How to Determine the Rght Lens and Resolution
So what do you want to see with your camera?
The resolution of the camera and the lens define what you will see. The resolution of the camera defines what detail you will see while the lens will determine how far away the image is from the camera and how wide an area (the Field of View) you can see. IP cameras provide a wide choice of resolution, but how much resolution do you need? It just depends on your requirements. Do you want to be able to identify a personís face or just know how many people are in a large room? Once you know what you want to see, you can specify the resolution of the camera and the type of lens you will need. This article describes how to select the right camera resolution and lens.
Historically TV was measured in Lines of Resolution; today digital IP cameras are defined by the number of Pixels in the camera sensor. Both of these measurements are related. TV lines are classically defined as the maximum number of black and white lines that can be seen on a monitor when you view a special test pattern. The measurement was only defined for black white video (monochrome) and not color. The number of TV lines you can see depend on the number of pixels as well as the resolution of the monitor.
Today Digital IP Camera Resolution is defined by the horizontal (H) and vertical (V) number of pixels (i.e. 640 x 480) or by the total number of pixels in the sensor (H x V = 336K Pixels). Megapixel cameras use sensors that can provide resolutions of 2592 x 1944 (5 Megapixels) or more.
How much resolution do we need?
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 personís face? The more detail we need the more resolution we require. The resolution (or detail) is defined by the number of pixels in the camera sensor.
The best way to provide a measure of resolution is to use the number of pixels across an object you want to view. For example how many pixels are required across a personís face to identify the person? To standardize this we can also ask the question, how many pixels per foot or per meter are required to identify something? How many pixels do we need to identify the license plate number? How many to identify a person? We can do some calculations to figure this out, or we can do some testing with different cameras and lenses.
The Real World View
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 per face is probably the minimum number of pixels necessary to identify a personís face. If a face is 12 inches wide, then we need 40 pixels per foot.
The Field of View: How wide an area do you want to look at with the camera?
The field of view determines how wide an area you will see. If you need to identify a personís face, then using the recommendations above, we need to select a field of view that provides a minimum of at least 40 pixels/ft. If the field of view is wider, we need higher resolution to see the detail. If the field of view is wider than the number of pixels available, we wonít have enough pixels to identify the person.
Suppose we want to look at an area thatís 40 ft wide. We use the following formula to determine the horizontal resolution of the sensor. Ph is the number of horizontal pixels, and W is the maximum field of view.
Ph = 40 X W
For example, if we want to view an area that is 40 ft wide, then the number of pixels required is 40 x 40 = 1600 pixels. We need a camera that has a resolution of at least 1600 horizontal pixels.
If we already have the camera, we can determine the maximum width we can see. We use the following formula.
W = Ph/40
For example suppose we have a camera with a resolution of 1280 x 1024, then W = 1280/40 = 32 ft
The lens determines the field of view you will see. You can select a wide angle lens to view objects that are close to the camera, or select a telephoto lens to see objects further away. For example, a telescope provides enough magnification to see the craters on the moon. You may not need that much magnification, but there are some lenses with enough magnification to see peopleís faces over 1,000 ft away. The higher the focal length (mm) of the lens the more magnification you get. The lower the number the wider the angle of view you will get.
When you use a high resolution camera you must select a high resolution or Mega-pixel lens that will maintain the high resolution.
Selecting the Lens
As an example, letís select a lens for our 1600 x 1200 resolution camera. The closer to the camera the wider angle (or smaller mm) lens we require, the further away the narrower the angle and the larger the mm of the lens. The formula for calculating the distance and width based on the focal point of the lens is as follows. This formula has some assumptions about distance between the lens and the sensor, so itís really a rough calculation, but it will get us close enough to select the right lens:
D = fw x W / CCDw
fw = focal length of the lens in mm
D = Distance from the camera to the field of view
CCDw = width of the CCD
W = width of the field of view
ē 1/3 inch 4.9 mm horizontal 3.7 mm vertical
ē Ĺ inch 6.4 mm horizontal 4.8 mm vertical
ē 2/3 inch 8.8 mm horizontal 6.6 mm vertical
ē 1 inch 12.7 mm horizontal 9.6 mm vertical
It is much easier to use a lens calculator. The lens calculator can help you calculate the distance to the object, width of the field of the view and lens focal length (f in mm). The lens calculators available use similar types of calculations, but you will get different results depending on which calculator you use. Thatís why itís best to select a variable lens that covers the range that you have calculated.
For example, using the lens calculator, I selected the Ĺ inch sensor, entered 4 mm in the size of the lens and entered 40 ft in the width of the picture, and it indicated that the camera should be 25 ft away. When I reset the calculator and entered a 10mm lens, the field of view then is 95 feet from the camera.
The resolution of the camera and the lens determine what we will see. A higher resolution camera provides better detail. The lens determines 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 in our previous newsletter.
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.
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