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# How to Choose a Fixed Magnification Lens

This is Section 7.6 of the Imaging Resource Guide

At first glance, lenses such as telecentrics or microscope objectives can seem intimidating to specify into an imaging system as they do not behave the same way as traditional fixed focal length lenses. However the selection process is actually much more straightforward than a traditional lens.

With a few exceptions, fixed magnification lenses generally only function properly at a single working distance. They are also specified by their magnification; for example, a 2.0X telecentric lens. Because the magnifications are physically listed on the lenses, that is where they always work, and their field of view can by described simply by rearranging Equation 1 from 6 Fundamental Parameters of an Imaging System to:

(1)$$\text{Field of View} = \frac{\text{Field of View}}{\text{PMAG}}$$

where PMAG is the magnification specified for the lens. This equation shows that regardless of the sensor size, the magnification will remain the same; only the field of view changes.

For example, an application requires the measurement of a bore into a machined part. The bore is 20mm in diameter and the part can vary slightly in placement underneath the imaging system, so a field of view of 24mm is required. A camera has been chosen that has a 1/1.8” sensor (7.2mm horizontal sensor size), so using Equation 1, the PMAG should be 0.3X. Since this is a measurement application, a telecentric lens should be chosen with that magnification.