Lens for the Network Attached IP Camera
The resolution of your IP camera system is determined by both the camera and lens. The lens must have enough resolution to match the megapixel sensor in the camera.
How Much Resolution is Required from Your IP Camera System?
The amount of resolution depends on what you want to see. Do you want to see a tree, or be able to pick out the shape of leaves? Do you want to be able to see a crowd of people, or be able to identify a person’s face? The more detail we need the more resolution we require.
The best way to provide a measure of resolution is to determine the number of pixels across the object you want to view. For example, we determined that about 50 pixels across a person’s face will allow you to recognize a person you know. You will need a higher number of pixels to identify a person that you don’t know. We need about 50 pixels/ft. to identify a license plate number, that is if the lighting is good and the vehicle is not moving too fast.
The Real World View of IP Cameras
IQinvision did some testing and published their results on their web page. The following test was done by IQinvision.
Methodology: Using an IQeye camera and a 6.5 mm lens they increased distance and took an exposure until the face was unrecognizable. Then the head and shoulders were cropped from the large image. They used Photoshop to copy the face and enlarged it to 92 by 110 (the native resolution of the largest face) using bicubic interpolation. No sharpening or other manipulation was used.
As you can see in the pictures above, 40 pixels across the face is probably the minimum number of pixels necessary to recognize a person that you know. If a face is 6.0 inches wide, then we need 80 pixels per foot. Axis also wrote a white paper on the same subject and determined that there were 3 classifications to consider, detection, recognition, and identification. In this case, you only require a few pixels (3 pixel across the object to detect a person. You would require only 17 pixels across the face to recognize someone. The reason you need less resolution is because recognition assumes that you know them. You would make a determination using more than just the face. You could use the clothes, the way they walk, their size and shape to recognize them with very good accuracy. On the other hand, if you didn’t know them at all, you would require a lot more information. In this case, Axis recommends 40 pixels across the face for positive identification.
The Right Lens
The lens is a very important part of the camera. If you don’t have the right lens the resolution won’t be as good as you expect. A lens is more than just a single glass, it contains an iris, and multiple lens adjustments that allow it to be focused and the zoom adjusted. The lens is actually a complete system that works together to provide a sharp image. The resolution (or detail) you see is determined by the number of pixels in the camera sensor, the electronics, compression and the quality of the lens. You can have a 5-megapixel sensor, but if your lens only allows you to see 1 megapixel of resolution, then all you will see is the lower resolution. Besides the resolution of the camera, the lens system also determines how far away you can see something. It defines how wide an area (the Field of View) you can see.
The angle of the lens is very important. It determines the size of the viewing area (or field of view). Take a look at our discussion about the field of view and lens angles.
Important Lens Specifications
The f-number specification defines the amount of light that can pass through the lens. The lower the f-number the better it is for low light applications.
Lens Resolution or Clarity:
The resolution of the lens is measured in line pairs per millimeters. The lens should have roughly double the resolution of the camera sensor. Computar and others now provide resolution quality in megapixels. If you have a 5 megapixel IP camera then select a 5-megapixel lens to get the best performance.
A lens system used on an IP camera provides many controls and functions for getting the best picture possible. There are a number of other lens characteristics including distortion, IR capability, and iris control that should be considered. Many cameras are designed to operate at night using IR illumination. If the lens is not made correctly the IR light will be distorted. If you plan to use the camera at night select a lens that says it will work with IR.
The specifications of the lens and camera determine what we will see. The higher the resolution of the IP camera the more detail we will see and the wider field of view we can see with the same resolution. The lens determines the quality of the image and the field of view. Both factors work together to provide the view we need for the application.
You can get more information about the other important camera specifications such as light sensitivity and frame rate in the article Understanding Camera Specifications.
Need some help selecting the right lens and resolution, just contact us. We will be happy to help. 1-800-431-1658 (in the USA) or 914-944-3425 (outside the USA) or send us a message.