Modulation transfer function or MTF is designed to evaluate an imaging system’s spatial resolution performance. If you want to determine which between two images or imaging systems is better, technical-wise, you would need to do an MTF Measurement and MTF Testing.
Understanding Resolution
Resolution is a key component that affects the quality of an image. When you talk about resolution, you are referring to how well an imaging system preserves spatial details.
If you look at the bigger picture, these so-called spatial detail and preservation are not just single measurements — instead, they are a streak of measurements, which when plotted can produce a functional curve known as the MTF.
So, How Can MTF Be Measured?
Labeling the key aspects mentioned above, frequency refers to the measure of a spatial detail while modulation transfer refers to the metric that determines how preserved the details are.
MTF is a measurement that incorporates these two components into a single specification.
For every frequency, there is an input modulation and an output modulation. The series of ratios between these two modulations will result in a curve known as the MTF.
Why Measure MTF?
Doing an MTF testing or MTF measurement is a fundamental and direct way of quantitatively measuring image quality. It is objective and can be universally used to measure different optic systems.
Nowadays, several optic systems are available to transfer various levels of detail an of an object into an image — from photographic optics and fax-and-copy optics to contact lenses and video systems. If you want to remain faithful in capturing the details of an object (may it be for records, aesthetic, or scientific purposes), you need to choose an optic system with good MTF statistics.
Ways of Measuring MTF
An MTF measurement can be determined through commercial testers. But while they are accurate and precise, you would need to purchase one or hire someone to do the testing for you. Here are some more alternative ways:
Computerized Reverse Projection and Resolution Chart. In reverse projection, a resolution chart is placed under the lens being tested. The chart is then projected onto a screen — the limiting resolution can, therefore, be read off the screen. In Computerized Reverse Projection and Resolution Chart, a digital camera is added on the projection side to take a photo of the projected target. Once the image is run through an image analysis software, the modulation found at different frequencies will be used to determine the MTF.
Slanted Edge MTF. In this technique, an image of the slanted edge will be taken — the points on the image will then be projected onto a line that is normal to the SE. This will be used to determine the edge spread function and subsequently, the line spread function and the fast Fourier transform. All these stats will be used to compute the MTF.
Point Source MTF. This method includes pointing a light source at the object and then taking a high-resolution image of the pout source through the lens being tested. After getting the point spread function, the LSF and FFT can be determined and the MTF can be computed.
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