and sharpness when you get very close to large subjects.
Finally, it is you who will decide which type of lens fits your needs and applications well.
Adapted from a&s Magazine