Machine vision lenses

How to choose a machine vision lens

A machine vision lens has three jobs: cover the whole sensor, give the field of view you need at the distance you have, and resolve detail as fine as the camera's pixels. Choose in that order - sensor format and mount, focal length, then resolution, distortion and the mechanical details. This category holds fixed focal length lenses for area-scan cameras plus zoom, autofocus, macro and anti-vibration versions.

1. Sensor format and mount

The lens must be rated for the camera's sensor format or larger. A lens made for a smaller sensor leaves the corners dark (vignetting) and soft. A 1 inch machine vision lens covers a 1" sensor and anything smaller; a 1/1.8 inch lens will not cover a 2/3" or 1" sensor. Check the image circle against the sensor diagonal:

Sensors sold under the same format name differ slightly, so use the camera's stated active area. The range also covers small formats from 1/2.7" and 1/2" up to APS-C, medium and large format. The mount follows the camera: C-mount (17.526 mm flange distance) for most cameras up to 1.1", CS-mount (12.5 mm) on some compact cameras, and F-mount, M42, M58 or M72 for large sensors and high resolution. A C-mount lens fits a CS-mount camera with a 5 mm spacer; a CS-mount lens cannot focus on a C-mount camera.

2. Focal length from field of view and working distance

For working distances much longer than the focal length: focal length = sensor width x working distance / field of view width. Example: a 2/3" sensor (8.8 mm wide) that must see 100 mm at 300 mm needs about 26 mm, so a 25 mm lens. Common steps are 6, 8, 12, 16, 25, 35, 50 and 75 mm, with shorter wide-angle and longer lenses at both ends. The focal length calculator does the arithmetic for your sensor, and this article explains the trade-offs between short and long lenses.

Check the minimum object distance (MOD) too: most lenses here focus from 0.1-0.3 m to infinity. To work closer, add a C-mount extension tube, which raises magnification and shortens the focus range, or pick a macro lens.

3. Resolution: match the pixel size

A lens rated in megapixels or line pairs per millimetre must resolve the camera's pixel. The limit a sensor can use is 1000 / (2 x pixel size in µm) lp/mm: about 145 lp/mm for 3.45 µm pixels, about 180 lp/mm for 2.74 µm. Choose a lens class at or above the camera's resolution - the range runs from lenses for 1-2 MP cameras to 150 MP lenses for large formats - and compare centre and corner resolution where it is stated. A sharp centre with soft corners fails edge measurements near the image border.

4. Aperture, depth of field and distortion

Most lenses here open to F1.4-F2.8 and close to F16 or F22. Stopping down increases depth of field but costs light and, past about F8 on small pixels, sharpness (diffraction). The depth of field calculator shows how much height variation a given aperture tolerates. For gauging, look for a low distortion lens: many are specified below 0.1%, some below 0.01%. Where the part moves in height or must be measured at true size regardless of distance, use a telecentric lens instead.

5. Spectrum and mechanics

Standard lenses are corrected for visible light (about 400-700 nm); some are corrected from 400 to 1000 nm or beyond for near-infrared lighting, so focus does not shift when you change from a red to an 850 nm light. Filter threads (from M25.5 x 0.5 upward) take band-pass filters and polarizers. On machines that vibrate or move, choose lenses with locking focus and iris screws or an anti-vibration design. Autofocus and liquid-lens models refocus electrically for parts at different heights.

Zoom and varifocal lenses

Zoom lenses here include 10-50 mm for 1/1.8" sensors and 12-36 mm and 16-48 mm for 2/3". A zoom is useful during development or where the field of view changes between products; in production, a fixed lens holds focus and calibration better and is usually sharper. Varifocal lenses for security cameras are in CCTV lenses.

Typical applications

• Part presence, assembly checks and label inspection with 8-25 mm lenses on 1/1.8" to 2/3" cameras.
• Dimensional measurement with low-distortion lenses and 5 MP+ cameras.
• Large fields (pallets, sheets, web inspection) with 16-35 mm lenses on 1.1" to full-frame sensors.
• Small features (pins, solder joints, print) with 50-75 mm lenses, extension tubes or macro lenses.

Common mistakes

• Choosing by focal length alone and finding the lens does not cover the sensor.
• Pairing a 20 MP camera with a 5 MP lens: the extra pixels add data but no detail.
• Ignoring the minimum object distance and finding the lens cannot focus at the planned working distance.
• Running wide open for brightness and losing depth of field on parts that vary in height.
• Leaving focus and iris unlocked on a vibrating machine.

Questions

Can I use a lens made for a larger sensor?
Yes. A 1.1" lens works on a 2/3" camera; the field of view is that of the smaller sensor, and you use the sharpest centre of the image circle.

What is the difference between a 1 inch and a 1/1.8 inch machine vision lens?
The image circle: about 16 mm for 1" and 9 mm for 1/1.8". At the same focal length both give the same magnification, but only the 1" lens covers a 1" sensor.

Fixed focal length or zoom?
Fixed for production: sharper, lower distortion and no setting that can drift. Zoom when the field of view must change or is not known yet.

Which lens fits my camera?
Match mount and sensor format first, then focal length. The system configurator lists the lenses that fit a chosen camera.