Lens Calculator

Key Takeaways

• Choice from over 190+ entocentric lens models from seven renowned lens manufacurers: Computar, Fujinon, Kowa, Optowl (former Ricoh or Pentax), Schneider-Kreuznach, Tamron, and VICO imaging, with new models added continuously
  
  
• Free working distance (FWD) is calculated from the front of the lens, alongside the distance from the object-sided principal plane and the total object-image distance
  
  
• Results come from geometrical optics applied to optical data specific to each lens, including floating-element behaviour where it applies
  
  
• Full system specifications: depth of field, Airy disc diameter, magnification, angular FoV, maximum recommended F-stop and the MTF the lens has to reach
  
  
• Calculates the extension rings or spacers needed when the required working distance falls below the lens minimum object distance (MOD)
  
  
• Available to registered customers through the STEMMER IMAGING customer portal

What a Lens Calculator Actually Calculates

A lens calculator resolves the relationship between four quantities that cannot be chosen independently: sensor size, field of view, working distance and focal length. Fix any three and the fourth follows. A tool that models this properly also accounts for the optical properties of the specific lens, because two models with the same nominal focal length can behave differently in a real setup.

The STEMMER IMAGING Lens Calculator works from the application side. You enter what the inspection has to achieve, and the calculator identifies the lens models that reach it, then reports what the resulting system will do in practice.

Focal Length Alone Does Not Specify a Setup

An enquiry that arrives as “we need a 12 mm lens” leaves important questions open. Which sensor is  the lens used with, and does the lens’ image circle cover the sensor diagonal? Is the working distance measured from the front of the lens or from the sensor plane? Is there enough  depth of field for a part that varies in height? Does the lens provide sufficient resolution for the pixel size of the camera sensor?

These characteristics are not properties of the lens alone but of the complete camera–lens system and its application. Sensor size and pixel size determine whether the lens provides sufficient image coverage and resolution; a lens designed for a smaller sensor, for example, may vignette on a larger one. Depth of field also depends on the setup, including magnification, aperture and the acceptable circle of confusion.

Working distance causes particular confusion. For some users, it refers to the free working distance from the mechanical front of the lens to the object.  In some applications, however, the relevant distance is from the sensor plane to the object. Many other optical tools don’t compute either of these, but a distance from the object-side principal plane. These figures can differ by a significant margin, and using the wrong one might require a mechanical redesign after the delivery of a wrong lens. 

You configure the setup in three steps. 

1. Choose an image sensor from the list or enter a custom pixel count and pixel size, and the tool derives the horizontal and vertical sensor dimensions and the image circle (i.e. sensor diagonal). 
2. Define the field of view you need on the horizontal axis, and the calculator returns the vertical and diagonal field of view together with the lateral resolution in µm per pixel. 
3. Enter the desired working distance, and a recommended focal length is estimated, thenset a tolerance around that focal length, and the lens list filters down to the models that fit.

Selecting a model then produces the complete specification of the configured system:

• Free working distance (FWD) measured from the front of the lens
• Distance from the object-sided principal plane [OH] and total object-image distance
• Depth of field for a circle of confusion you can set yourself
• Magnification, opening angle and Airy disc diameter
• Maximum recommended F-stop before diffraction limits contrast
• The MTF the lens has to reach across the full sensor
• The extension rings or spacers required, where applicable (i.e. FWD < MOD)

Manufacturer-independent lens database

The database holds more than 190 lens models from seven renowned manufacturers in the STEMMER IMAGING optics portfolio and grows as models are added and validated. Because it is not tied to a single supplier, the shortlist for a given application can span several brands, and the comparison happens on optical merit.

Optical data from the manufacturers

Lens specifications are supplied by the manufacturers and maintained by our optics specialists rather than estimated from approximate catalogue values. This is what allows the calculator to report the free working distance from the front of the lens instead of from a reference plane somewhere inside it, and to account for models whose internal element spacing shifts with focus (floating design).

Complete system specification

The output describes the camera-lens system in the application, not the lens in isolation. Depth of field, magnification, opening angle and required MTF all respond to the sensor and aperture you configured, which is what an engineer needs when checking whether a station will work before anything is ordered.

Adjustable circle of confusion

Depth of field depends on how much blur the application tolerates, so the circle of confusion is yours to set rather than fixed. Colour sensors with a Bayer filter array need a larger value than monochrome sensors because of the interpolation, and the tool suggests a working figure for each.

Extension ring calculation

When the required free working distance (FWD) falls below the minimum object distance (MOD) of the lens, the calculator determines the minimum extension ring length needed between lens and camera flange. The free working distance is adjusted automatically, which avoids mounting problems that appear once the parts are on the bench.

Warnings on suboptimal configurations

If a setting pushes the system into a range where imaging performance degrade, such as an aperture closed far enough for diffraction to dominate, the tool flags it rather than returning a number without context.

Integrated optical glossary

Every computed value links to a glossary entry explaining what it means and how it is derived, which makes the tool usable by engineers who work with optics occasionally rather than daily.

The Lens Calculator covers entocentric lenses with a standardised flange focal distance of 17,526 mm, which takes in the common C-mount and TFL-mount optics used across industrial imaging. Telecentric lenses, SWIR optics, prism and 3CMOS lenses, microscope optics and zoom lenses currently sit outside it.

Not every lens in the STEMMER IMAGING portfolio is in the database yet, and models are added as their optical data is validated.

Results carry the tolerances inherent to any optical calculation. Production variation shifts effective focal lengths between individual lenses, and geometrical optics is an approximation that becomes less exact at short focal lengths. The performance of the assembled system might therefore differ slightly from the calculated figures.

The lens must produce an image circle at least as large as the sensor diagonal to illuminate the full sensor. If the image circle is smaller, the sensor corners may be dark or not covered at all. For this reason, image circle is used as an initial filter when selecting suitable lenses.

STEMMER IMAGING

Supporting Your Optics Selection

The Lens Calculator narrows the choice. Our optics specialists and Technical Competence Centre take it from a shortlist to a tested and application-ready system.

Operational Services
Camera-lens assembly, focusing to the configured working distance, sensor cleaning and intrinsic calibration, delivered ready to mount with documentation. 
Learn more >

Engineering Services
Feasibility testing with sample parts, illumination concepts for the required image contrast and exposure, taking into account the aperture needed to achieve the target depth of field, and one-to-one coaching on optical system design.
Learn more >

Special Solutions & Applications
Custom optical subsystems for requirements the standard portfolio does not cover, developed with you from qualification through to a series-ready solution.
Learn more >

What is the STEMMER IMAGING Lens Calculator?

It is a web-based tool that determines which machine vision lenses fit a given combination of sensor, field of view and working distance. It returns a filtered list of suitable lens models and the full optical specification of the resulting camera-lens system, including free working distance, depth of field and required MTF.

Which lens manufacturers are covered?

The database is manufacturer-independent and currently holds more than 190 entocentric models from seven renowned manufacturers in our optics portfolio, including Computar, Fujinon, Kowa, Optowl (former Ricoh or Pentax), Schneider-Kreuznach, Tamron, VICO imaging.

Do I need an account to use it?

Yes. The Lens Calculator is available to registered customers through the STEMMER IMAGING customer portal. 

Which lens types and mounts are supported?

The calculator currently supports entocentric lenses designed for a 17.526 mm flange focal distance, including C-mount, TFL-mount, and selected M42-mount lenses. Telecentric, SWIR, prism, microscope and zoom lenses are not currently included.  The supported mounts and image circle range are shown directly in the tool. Support for telecentric lenses is planned.

How accurate are the results?

Calculations follow the established principles of geometrical optics applied to the measured specifications of each lens model, which is why free working distance is reported from the front of the lens rather than from an internal reference plane. A system built on the bench will vary slightly from the calculated values, mainly due to lens production tolerances and the limits of the optical model, particularly for wide-angle or strongly non-paraxial setups. 

Does it calculate depth of field and extension rings?

Both. Depth of field is calculated for a circle of confusion you set.. Where the required working distance is shorter than the minimum object distance of the lens, the tool calculates the extension needed and distinguishes threaded rings from thin spacers.

Can I use a lens that is not in the STEMMER IMAGING database?

No, currently not. We might add this function in a later release of the tool. 

What if my application needs line scan or telecentric optics?

Those currently sit outside the scope of the present version of the lens calculator. We continuously extend its capabilities, and our optics specialists can work through those requirements with you directly. Just call or fill in the support/contact form.