Selecting a Machine Vision Lens
When selecting your machine vision lens there are a number of specifications you will want to keep in mind. The lens you choose will determine both the image quality and processing speed of your image, and thus measurement accuracy, reproducibility, and the ultimate success of your application.
A few of the factors you will want to keep in mind while choosing your lens are the sensor size of your optical system, your ideal working distance (and thus focal length), and the optical quality as determined by the resolution limit and MTF. You will also need to ensure that the machine vision lens you select has the correct mount for smooth installation in your optical system.
We define the optical quality of our lenses in terms of MTF (modulation transfer function) which relates to both resolution and optical distortion. While in an ideal world the image produced by a machine vision lens would match the object perfectly, in practice every lens will act as a low pass filter and the resolution and detail level it can provide is limited. The point at which modulation is zero, expressed in line pairs per millimeter (lp/mm), is called the resolution limit. This can be measured by testing the image of a test pattern, in which black lines are printed closer and closer together. The point at which the lines are non-distinguishable from each other is the resolution limit.
Standard resolution lenses are typically expected to have a MTF of 70-90 lp/mm. High resolution lenses should have an MTF greater than 120 lp/mm, and macro lenses also require very high MTF characteristics.
Understanding an MTF Plot
A MTF plot quantifies how much image contrast is lost as light passes through a lens, and will typically plot the percentage of contrast loss against the resolution (measured in line pairs per millimeter, or lp/mm). A comparison of MTF curves can be very helpful with lens selection; but one should be aware that only lenses with identical or very close focal lengths should be compared. In general, all lenses will have a MTF of nearly 100% at low frequencies. As frequencies increase, the closer the graph stays to 100% the better the contrast and resolving power of the lens.
System integrators should note that every optical component in a given system will have its own MTF curve. The product of these MTF curves is the MTF of the final optical system.
Shading and vignetting occur when a lens is too small for the sensor used, and is unable to illuminate the complete sensor area. When the lens you select matches the size of the sensor used in your application, the complete sensor area will be in full light and image quality will not fall off at the edges.
The working distance between a machine lens and object being surveyed will depend on the sensor size, the length of the object in question, and the focal length. This makes focal length one a key factor to consider when selecting a lens that will perform well at a desired working distance.
Although not an integral part of the lens, it is essential that the mount a given machine vision lens is equipped with matches the mount on the camera system. The most common mounts are C mounts, with a 1-inch cylinder threaded with 32 threads per inch. The male threading of the lenses connect with an analogous female thread on the camera. These lenses are standard for closed circuit television cameras and microscope phototubes as well as other machine vision cameras.
F-mount lenses are used especially for high resolution applications. These lenses feature three lug bayonet mount with a 44 mm throat and a mechanical locking system; they lock by turning counter-clockwise and unlock clockwise.
At Shanghai Optics, our engineering team is always ready to discuss available machine vision lenses with you and help you make an informed decision on which lens is most appropriate for your application.