Machine Vision Guide - Lens Fundamentals

Lens Fundamentals

First-Order Design Theory

To establish an understanding of theoretical principles, we will first review a few basic lens definitions and parameters. We then examine the thin-lens model. The thin-lens model describes a lens with no limitations � one that can be used at any magnification and work at any conjugate. However, since real lenses do have limitations, the thin-lens model does not provide the complete picture. Following this theoretical discussion, we will examine real lenses and their parameters, as well as special lenses. Camera Format The camera format provides the dimensions of the image sensor. Lenses, by design, provide images over a limited area. Be sure the lens covers an area as large or larger than the camera format. Field of View (FOV) The FOV is the object area that is imaged by the lens onto the image sensor. It must cover all features to be measured, with additional tolerance for alignment errors. It is also good practice to allow some margin (e.g., 10 percent) for uncertainties in lens magnification. Features within the field of view must appear large enough to be measured. This minimum feature size depends on the application. As an estimate, each feature must have 3 pixels across its width, and 3 pixels between features. If there are more than 100 features across a standard camera field, consider using multiple cameras. Magnification The required magnification is:

where W_camera is the width of the camera sensor and W_FOV is the width of the FOV. Note that the required magnification depends on the camera sensor size.
Working Distance The working distance is the distance from the front of the lens to the object. In machine vision applications, this space is often needed for equipment or access. In general, a lens that provides a long working distance will be larger and more expensive than one that provides a shorter working distance.

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