Resolution and MTF Testing

In the past, when sensors only offered at most 640 pixels, it was easy to make an imaging lens that could outperform the camera’s sensor. Today, with pixel sizes less than 1 micron,  pixel count on sensors have significantly increased —sensors now regularly have over six million pixels. To keep up with this technological advancement and provide superior image quality, lens performance must increase, and measuring how well a lens performs requires the correct metrology. This is where resolution and modulation transfer function (MTF) testing comes in.

Read on to learn more about MTF testing, its benefits, common methods of evaluating lens resolution, and MTF and resolution testing components.

What Is MTF Testing?

Since the introduction of linear system analyses to the optics field, demand for higher quality, higher resolution optical systems has increased. Designers and metrology scientists have turned to modulation transfer function (MTF) testing as a standardized method of optical system characterization.

MTF testing is the measurement of the ability of optical systems to transfer various levels of detail from an object to an image. Performance is measured in terms of contrast (the intensity difference between the image’s black and white regions) and accounts for real-world factors like optical blurring.

An MTF test bench’s calculations are contrasted as a function of spatial frequency within a two-, or three-dimensional coordinate system. This function reveals how well-resolved an object is based on its size.

Common Methods for Evaluating the Resolution of a Lens

There are three common methods of evaluating lens resolution:

• Reverse projection testing: This is a lens-only test where the lens is used as a projector, reversing the typical image. This method is a simple, low-cost, and fast resolution test. However, reverse projection testing can’t effectively measure contrast levels since it depends on the operator’s eyesight.
• Point-spread function (PSF) testing: This method relies on projecting a pin-point of light into the lens system and measuring the blurring at the image plane. From the measured PSF, the MTF is calculated and is the most direct way to measure MTF of a lens.
• Slanted-edge MTF testing: This is a system-level test that typically includes the impact of the image sensor MTF. Compared to PSF testing, slanted-edge MTF testing is quicker, more adaptable, and can capture the MTF across an entire image in one measurement.
• Camera testing: This test isolates the image sensor MTF from the lens MTF. Camera tests can test multiple field points simultaneously, are highly adaptable, and can be used to measure system-level performance, including the lens, camera, illumination, and image processing algorithms. However, camera testing results can be difficult to correlate.

Benefits of MTF Testing

There are several benefits of measuring MTF:

• Provides a direct, quantitative measure of image quality: MTF testing helps to quantify the overall imaging performance of a system in terms of contrast and resolution.
• Objective and universal: The test engineer doesn’t have to make subjective judgments about contrast, resolution, or image quality. With modulation transfer function testing, there are no standardization or interpretation challenges.
• Enables system testing that mimic application conditions: MTF testing is performed on the image or wavefront produced by an optical system, allowing the parameters influencing lens performance and design to be recreated precisely during testing. You can model the field angle positions, image plane architecture, conjugate ratios, and spectral regions in the test.
• Accurately predicted and toleranced: Almost all modern lens design software allows for precise graphical simulation  and the impact of tolerances on the polychromatic modulation transfer function. Measured MTFs can be compared to simulated MTFs.
• Testing versatility: MTF testing offers a method for directly measuring the image features related to overall system performance, such as field curvature, blur spot size, and distortion.

Components of MTF and Resolution Testing

These are the three main components of MTF and resolution testing:

• MTF vs Field of View: This involves measuring the MTF across different image points and observing the resolution changes
• Depth of field testing: This involves evaluating the MTF at different distances above and below the best focus.
• MTF vs Wavelength: This involves quantifying how the MTF of a lens changes across different wavelengths of light, including visible and near infrared spectrums.

Contact The Metrology and Testing Experts at FISBA

Resolution and MTF testing are critical in constructing the proper lens specifications. At FISBA, we specialize in metrology and are committed to delivering products that have undergone rigorous testing to ensure they meet the highest quality and performance requirements. To learn more about our metrology capabilities visit this page: Metrology & Test. Our highly experienced engineers and precision assembly teams use industry-standard test methods and can design and build custom solutions to meet your unique needs. Talk to an expert to get a conversation started.