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By Kristin Vogt
Specifying magnification for machine-vision systems can be tricky. A lens' primary magnification is an important design consideration, but what it means to the application is frequently misunderstood.
As Figure 1 shows, in the machine vision world there are actually two different magnifications – primary magnification (the magnification created by the lens itself) and system magnification (the ratio of the image size to the object size). Intuitively, engineers and scientists think in terms of system magnification.
Imagine you want to image an object that is 40 mm x 30 mm and display it on a 21-inch monitor such that it more or less fills the monitor. This object has a diagonal of 50 mm, and a 21-inch monitor has a diagonal of 533 mm, so it seems that a lens with 10x magnification should be chosen. A 10x lens coupled with that monitor, however, actually gives a system magnification of about 500x.
Each system component (lens, camera, and display device) contributes to system magnification. The lens' magnification is a ratio of the size of the real image it projects onto the camera's sensor to the size of the object. Mapping the image onto the monitor magnifies it again. This display magnification is a ratio of the image size on the monitor to the sensor size.
All you need to know to calculate the primary magnification is the object size (in this case, 50 mm) and the sensor size. Assuming a 2/3-inch sensor (11 mm diagonal), the primary magnification is