Basics of Lens Selection for Machine Vision Cameras

To optimize your high performance machine vision camera; it needs to be matched with the appropriate lens.  We specialize in cameras, but from our years of experience can offer some basics on lens selection.  The details and example provided are just a simple overview – please contact us with any complex questions.

We can classify the machine vision lenses into two unofficial broad categories:

1. Lens for Field of view (image size) that is much larger than camera sensor size
2. Lens for Field of view that is smaller or near to camera sensor size.

When the field of view (image size) is much larger than camera sensor size

For applications in this scenario, the field of view (FOV) ranges from about 20-30 millimeters to as large as 100 meters (outdoor applications).  The lenses for these applications come with either fixed focal distance or a variable focal distance.

There are 3 important factors that contribute in the selection process:

1. Field Of View
2. Working distance
3. Sensor Size of the camera

Magnification of a image acquired = (Sensor Size of the camera)/(Field of View)

To estimate the required focal length for your application:

Focal Length = (Magnification)*(Working Distance)/(1+Magnification)

For Example:

The Adimec Opal-2000 camera has a sensor size of 2/3”. If an application require the camera to look at a FOV of 50mm (Horizontal) and a working distance of about 200mm, by using the equations mentioned earlier we should be able to estimate a lens suitable for this application.

FOV                            =      50mm

Sensor Size                 =      8.8mm (based on 2/3”sensor size)

Working distance         =     200mm

Magnification               =     0.176

Estimated Focal Length =    29.93mm

Lenses come in certain fixed focal distance and you will not be able to find a 29.93mm lens in this case.  There are basically 3 options available here:

1. Select a 25mm lens to provide your application with a bigger field of view in the expense of resolution.
2. Increase your working distance by selecting a 35mm lens (moving your camera and lens further away from the object you are looking at increases FOV).
3. Select a 25mm lens and add an extension ring to it. (Calculating and selecting the right extension ring is then another consideration).

When the field of view is smaller or close to camera sensor size

These lenses come with magnification from about 0.01x to several hundred times that of the image sensor. Most such application uses telecentric lenses and these lenses provide low distortion and near exact image reproduction. Telecentric lenses are seldom or never used in large FOV application, this is because in order to get low distortion, the physical lens has to be as big as the image you are looking at. These lenses are specified in terms of magnifications (image size / Sensor size) and working distance is usually rigidly fixed by the lens maker.  To determine an appropriate lens, calculate the appropriate magnification and select the nearest magnification that matches your requirement.

Additional Considerations

In both cases, it is still not a simple answer of purchasing the cheapest lens meeting the calculations.  You may be disappointed with the resulting sharpness if you do so. Now you need to match the resolving power to the quality of the image sensor/camera. Every camera has a certain pixel pitch.  In case of the Adimec Opal series it is 5,5 micrometer per pixel.  The lenses resolving power (ability to image small details per mm) is indicated as Modulation Transfer Function. This is mostly indicated as a graph showing the amount of light passing the lens for a given linepair per millimeter.

With a 5,5um pixel you have 90 line pairs per mm (1/(2*5,5um) is the number of linepairs per mm) so this is the minimum (and maximum) number you need for the best sharpness.  More Lp/mm is useless, the sensor cannot “see” it – it just adds a lot of money for the lens.

If your lens can do better than 90lp/mm you need to limit the lens by placing a optical low pass filter between the Lens and the sensor. This way you will not get aliasing/moiré effects visible in your image.