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Tests and Configurations

Tests and Configurations

Common Configurations
Measuring Image Quality
Other Measurements

Common Configurations

Object at Infinity
Negative Lens
Object at a Finite Distance
Afocal Module
Testing a Lens in Projection

Common Configurations

Object at Infinity

This test setup is best suited for
Camera lenses, or any lens designed to operate with an object at infinity. This test configuration is also suitable for lenses designed to image objects at large (but not infinite) distances.


This is a very common test configuration. The collimator creates a virtual image of the test reticle. The lens under test refocuses this to a real image which is captured by the microscope.

It is impractical to use an object that is literally infinitely far away. However, it is easy to simulate this situation with a collimator. If you were to look into the collimator you would see a bright image of the reticle test pattern. Light rays from any single point on the reticle emerge from the collimator essentially parallel.

Typical Setup

The collimator is visible on the left. A lens from an inexpensive APS camera is held in the 3-jaw chuck. The video microscope is visible on the right. In the photo above, the lens has been rotated to test its off-axis performance. The vernier shows that the chuck has been rotated by 20.8 degrees

Schematic Representation

OpticStudio™ hardware works in conjunction with PixelScope™ software. While no programming is needed, it is necessary to enter the parameters of the setup and the nominal parameters of the lens under test. The picture above shows how these parameters are entered.

Limitations

  • The one meter optical rail makes it impractical to test lenses whose focal length is longer than about 800 mm.
  • The clear aperture of the lens should not be larger than the collimator. The standard 200 mm collimator is suitable for lenses with clear aperture up to about 20 mm.
  • The numerical aperture of the lens under test must be smaller than the microscope objective. A good rule of thumb is that the 4X lens is suitable for F6 or slower lenses, and the 10X lens is suitable for 2.5 or slower lenses. How can I test lenses faster than F2.5?

    If the NA of the microscope objective is too small then only the central part of the lens under test contributes to the measurement. Since most lenses "work better" in the center, ignoring the periphery generally leads to misleading results!

  • The classical "lens bench" architecture has some limitations with off-axis testing of lenses with external pupil locations. Such lenses may be tested off-axis with the optional collimator mount. more information

Tests possible with this setup

On-axis and off-axis tests

The first step is to examine the performance of the lens on (or near) the optical axis. However, most practical applications require that the lens also image well over a specified field-of-view. Sometimes a lens achieves good performance over a surprisingly small FOV, so it is important to check operation across the full field that will be used in the application.

A lens is tested at off-axis field points by rotating the lens holder. For example, the lens in the photo above is being tested at a field angle of 17.5 degrees.

Note that there is a second axial slide on top of the lens holder rotation mount. This "nodal slide" allows the lens to be positioned so that the image remains stationary when the lens mount is rotated. (The image is truly stationary only if the lens is free from distortion.)

Plane of best focus
For a well corrected lens this is simple—adjust the microscope axial position until the image is sharpest. The location of the plane of best focus may be read directly from the digital micrometer.

If the lens has aberrations then the concept of a simple "plane of best focus" may not apply. For example, if the lens has astigmatism, there will be two distinctly different points of optimum focus, one for X features and one for Y features.

Image quality measurements
There are many ways to evaluate image quality, including MTF, 3-bar contrast, and visual examination of the PSF, just to name a few. These measurements are common to many different OpticStudio™ setups, so they are discussed in a separate chapter on measuring image quality.

Measuring EFL
"EFL" is the effective focal length of the lens. EFL can be calculated by measuring the size of a known feature. (Recall that short focal length lenses form smaller images.) The EFL wizard in PixelScope™ makes it easy to do this with a few clicks of the mouse. For more information see measuring EFL.

Measuring BFL
"BFL" or Back Focal Length is the distance to the plane of best focus, measured from the mounting surface of the lens. While this is obviously related to EFL it is not identical. BFL is read directly from the digital micrometer. For more information see measuring BFL.

Measuring Magnification
When a lens operates at infinity linear magnification is not defined. Instead, magnification must be specified in angular terms, with units of [mm of image height per radian of object height]. Stated in those terms the angular magnification is exactly equal to the EFL in mm. For more information see measuring magnification.

Axial color
"Axial color" means that the best focus plane is not the same for all colors. In a perfectly corrected lens, the plane of best focus remains constant over a range of wavelengths. However, in most real-world lenses, changing the narrow band filter in the collimator will cause a shift in image location. This shift may be read directly from the digital micrometer. For more information see measuring axial color.

Lateral color
"Lateral color" means that the location of the image is not the same for all colors. Because of symmetry, lateral color is not a problem on-axis. However at off-axis field points the image location may be slightly different for different wavelengths. Any shift is due to color aberration in the lens, and may be measured directly with PixelScope™. For more information see measuring lateral color.

Distortion
"Distortion" means that the magnification is not the same at all field points. PixelScope™ does not have a tool for direct distortion measurement. However, distortion may be inferred from measurements made in PixelScope. For more information see measuring distortion.

Field Curvature
"Field curvature" means that the plane of best focus is not the same at all field angles. PixelScope™ does not have a tool for direct measurement of field curvature. However, field curvature may be inferred by plotting the plane of best focus vs. field angle. For more information see measuring field curvature.