Measuring at the Correct Conjugate

When evaluating lens performance it is critical for the test system to properly simulate the lens’s image and object conjugates. Testing off conjugate will produce results that can give a false sense of confidence and when it comes time to use the lens as intended you can get an unpleasant surprise. In that light, the two most common errors we have seen people make when measuring MTF are:

  1. Testing an infinite conjugate optic with a finite conjugate target. Simulating an infinitely distance object can be difficult. Often, the assumption is made that an infinite object can be approximated by a finite, but relatively large object distance, such as the hyper-focal distance. In many cases this can be shown to be a poor assumption.
  2. Testing a finite conjugate optic with an infinite conjugate target. Configuring a system to measure MTF at a finite conjugate involves other practical difficulties, such as challenging alignment procedures and limited system flexibility. It can be tempting to test a finite conjugate optic using an existing system with an infinite conjugate source.  However, this will again produce misleading data.

You need to consider the conjugates for which your system was designed and how it will be used when designing a test configuration. When testing an infinite conjugate system with a finite source, it is important to model the MTF performance to determine an appropriate object distance rather than relying on rules of thumb.

Regardless of your system’s conjugate needs, Optikos offers both finite and infinite conjugate lens testing systems. Optikos Corporation’s standard OpTest® and LensCheck systems are designed not only for infinite object conjugate lenses (even afocal systems!), but also offer finite conjugate variants for lenses designed with a finite conjugate in mind. 

For versatile camera testing, Optikos has you covered with our Meridian® camera testing systems that can accommodate both finite and infinite conjugates. 

And when your budget is tight and time is of the essence, the metrology engineers in our IQ Lab™ testing services group can perform the testing for you using Optikos products in our state-of-the-art metrology lab. If you have an imaging system that requires a unique approach to testing, our engineering team can help design, prototype, and build a test system for your unique application.

If you’d like to learn more, read about “MTF Testing at Proper Conjugates” on our articles page.

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Written by David P. Biss, Ph.D., Optikos Corporation

David P. Biss, Ph.D.

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