• Varifocal offers a range of focal lengths and different manually adjustable fields of view. Whenever you change the field of view, you need to manually refocus the lens. Remote focusing capabilities have been added to the latest generation of network cameras, enabling you to focus the camera from a remotely-located computer instead of while up on a ladder or lift.
• Zoom offers a range of focal lengths and different adjustable fields of view. Unlike varifocal lenses, zoom lenses do not require refocus of the lens if you change the field of view. They automatically maintain focus within a range of focal lengths, such as 6 mm to 48 mm. You can adjust zoom lenses manually or remotely through motorized control.
Deciding on an Iris Control
Another important factor in image quality is how you control a camera’s iris opening. The iris — which lets in a specific amount of light depending on the size of its opening — controls the depth of field and maintains the optimum light level to the image sensor so that the image appears sharp, clear and correctly exposed with good contrast and resolution. Iris control can be fixed or adjustable.
• Fixed iris control is set at a certain f-number and the opening cannot be adjusted. It is most appropriate for indoor environments with constant light levels.
• Manual control allows you to adjust the iris opening by turning a ring on the lens. Like a fixed control, it works best where light levels are constant.
• DC-iris, also known as auto-iris control, uses the camera’s processor to automatically adjust the aperture (opening of the iris) to changing light conditions. The DC-iris only has two aperture settings — small and large. It cannot be fine-tuned to an exact iris position.
• P-iris control provides an automatic, more precise aperture as it uses special software to communicate with and control the iris’ exact position to manage even the slightest change in light. This provides better, “precise” control to optimize contrast, clarity, resolution and depth of field. The P-iris control also eliminates the blurring problems that may occur with DC-iris control, which is especially important in high-resolution cameras such as 3- and 5-megapixel.
Choosing the Right Resolution
Different surveillance objectives require different resolutions, so you need to select a camera and lens that match the field of view to the size of the scene you want to capture. If your only goal is to detect an individual in a scene, a lens with a linear resolution of 20 pixels per meter would probably be sufficient. For forensic detail such as a license plate number, the linear resolution would need to be around 200 pixels per meter. For facial identification purposes, the resolution would have to be closer to 500 pixels per meter.
Check that the megapixel rating for the lens matches the megapixel rating of the camera. Image quality suffers when you use a non-megapixel rated lens on a megapixel camera or even a lower-resolution megapixel lens on a higher-resolution megapixel camera.
Lenses often experience color distortion called chromatic aberration. This occurs because lenses have different refractive indices for different wavelengths of light and every color in the optical spectrum cannot be focused on a single common point. The aberration often manifests itself as fringes of color along boundaries that separate dark and bright parts of the image. Choosing an achromatic lens — a compound of different materials with differing light dispersions — reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not eliminate the distortion altogether.
If you are using a day/night camera, be sure the lens includes an IR cut filter on the lens that can be automatically or manually removed depending on lighting conditions. That way, when surveillance occurs under infrared lighting conditions, the lens will not trigger a shift in focus and color distortion.
Specialty lenses compensate for a host of problems. For instance, a wide-angle, low barrel distortion lens offsets barrel distortion caused by lens magnification being smaller at the edges of the field of view compared to the center of the image. This phenomenon is often more pronounced at short focal lengths, making a standard wide-angle lens less suited for identification purposes.