An MTF (Modulation Transfer Function) chart is a graphical representation of a lens's ability to transfer detail from the subject being photographed to the image captured. The chart provides a quantitative measure of a lens's performance and allows you to compare different lenses and evaluate their relative strengths and weaknesses. 

The MTF chart is critical because it provides an objective measure of a lens's ability to resolve fine details and distinguish between different tones in the image. Understanding how to read an MTF chart can help you make informed decisions about lens selection. This will help you improve your overall photography skills by helping you understand the limitations of a lens and how it may affect the final image quality. 

It's also imperative to keep in mind that the MTF chart only provides part of the picture. Other factors such as lens aberrations, chromatic aberrations, and distortion can also affect the final image quality. Nevertheless, an MTF chart provides a useful starting point for evaluating lens quality and can help you make more informed decisions about lens selection. 

What does an MTF chart show? 

An MTF chart provides a graphical representation of a lens's ability to transfer detail from the subject being photographed to the image captured. The chart has two axes: the horizontal axis represents spatial frequency, which is the number of line pairs per millimetre, and the vertical axis represents the level of modulation, or the amount of detail that is preserved. 

The level of modulation is expressed as a percentage, with 100% representing complete preservation of detail and 0% representing no detail. The MTF chart displays the amount of detail that is preserved as a function of spatial frequency. This allows you to understand how the lens performs at different levels of detail. 

For example, if a lens has a high level of modulation at high spatial frequencies, it indicates that the lens can preserve fine details in the image. Conversely, if the level of modulation decreases as spatial frequency increases, it means that the lens is not able to preserve fine details as effectively. 

The MTF chart provides a useful tool for evaluating lens quality and allows you to compare different lenses and understand the limitations of each lens in capturing fine details. By examining the MTF chart, you can get a better understanding of how a lens performs in terms of detail preservation and make informed decisions about lens selection. 

How to read the horizontal axis of a MTF chart? 

The horizontal axis of an MTF chart represents the spatial frequency of the lens, measured in line pairs per millimetre (lp/mm). Spatial frequency refers to the number of alternating light and dark lines that can fit in a given area. It is a measure of the level of detail in the image. 

The spatial frequency is typically plotted on a logarithmic scale, with the lowest value on the left and the highest value on the right. This logarithmic scale allows you to see how the lens performs at different levels of detail. This is from the coarse details on the left side of the chart to the finer details on the right side. 

For example, a spatial frequency of 10 lp/mm represents coarse details, while a spatial frequency of 100 lp/mm represents finer details. By examining the horizontal axis, you can get a sense of the level of detail that the lens is capable of resolving. 

It's important to keep in mind that the horizontal axis only provides information about the spatial frequency of the lens. It does not provide information about the level of detail preservation. To understand how well the lens is able to preserve detail, you need to look at both the horizontal and vertical axes of the MTF chart. 

How to read the vertical axis of a MTF chart? 

The vertical axis of an MTF chart indicates the level of modulation, or the amount of detail that is preserved from the subject to the image. The level of modulation is expressed as a percentage, with 100% representing complete preservation of detail and 0% representing no detail. 

The MTF chart displays the level of modulation as a function of spatial frequency, which is plotted on the horizontal axis. The chart typically has two curves: one for the lens's ability to preserve high spatial frequency detail, represented by fine lines or small details, and one for the lens's ability to preserve low spatial frequency detail, represented by coarse lines or large details. 

The higher the curve, the better the lens is able to preserve the corresponding level of detail. If the curve is flat and close to the 100% level, it means that the lens is designed to preserve a high level of detail at all spatial frequencies. Conversely, if the curve drops off rapidly, it means that the lens is not able to capture fine details as effectively. 

By examining the vertical axis, you can get a sense of the lens's ability to preserve detail. You can also understand how well the lens is able to capture fine and coarse details in the image. This information can be useful in making informed decisions about lens selection and understanding how a lens may affect the final image quality. 

What to look for on an MTF chart 

When examining an MTF chart, there are several key factors to consider in order to understand the lens's ability to preserve detail: 

• High spatial frequency performance: Look at the high spatial frequency curve, which represents the lens's ability to preserve fine details in the image. If the curve is close to the 100% level, it indicates that the lens is able to preserve a high level of fine detail. 

• Low spatial frequency performance: Look at the low spatial frequency curve, which represents the lens's ability to preserve coarse details in the image. If the curve is close to the 100% level, it indicates that the lens is able to preserve a high level of coarse detail. 

• Sharpness: A sharp lens is one that can preserve both high and low spatial frequency detail well. A sharp lens will have MTF curves that are close to the 100% level at both high and low spatial frequencies. 

• Contrast: The MTF chart can also provide information about the lens's ability to preserve contrast. If the MTF curves are flat and close to the 100% level, it means that the lens is able to preserve a high level of contrast in the image. 

• Colour accuracy: While an MTF chart provides information about detail preservation, it does not provide information about colour accuracy. To assess colour accuracy, you need to examine other lens performance metrics, such as colour fringing or chromatic aberrations. 

By considering these factors, you can get a clear understanding of the lens's ability to preserve detail. You can also make informed decisions about lens selection based on your specific needs and requirements. 

Interpreting MTF curves 

The MTF curves on an MTF chart represent the lens's ability to preserve detail as a function of spatial frequency. To interpret the curves, it's helpful to understand the following key aspects: 

• Shape of the curve: The shape of the MTF curve is indicative of the lens's ability to preserve detail. If the curve is flat and close to the 100% level, it means that the lens is able to preserve a high level of detail at all spatial frequencies. If the curve drops off rapidly, it means that the lens is not able to preserve fine details as effectively. 

• Position of the curve: The position of the curve on the MTF chart can also provide information about the lens's detail preservation ability. If the curve is shifted to the left, it means that the lens is able to preserve detail at lower spatial frequencies but not as effectively at higher frequencies. If the curve is shifted to the right, it means that the lens is able to preserve detail at higher spatial frequencies but not as effectively at lower frequencies. 

• Height of the curve: The height of the curve is an important indicator of the lens's precision. The closer the curve is to the 100% level, the better the lens is able to preserve detail. 

By interpreting the MTF curves, you can get a sense of the lens's ability to preserve detail. You can also understand how well the lens is able to capture fine and coarse details. You can make informed decisions about lens selection based on your specific needs and requirements. 

It's important to keep in mind that while the MTF chart provides valuable information about the lens's detail preservation ability, it's just one aspect of lens performance. To get a full picture of lens performance, you need to consider other factors, such as colour accuracy, distortion, and chromatic aberrations. 

Understanding lens limitations 

While an MTF chart provides valuable information about the lens's ability to preserve detail, it's helpful to understand that no lens is flawless, and all lenses have limitations. Understanding these limitations can help you make informed decisions about lens selection based on your specific needs and requirements. 

Some common limitations to be aware of include: 

• Center vs. corner sharpness: Lenses often have better detail preservation ability in the centre of the frame compared to the corners. This is because the image is projected onto the camera sensor at an angle, which can cause some degree of distortion and a reduction in detail preservation ability. 

• Aperture: The aperture of a lens can also have an impact on its detail preservation. As the aperture is increased, the amount of light entering the lens decreases, which can result in a reduction in detail preservation ability. 

• Focal length: The focal length of a lens can also impact its ability to capture details. Longer focal lengths can result in a significant degree of distortion and a reduction in detail preservation ability. 

• Optical aberrations: Optical aberrations, such as chromatic aberrations and spherical aberrations, can also impact a lens's image quality. These aberrations can cause a reduction in contrast and a reduction in the lens's ability to preserve fine details. 

By understanding these limitations, you can make informed decisions about lens selection and ensure that you're getting a lens that meets your specific needs and requirements. Additionally, by understanding these limitations, you can also make better decisions about lens usage, such as selecting the appropriate aperture or focal length for a given situation. 

In conclusion, MTF charts are a valuable tool for evaluating a lens's detail preservation ability. By understanding the key elements of an MTF chart, including the horizontal and vertical axes, the MTF curves, and lens limitations, you can make informed decisions about lens selection and use. Whether you're a professional photographer or a hobbyist, understanding MTF charts can help you get the most out of your lenses and capture the images you envision.