10 key lens specs—and magnification's not one
Gregg Fales, Edmund Industrial Optics -- Test & Measurement World, 11/1/2003
If you're new to optics, you might find that choosing among machine-vision lenses is a daunting task. But if you understand 10 key specifications, you can easily select the appropriate lens to optimize your system's performance.
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Basic optical parameters of a machine-vision system include the field of view, working distance, resolution, and depth of field. (Magnification is not a basic parameter.) |
Specifying the field of view rather than the magnification also ensures that the system will have the appropriate resolution (key spec 2)—the minimum distinguishable feature size of the object under inspection. A system's resolution depends on the modulation transfer function (MTF) of the optics, camera, cabling, and display hardware. MTF qualifies the overall imaging performance of a component in terms of resolution and contrast.
The working distance (key spec 3) is the distance from the lens front to the object under inspection. The longer the required working distance, the more difficult and more costly it becomes to maintain a small field of view. If a long working distance isn't necessary, don't complicate matters needlessly.
For 3-D objects, you must also consider the depth of field (key spec 4)—the ability to maintain a desired resolution as the object is positioned closer to and further from best focus. Keep in mind that although the lens will maintain the minimum resolution over the specified depth of field, the lens won't necessarily maintain the same field of view over that depth. This change in magnification can have disastrous results on machine-vision measurement applications. (Telecentric lenses minimize this problem.)
These "big four" specs are sufficient for specifying a lens for your machine-vision system. But you can reduce setup costs and system downtime while optimizing reliability and repeatability if you consider six other specs as well: f-number, maximum chip format, distortion, zoom/focus, design conjugate, and telecentricity. For additional information, see the complete version of this article.
