Camera Selection

In this article we will look at how to read the Optical Charts provided by the lens manufacturers.

Definitions:

Camera Distance (CD) - The distance from the base of the lens to the plane level with the object surface to be inspected.

Field of View (F.O.V)- The total area that will be in view of the camera, measured as  X (mm) x Y (mm)

Horizontal Pixels (H) - Number of pixels in the Horizontal Dimension of the image

Vertical Pixels (V) - Number of pixels in the Vertical Dimension of the image

Extension Tube - An extension that is placed between the lens and the camera to physically adjust the focal length of the camera. 

Chart Axes - The X Axis of the Optical Chart represents the Y Dimension in the image F.O.V

 

 

For this example we will use a F.O.V requirement of 200mm in the Vertical (Y) dimension of the image and minimum 300mm in the Horizontal (X). We will also assume a mounting restriction of a maximum 500mm distance to the object.

 

Selecting a Camera:

Given the above requirements we can determine a resolution that would be suitable to use for this application. Each resolution will provide a different ratio in the image and therefore different F.O.V. (Example of this below)

2MP:

H = 1088 pixels

V = 2040 pixels

1. Figure out the ratio of V to H

H/V = 2040/1088 

        = 1.875

2. Use the ratio of pixels to find the X dimension of the image based on the required 200mm in the Y dimension

V * 1.875 = H  -→ Y * 1.875 = X

 200 * 1.875 = 375mm ✔️

3.2MP:

H = 2040 pixels

V = 1536 pixels

1. Figure out the ratio of V to H

V/H = 2040/1536

        = 1.33

2. Use the ratio of pixels to find the X dimension of the image based on the required 200mm in the Y dimension

V * 1.33 = H  -→ Y * 1.33 = X

 200 * 1.33 = 266mm ❌

 

From this we can conclude that the 2MP resolution is going to suit this application better as we can have a 200mm Y dimension and also achieve the 300mm dimension in the X.

 

Selecting a Lens:

Example Chart:

 

 

1. Looking at the X axis of the Optical Chart (noting that it is a logarithmic scale) we can find the point that gives us 200mm in the F.O.V
2. Follow this line vertically and we can see that all of the available lenses intersect with this line, meaning we can achieve this with any lens, the main variable is now the Camera Distance.
3. Using the maximum mounting distance specified we can see that we have 3 lenses that will give us the 200mm in the Y axis of the image.
4. For this example we will work with an optimal distance of ~ 300mm, the chart shows us that we will need to use an 8mm lens for this application.
   1. Zooming into the chart we can see that this is not exactly 300mm, perhaps 325mm. However, as we have room up to 500mm and we are able to achieve the F.O.V, this is acceptable. 

 

Extension Tubes Explained:

In the above optical chart there are regions of the lines that have a t number associated with them.

These “t numbers” are indicating where you will need to use an extension ring for the camera and what size you will require. In the region that we selected the lens in the above example we can see that we have “t0” indicating that this does not require an extension tube to be used. 

In another example, seen in the above images, to achieve a narrow F.O.V at close range (with a long focal length lens, 100mm) we require the use of 20, 25, 30, or even 40mm extension tubes.