Fundamentals of MTF and Image Quality Evaluation Metrics
The Modulation Transfer Function (MTF) is a representative metric used to quantify the resolution performance of lenses and imaging systems. By illustrating the relationship between spatial frequency and image height, it enables quantitative evaluation of how contrast and resolving power vary across the image field.
In addition, MTF serves as an important index for improving the visual performance of display devices and for selecting lenses, making it a key parameter for objectively assessing product performance. This article explains the fundamentals of MTF, how to interpret it, and its practical applications.
MTF represents the amplitude component of the Optical Transfer Function (OTF) and can be derived from various measurement methods such as the slit method, edge method, and chart method. In particular, the chart method is widely used in Japan, providing valuable insights for quantifying the characteristics of imaging and display systems, as well as for comparing different types of equipment.
What is MTF (MTF Curve)?
Let’s begin by understanding MTF itself and what its curve represents.
MTF stands for Modulation Transfer Function, which quantifies the contrast reproduction capability of an optical system at different spatial frequencies. The visual representation of this is the MTF curve, where the vertical axis typically represents contrast reproduction and the horizontal axis represents spatial frequency or image height.
In regions where the MTF values are high, fine details of the subject are clearly rendered and contrast is well maintained. Conversely, in regions where the MTF decreases, fine details may blur and contrast may drop, revealing weaknesses in lenses or display devices.
While other factors besides MTF also influence image quality in actual photography or display performance, the MTF curve is extremely useful for obtaining an overall view of resolution and contrast characteristics.
Definition of MTF and Its Impact on Image Quality Evaluation
MTF is a plot of the amplitude component of the Optical Transfer Function (OTF) against spatial frequency, and evaluating in this frequency domain allows analysis down to extremely fine patterns that are imperceptible to the human eye. Although different measurement methods exist, such as the slit method, edge method, and chart method, the fundamental principle remains the same: quantifying how well contrast is preserved.
In image quality evaluation, a high MTF indicates strong sharpness and resolution, which generally translates into images with clear detail and well-defined contrast during actual photography or display. However, MTF tends to decrease at higher spatial frequencies, and the performance at the edges of lenses or in high-resolution regions significantly affects the overall system evaluation.
In particular, for fields such as X-ray imaging, it is necessary to consider the overall MTF, including the characteristics of intensifying screens and film. By combining the MTF of each system component through multiplication or other relationships, the final image quality can be estimated. This approach is also applied in conventional camera and display evaluations, contributing to multifaceted quality management.
Basics of Reading an MTF Curve
To properly interpret the information provided by an MTF curve, it is important to focus on several key points.
An MTF curve typically shows contrast transfer in both the sagittal (S) and tangential (T) directions for multiple spatial frequencies (e.g., 10 lines/mm, 30 lines/mm). By comparing the different curves on the graph, one can assess the resolution characteristics at the center versus the edges of the lens or display, as well as across different spatial frequencies.
Generally, a high curve at 10 lines/mm indicates good performance in the low-frequency region, while a high curve at 30 lines/mm or other high frequencies reflects strong capability to reproduce fine details. Conversely, a rapid drop in MTF values reveals a decrease in image quality at the edges of the lens or a reduction in display contrast.
In real equipment, including lenses and displays, MTF performance can vary depending on the measurement environment and design characteristics. Comparing a set of products under the same conditions can help deepen understanding. To find the characteristics best suited to your application, it is important to examine the information indicated by the curve across a broad range of spatial frequencies and directions.
Difference Between Tangential (T) and Sagittal (S)
The sagittal (S) direction is evaluated along lines radiating outward from the center, while the tangential (T) direction is evaluated along lines perpendicular to this, in a circumferential orientation. These two directions are affected differently by lens asymmetry and optical aberrations, which is why differences often appear in the MTF curves.
Generally, the sagittal direction is relatively less influenced by spherical aberration and astigmatism, whereas the tangential direction tends to show a greater decrease in MTF toward the edges of the lens. These differences provide valuable insights into the lens design philosophy and the characteristics of peripheral image quality.
In practical photography, lenses that consider both sagittal (S) and tangential (T) performance are regarded as high-performance. Understanding these differences helps in selecting a lens that is best suited for the subject composition and the intended shooting purpose.
What Can Be Learned from an MTF vs. Image Height Graph?
An MTF vs. image height graph visualizes the contrast transfer from the center of the image (around 0 mm) to the edges (near the maximum image height). It is common for most lenses to exhibit high MTF values at the center, with a gradual decrease toward the periphery.
The rate and extent of this decline vary significantly depending on the lens design. For example, high-performance lenses typically maintain relatively high MTF even at the edges, showing only a gentle drop. Conversely, a steep decline may occur in wide-angle lenses, where peripheral image quality deteriorates noticeably, making them less suitable for shots that require sharpness near the corners.
When producing large prints or displaying images on big screens, peripheral resolution can have a greater impact on perceived image quality than expected. Therefore, it is ideal to carefully examine the MTF vs. image height graph and choose a lens or display with characteristics that suit your shooting style and intended display method.
Display Resolution Evaluation and MTF Measurement
We will examine the effectiveness of MTF measurements in evaluating the visibility and resolution of displays.
Even in display evaluation, MTF serves as an important metric, enabling measurement of actual visual clarity that cannot be fully captured by pixel density (ppi) or contrast ratio alone. In particular, when displaying fine text or lines, areas with high MTF values are more likely to produce sharp and easily readable images.
At the same time, display-specific factors such as viewing angle or peripheral dimming can make it surprisingly difficult to isolate and assess pure resolution characteristics. Even in these situations, using MTF allows for an objective evaluation based on frequency response.
In practice, many factors—such as the quality of the display panel, the backlight, and pixel structure—interact to determine overall performance. While MTF alone cannot capture everything, it has become an essential measurement method in development and quality control environments.
Approaches to Improving Display Visibility
To enhance display visibility, it is important not only to increase resolution but also to optimize contrast and color reproduction. If MTF measurements reveal a drop in contrast within specific frequency ranges, investigating ways to control these regions can help improve overall display quality.
For instance, when displaying text or line art, the MTF in the mid-to-high frequency range becomes particularly important. Improving these frequency characteristics can involve redesigning the display’s backlight, adjusting the pixel layout, and combining measures to maintain sharpness.
Ultimately, the design should maximize panel performance while taking human visual characteristics into account. Using MTF curves as reference data allows for improvements based on objective, quantifiable evidence.
Case Studies of MTF Measurement for Display Image Quality
In recent years, methods using dedicated optical measurement equipment to accurately calculate the MTF (Modulation Transfer Function) of patterns displayed on screens have become increasingly common. For example, test images such as square-wave or edge patterns are displayed on the display and captured using a high-resolution camera. By performing image analysis, including Fourier transforms, on the acquired image data, it becomes possible to evaluate the MTF of the display and optical system with high precision.
This approach provides not only information about resolution or optical characteristics but also the MTF as seen through the actual displayed image, yielding evaluation results closer to what users visually perceive. Additionally, measurements from different viewing angles can be conducted relatively easily.
For instance, when measuring two displays with the same nominal resolution, the numerical ppi values may be nearly identical, yet the MTF comparison can reveal significant differences in contrast at high spatial frequencies. The ability to quantitatively capture such differences highlights the value of MTF measurement.
Utilizing MTF in Lens Performance Evaluation
When evaluating the performance of camera lenses, objective indicators such as MTF curves are extremely important.
For camera lenses, MTF curves are often published in manufacturer catalogs or on official websites, serving as an entry point for users to assess lens performance. By comparing the tangential and sagittal curves at different spatial frequencies, such as 10 lines/mm and 30 lines/mm, the characteristics of a lens can be understood at a glance.
In actual shooting, image quality may vary depending on conditions and subjects, so MTF is not absolute. However, when comparing multiple lenses, referencing curves measured under the same standards is highly effective. High MTF values are particularly valuable for applications that demand fine detail reproduction, such as photographing autumn foliage or landscapes.
Furthermore, knowledge of MTF curves is useful for estimating changes in image quality when using filters or teleconverters. When considering the overall image quality of a lens system, MTF is an indispensable tool.
Practical Use of MTF Curves in Lens Selection
When purchasing a lens, comparing the MTF curves provided on product websites or in catalogs allows you to estimate which focal lengths and aperture settings will yield the sharpest results. For example, in the case of a zoom lens, different MTF curves are often shown for the wide-angle, mid-range, and telephoto ends, enabling you to choose the optimal settings according to your shooting situation.
However, relying solely on the curves for selection can sometimes be premature. Factors such as the smoothness of bokeh or tendencies in chromatic aberration—elements that MTF alone cannot represent—also affect the final impression of an image. Therefore, MTF curves should be used as one quantitative indicator of lens characteristics rather than the sole deciding factor.
Additionally, measurement conditions may vary slightly between manufacturers. When choosing lenses from different brands, it is advisable to look for data tested under comparable conditions or to make a comprehensive judgment by also referring to sample images taken with the lenses.
Key Points for Evaluating Resolution and Image Contrast
Lenses with high MTF values tend to render fine details sharply and provide rich contrast. However, depending on the subject, a slightly softer rendering may create a more pleasing atmosphere, so a higher MTF does not always equate to “better” in every situation.
When considering image quality in terms of “resolution” and “contrast,” some lenses may exhibit high resolution in the high-frequency range while showing slightly weaker contrast in the low-frequency range. It is important to assess the balance that best suits the shooting scene and your preferred visual style.
Ultimately, the final image quality depends not only on resolution but also on color reproduction and the quality of bokeh. Therefore, evaluation should be comprehensive, taking into account not only the MTF curves but also practical aspects such as real-world shooting results and ease of RAW processing.
Key Points to Keep in Mind When Interpreting MTF Curves
When examining MTF curves, it is important to remember that they provide an objective measure of a lens or system’s ability to reproduce contrast at different spatial frequencies, but they do not tell the whole story about image quality.
First, MTF curves are usually measured under specific conditions (such as aperture, focal length, and focus distance), so comparing curves from different sources or manufacturers requires caution. Measurement methods and standards may vary, which can lead to apparent differences that do not reflect actual performance.
Second, MTF values represent only contrast and resolution. Other factors, such as color aberration, bokeh quality, flare, and diffraction effects, also influence the final image and are not captured in MTF curves.
Finally, real-world shooting conditions—such as lighting, subject motion, and camera stability—can affect perceived image quality, so MTF curves should be used as one of several tools for lens evaluation rather than as the sole criterion.
MTF as a Reference Metric: Comparison with Other Indicators
MTF should be regarded primarily as a reference metric rather than a definitive measure of overall image quality. While it quantitatively evaluates contrast reproduction and resolution, other indicators—such as color accuracy, bokeh quality, flare resistance, and dynamic range—also play essential roles in determining the final visual impression.
In practice, combining MTF data with these complementary metrics provides a more comprehensive understanding of lens or display performance. Relying solely on MTF may overlook important qualitative aspects that affect how images are perceived in real-world use.
How to Utilize MTF Characteristics by Lens Type
Let’s also pay attention to the fact that MTF characteristics can vary significantly depending on a lens’s focal length and design.
Lenses can be broadly categorized by focal length—wide-angle, standard, and telephoto—each exhibiting different characteristics. By checking the MTF curves in advance, you can understand, for example, whether a wide-angle lens shows noticeable image quality degradation at the edges, or how well a telephoto lens maintains high-frequency performance.
For zoom lenses, it is difficult to maintain the same level of MTF across the entire zoom range. Therefore, by interpreting the MTF curves, you can determine whether a lens is optimized for the wide-angle end or prioritizes image quality at the telephoto end, which in turn helps you choose the lens and lighting setup according to the shooting scenario.
Prime lenses, lacking a zoom mechanism, are generally said to achieve higher MTF at a specific focal length. However, it is important to keep in mind that actual performance can vary depending on the design and price range.
MTF Characteristics of Wide-Angle Zoom Lenses
With wide-angle zoom lenses, particular attention should be paid to decreased resolution and distortion at the edges of the frame. Referring to the MTF curves often reveals cases where the values drop sharply near the maximum aperture.
For example, when shooting architectural photography, edge sharpness is important, so it is advisable to choose a lens design that maintains higher MTF in the peripheral areas. To achieve the dynamic composition unique to wide-angle lenses, selecting a lens with good edge performance is essential.
On the other hand, for casual snapshot photography where edge quality is less critical, prioritizing the convenience of a wide-angle zoom may be perfectly acceptable. By understanding MTF characteristics in relation to the shooting scenario, you can greatly expand your creative flexibility.
MTF Characteristics of Standard Zoom Lenses
Standard zoom lenses cover the most frequently used focal length ranges, so many of them exhibit strong MTF performance at the center of the frame. However, it is not uncommon to see a gradual decrease in MTF toward both ends of the zoom range.
Additionally, depending on the product, some lenses are designed to emphasize performance at the wide-angle end, while others prioritize the telephoto end. If MTF curves are published, it is advisable to check the characteristic differences at each focal length and make a choice that suits your actual shooting scenario.
For highly versatile standard zooms, the wide coverage of focal lengths makes it challenging to maintain high MTF across the entire range. Therefore, understanding which range is the sharpest allows you to choose appropriately—for example, whether you are focusing on capturing delicate landscapes or mainly shooting portraits.
MTF Characteristics of Telephoto Zoom Lenses
Telephoto zoom lenses allow you to capture subjects up close, but they tend to have difficulty maintaining contrast in the high-frequency range. In particular, near the maximum telephoto end, the MTF at wide-open apertures can drop compared to other parts of the zoom range.
In scenarios such as sports or wildlife photography, where subjects are at a distance, the sharpness of a telephoto zoom lens is a critical factor that can determine the outcome. By referring to MTF curves, you can estimate how close you can get to the subject and which aperture will provide optimal contrast.
Additionally, whether the lens has built-in image stabilization and the overall stability of the system also affect image quality. To fully leverage the MTF characteristics, it is important to consider both a shooting posture that minimizes shake and the choice of equipment.
Case Study: Comparison of MTF Across Various Lenses
Finally, through concrete comparison examples, we can summarize the practical insights that can be gained from MTF curves.
For instance, when comparing lenses from different manufacturers with the same focal length, there can be significant differences in how MTF drops at the edges and how contrast is maintained in the high-frequency range. The advantage of MTF comparison is that it allows you to check these subtle differences, which cannot be determined by numbers alone, in a comprehensive manner.
Furthermore, by comparing MTF curves from wide-open to stopped-down apertures, it becomes immediately clear how much performance improves. Some lenses become extremely sharp when stopped down by two stops, while others show little change in resolution even when stopped down.
By referring to such MTF comparison examples, you can choose lenses that match your shooting style and desired image quality, achieving the best results while keeping initial costs under control.
Summary
By correctly understanding and utilizing MTF, you can gain valuable insights for achieving high-quality results in both photography and display evaluation.
First, MTF is an objective indicator that shows resolution and contrast reproduction at different spatial frequencies, making it extremely useful for understanding the gap between ideal and actual performance. In photography, it helps with lens selection, while in display evaluation, it is applied to assess visibility, among other uses.
However, MTF often represents results under ideal conditions and is limited to independent evaluation of individual factors. Elements such as bokeh, color reproduction, and optical aberrations cannot be fully captured by MTF curves alone, so it is important to use them in conjunction with other indicators and real-world examples.
To achieve overall image quality, it is necessary to gather information from multiple perspectives and make a comprehensive judgment based on the characteristics and limitations of each metric. By leveraging the powerful tool that MTF provides, you can select the optimal lens or display that best suits your specific objectives.