Machine Vision Guide - Lens Fundamentals

Lens Fundamentals

Special Lenses

Zoom

Zoom lenses have focal lengths that are adjustable over some range. They are useful for prototypes where the focal-length requirement is not yet determined. They can be set at focal lengths between those available with fixed lenses. Zoom lenses are larger, less robust, more expensive, and have smaller apertures than similar fixed-focal-length lenses. Also, they frequently have more distortion.

Macro Lens

A camera lens optimized to work at magnifications near 1 is called a macro lens. A macro lens provides better image quality than a standard camera lens used with extension tubes.

Telecentric

Telecentric lenses provide constant magnification for any object distance (see Telecentricity). They are useful for precision gauging or application where a constant perspective angle across the field is desirable. Their object distance is generally less than that of standard lenses. The magnification of a telecentric lens is fixed by its design. Because the first element must be as large as the field width, telecentric lenses tend to be larger and more expensive than standard lenses.

Close-up Lenses

Close-up attachment lenses reduce the object distance of a standard lens. The nominal magnification of a lens with a close-focusing attachment is:

Teleconverters (Extenders)

Teleconverters are short relay optics that fit between the lens and the camera and increase the effective lens focal length. They are usually available at 1.5! and 2! power. The penalties for their use are increased image-side f-number (by the power factor), and increased distortion. Reverse Mounting

For magnifications greater than 1, a camera lens can be used in reverse, with the object held at the usual camera plane and the camera in the usual object plane. In this case, the object distance will be short, while the lens-to-camera distance is long. Adaptors are available to hold camera lenses in this orientation.