Basics of machine vision lens selection
How to select the correct machine vision lens for your application
It is critical to consider the required conditions for your application which is key for selecting the correct imaging devices.
1. Points for selecting a lens
Points for Step 1
- Sensor size = Pixel size (V) or (H) x Effective Pixel amount (V) or (H)
- FOV = Sensor size (V)) or (H) / Optical magnification
Points for Step 2
- The distance between the object and front of the lens.
- Object that inspect or analyze
- Type of applications/
- example: OCR
- The distance between the object and sensor
- Depth range in which the object will be in focus
- Type of light source, Lighting Method Lighting WD etc.
- Vibration, Shock, Unnecessary Light, Dust, Water, Temperature, Humidity
2. Selecting a lens
Calculate Opt. Magnification by using FOV and sensor size
Opt. Mag＝Ratio between FOV and Camera sensor size
Opt. Mag = Sensor size(V) or (H) / FOV(V) or (H)
※Camera dimensions ＝Pixels * Effective Pixels
- How to calculate Focal Length of a lens
f = WD * Opt. Mag(Sensor size(V or H) / FOV(V or H))
Compromise between many competing factors
- How to calculate Field of View (FOV)
FOV(V or H) = WD * Sensor size(V or H) / f(focal length)
※The focal length of a lens defines the lens’s angular field of view. For a given sensor size, the shorter the focal length, the wider the angular field of the lens. Additionally, the shorter the focal length of the lens, the shorter the distance needed to obtain the same FOV compared to a longer focal length lens
Type of Machine Vision Lenses
|Fixed Focal Length Lenses
|Large format and Line Scan Lenses
Telecentricity determines the amount that magnification changes with working distance. Better telecentricity means less magnification changes. Telecentric lens has parallel chief rays to its optical axis and bad telecentricity lenses produce images with higher magnification when the object is closer and the object can be seen differently between center and field of image. The degree of telecentricity is measured by the chief ray angle in the corner of the image field. You can easily check the telecentricity using a target as shown below. Telecentric lens is very important for gauging three-dimensional objects or objects whose working distance is not stable.
Image-side Telecentric lens
Object-side Telecentric lens
- Top view of slope
When you use telecentric optical system, you can recognize straight line on the slope from top view
- Top view of connector pins
Since the telecentric optical system recognize an image viewed from top view, it is easy to detect errors.
Macro lenses is specially designed for short working distance（around WD 100mm) without a focus adjustment and you need to set-up the camera position precisely. VST has fixed magnification models, variable magnification models and variable aperture models.
Fixed Focal Length Lenses
Fixed focal length lenses are commonly used optics in machine vision, being affordable products that are well suited for standard applications. The focus can be adjusted from a minimum working distance to infinity.
Large Format and Line Scan Lenses
Lenses developed especially for the use with line scan cameras. Used for inspection of steel, pulp, fiber/textile, printing, film and other flat materials and Web appearance testing equipment. Usually come with big camera mount.
The optics of the microscope objective are defined by the focal length, N.A., and field of view. It is a very high-powered magnifying glass, with very short focal length.
3. Important factors and lens accessories
Image Circle, Shading
A lens has the ability to support a certain sensor size to image. The maximum sensor area that the lens can support it defined as Image Circle. If the sensor size is too large, it causes “Shading” or “Vignetting”.
- 2/3″ sensor, Vignetting or Shading
only □ part will be imaged
Each camera mount has a different size thread and flange back (FB: distance from sensor to lens flange)Common camera mount FB are shown below. VST mainly offers C, F and M72 mount but also some special mounts for remote head cameras. Please contact us for more details if you have a special lens mount.
※1. FB size depends on the camera manufacturer. You need to make sure both mount thread and FB are correct to get the correct image.
Extension Tube and Rear Converter Lens
VST offers various accessories. Extension tube is attached between camera and lens to increase the magnification while decreasing WD. Rear converter lens is used to increase the magnification without changing the WD.
Extension Tube SV-EXR series
Tubes used between lens and camera when you need to focus in shorter WD than the lens MOD(minimum object distance).
FOV will be also smaller consequently. SV-EXR series are available in 12 length type from 0.2mm to 50mm.
Rear Converter Lens SV-X series
A optional lens used between lens and camera when you need to get higher magnification(smaller FOV) without changing WD.
SV-X series are from 1.5x to 5.0x