Introduction
The first part of this article focused on the specifications of the camera sensor that impact image quality. In Part II, we will examine the other component of a camera’s content capture mechanism critical to image quality.
Lens
Just as the sensor is critical to the quality of the image, so is the optics of the camera. The two work in tandem, and it is therefore important that the lens selection is made, keeping the sensor specifications in mind. We are not talking about lens specifications such as focal length, F-stop, Field-of-View here; but something more fundamental. The key parameters in lens selection, for an IP surveillance camera, are:
- Material
Glass lenses deliver significantly better image quality than plastic lenses. To be fair, since a lens is typically made of 3 – 4 elements, there may be cases where the required optical specifications can be fulfilled only with the use of a plastic or moulded element in the inner layer.For reasons of cost, some surveillance camera vendors offer plastic lenses (outermost element) on their cameras, and this has a deleterious effect on the quality of the captured image.
- Resolution
Lens resolution is usually specified in lp/mm (line pairs per millimeter) or TV lines. It is advisable that the resolution of the lens is always a little higher than the resolution of the sensor. To do the comparison, divide the number of vertical pixels of the sensor by the height of the sensor; to arrive at the pixels per mm (please note that some surveillance cameras report different resolutions for video and for still images, so the figure to go with is the video resolution). Divide this number by two, to allow a comparison with the lp/mm metric of the lens. You will see that the smaller the sensor, the higher needs to be the resolution of the lens.The problem is that lenses of good resolution (60 – 80 lp/mm and higher) cost a lot. Surveillance cameras typically use lenses that cost tens of dollars; while a good resolution lens (used in digital photography) typically costs hundreds, if not thousands, of dollars.
- Contrast
The Modulation Transfer Function (MTF) quantifies how well a lens preserves contrast information. The problem is that there is no standardization in how MTF is arrived at, among vendors, making it difficult to compare the MTF of lenses from different vendors. The best way to evaluate this parameter, then, is to compare the image quality of the surveillance cameras in-situ, under similar conditions.The attached link provides more information on lens quality, for those interested:http://www.cambridgeincolour.com/tutorials/lens-quality-mtf-resolution.htm
Conclusion
Remember, image quality is more than just megapixels. While a real-life test will help determine the suitability of a surveillance solution for current requirements, it is important to investigate, in detail, the sensor and lens specifications; in order to arrive at the headroom that the surveillance solution will provide, in terms of stable image quality, as surveillance norms become more demanding.
