The lens gathers the light reflected (or transmitted) from the part being inspected, and forms an image in the camera sensor. The proper lens allows you to see the field-of-view you want and to place the camera at a convenient working distance from the part.
To pick the proper lens you will first need to know the field-of-view (FOV) and the working distance. The FOV is the size of the area you want to capture.
Here is a typical example: If the part to be inspected is 4" wide and 2" high, you would need a FOV that is slightly larger than 4", assuming your staging can position the part within this FOV. In specifying the FOV you have to also consider the camera's "aspect ratio" - the ratio of the width to height view. The cameras used with Vision Appliances™ have a 4:3 aspect ratio. In the previous example, the 4" x 2" part size would fit in a 4:3 aspect ratio, but a 4" x 3.5" part would require a larger FOV to be entirely seen.
The working distance is approximately the distance from the front of the camera to the part being inspected. A more exact definition takes into account the structure of the lens.
From the FOV and working distance and the camera specifications, the focal length of the lens can be estimated. The focal length is a common way to specify lenses and is, in theory, the distance behind the lens where light rays 'from infinity' (parallel light rays) are brought to a focus. Common focal lengths for lenses in machine vision are 12 mm, 16 mm, 25 mm, 35 mm and 55 mm. When the calculations are done, the estimated focal length will probably not exactly match any of these common values. We typically pick a focal length that is close and then adjust the working distance to get the desired FOV.
There are other important specifications for lenses, such as resolution (image detail - depends on the camera and the lens), the amount and type of optical distortion the lens introduces and how closely the lens can focus.
Given all of these issues, we recommend that you work closely with your DALSA IPD distributor to choose the appropriate lens for your application.