How to Read MTF Charts

Hey there, fellow photography enthusiasts! If you’ve ever come across the term “MTF charts” and scratched your head, you’re in the right place. In this blog section, we’re going to demystify MTF charts and explain why they are a must-know for photographers.

Brief Explanation of MTF Charts

Let’s start by explaining what MTF charts are. MTF stands for Modulation Transfer Function, which may sound complicated, but don’t worry – we’ll explain it in simple terms.

MTF charts are a tool used to measure the quality of optical systems, like camera lenses. They tell us how well a lens can reproduce details, particularly at different levels of contrast and different spatial frequencies (basically, how fine or coarse the details are).

In simpler terms, MTF charts help us understand how sharp and clear our images will be when we use a specific lens. They provide insights into how well a lens can handle tasks like capturing fine textures or distinguishing between subtle shades of color.

Now, you might be wondering, “Why should I care about MTF charts?” Well, that’s where the importance comes in.

Importance of Understanding MTF Charts for Photographers

• Sharper Images: As photographers, our goal is to capture moments in the clearest and most detailed way possible. Understanding MTF charts helps us choose lenses that can deliver the sharpness we desire. When you know how to read these charts, you can make informed decisions about which lens to use for a particular shot.
• Optimal Aperture Selection: MTF charts also come in handy when deciding on the right aperture setting for a given situation. They can show you the sweet spot of a lens – the aperture where it performs at its best. This means you’ll know when to use a wide aperture for that beautiful background blur (bokeh) and when to stop down for maximum sharpness.
• Reduced Guesswork: Before MTF charts became widely accessible, photographers had to rely on trial and error to find the best lenses and settings. By understanding these charts, you can skip the guesswork and make more educated choices.
• Lens Comparisons: Have you ever been torn between two lenses, unsure which one to invest in? MTF charts allow you to compare lenses objectively. You can see how they perform in terms of contrast and resolution, making your decision-making process much easier.
• Avoiding Disappointment: There’s nothing worse than capturing a moment you thought was perfect, only to discover later that your image lacks the sharpness you expected. Understanding MTF charts helps you avoid these disappointing surprises and ensures your shots meet your quality standards.
• Saving Time and Money: Lastly, knowledge of MTF charts can save you both time and money. You won’t waste hours post-processing trying to sharpen a soft image, and you won’t make unnecessary lens purchases because you didn’t understand what your current lens was capable of.

In conclusion, MTF charts may seem like a technical aspect of photography, but they are a powerful tool that can elevate your skills as a photographer. Armed with this knowledge, you’ll be able to capture sharper, clearer, and more impressive images. So, stay with us as we delve deeper into the world of MTF charts and equip you with the skills to make the most of your lenses.

What is an MTF Chart?

What exactly are MTF charts, and why are they crucial for photographers? In this section, we’re going to answer those questions and shed light on this important aspect of photography.

A- Definition of MTF (Modulation Transfer Function)

Let’s start by explaining the acronym MTF. It stands for Modulation Transfer Function, which may sound complicated, but we’ll explain it in simple terms.

MTF is a quantitative measure that helps us understand how well a lens or optical system can transfer the details of a subject onto the image sensor or film. In other words, it tells us how effectively a lens can reproduce the fine details, contrast, and sharpness of the objects we photograph.

B- How MTF Charts Measure Lens Performance

MTF (Modulation Transfer Function) charts measure lens performance by plotting the contrast and resolution of a lens from the center to its edges against a “perfect” lens that would transmit 100% of the light that passes through it.

Contrast is the difference between the brightest and darkest areas of an image. Resolution is the ability of a lens to distinguish between fine details in an image.

The MTF chart is divided into two sections:

• Sagittal contrast and resolution, which is measured along the radial lines of the chart
• Meridional contrast and resolution, which is measured along the concentric circles of the chart

The horizontal axis of the MTF chart represents the distance from the center of the image to the edges, and the vertical axis represents the contrast and resolution.

A good lens will have a high MTF score, meaning that it will produce sharp, high-contrast images throughout the frame. A lens with a low MTF score will produce softer, lower-contrast images, especially at the edges of the frame.

In conclusion, MTF charts are powerful tools that provide photographers with valuable insights into lens performance. By understanding MTF charts, you can make informed decisions about your gear, capture sharper images, and elevate your photography skills. Stay tuned as we delve deeper into interpreting these charts and applying the knowledge to your photography adventures!

Interpreting MTF Charts

Understanding the Axes and Scales of MTF Charts

MTF charts might look intimidating at first glance, but they’re not as complex as they seem.

To start, let’s break down the axes and scales:

• Horizontal Axis:

This represents the distance from the center of your image. It’s measured in millimeters. Imagine your image as a circle, with the center at 0mm and the edges at the maximum distance your lens can cover.

• Vertical Axis:

On the vertical axis, one will observe indications ranging from zero at the lower end to “1” at the upper end. These indications serve to represent the relative fidelity and optical quality level. The superior performance of a curve on an MTF chart can be inferred by its higher positioning, as it signifies excellence in the aspects being measured by the chart.

Now, the lines or curves on the chart depict how well your lens can reproduce contrast at different distances from the center. The closer the line is to the top of the chart, the better the lens performs.

Differentiating Between Contrast and Resolution in MTF Charts

One of the trickiest aspects of MTF charts is distinguishing between contrast and resolution. Here’s the lowdown:

• Contrast is how well a camera lens shows the difference between dark and light areas in a picture. A good lens will make the transition from black to white very clear without any blurriness. Some lenses may not do this well, especially at the edges of a picture.
• Resolution is different from contrast. It measures how well a lens can show very fine details in a picture. However, a lens with high resolution but low contrast may still produce blurry pictures because it can’t show the difference between light and dark areas clearly. Look at the shape of the curve to assess resolution – a sharp, steep curve means excellent resolution.

How does MTF measure the contrasts and resolution?

• Contrast is measured by examining how well a lens performs with thicker and wider lines. These lines are spaced at a rate of 10 lines per millimeter, which may seem thin but are actually quite thick for testing purposes. The lens is repeatedly evaluated to see how well it can display a clear distinction between the lines and the background, starting from the center of an image and moving toward the corners.

• Resolution, on the other hand, is measured using finer lines that are spaced at 30 lines per millimeter. These extremely thin lines are crucial in determining a lens’ ability to capture and display fine details in real images. When a lens reaches its maximum resolving power, these lines will no longer appear distinct and will instead appear blurred when examined by engineers. Resolution is assessed separately from contrast, and the results of both evaluations are then plotted on MTF charts that can be found online and in other sources.

Sagittal and Meridional test patterns

It is reasonable to presume that a high-quality lens would yield identical levels of “sharpness” for parallel repeating line patterns, regardless of their orientation.

However, in practical circumstances, this is not the case, even with some of the most exceptional lenses available. Lines that run parallel to a diagonal line extending from the center of an image to one of its corners are frequently easier for a lens to accurately and consistently reproduce.

In fact, certain lenses exhibit a significant disparity in their ability to precisely replicate lines that are arranged perpendicular (90˚) to this diagonal direction. These variations are depicted in MTF charts and are a crucial component of the information conveyed by MTF data.

The majority of lenses exhibit varying degrees of contrast and resolution when it comes to Sagittal lines, which are arranged parallel to a diagonal line drawn from the center to the corner of an image, as opposed to Meridional lines.

To test for repeating patterns of both Sagittal and Meridional lines, the process begins at the center of the lens’ image and gradually progresses outward towards its corners. It is important to note that Sagittal lines can be likened to spokes on a wheel, emanating from the center of the image area, while Meridional lines are perpendicular to Sagittal lines and run in the direction of the outer, circular edge of the wheel. These lines are straight and positioned at a 90° angle to the spokes of the wheel.

How to read an MTF Chart

When analyzing MTF charts, it is important to approach them with a critical mindset in order to assess their implications for your photography. The following points should be taken into consideration when examining the MTF charts available online or in brochures:

• Generally, a higher curve on the chart indicates better optical performance in terms of contrast or resolution.
• It is important to note that a “perfect” lens, although purely hypothetical and non-existent in reality, would produce MTF results with all lines running as a completely straight horizontal line at the top of the chart, from left to right. All the blue and black lines, both solid and dashed, would be perfectly aligned. However, it is unlikely that any lens will achieve this level of performance in an MTF test in the near future.
• It is common for the curves of most lenses to decline as you move from the left to the right sides of the MTF chart. This indicates a decrease in optical performance towards the corners of the image. This phenomenon is particularly noticeable in wide-angle designs and is generally considered normal, without causing alarm or concern. The less steep the decline, the better the lens’ performance will be in terms of edge and corner sharpness.
• Meaningful comparisons between lenses of different focal length categories cannot be made solely based on MTF results. The MTF results obtained from wide-angle and ultra-wide lenses, in particular, exhibit significantly reduced MTF values towards the outer edges of the curves. This phenomenon is primarily attributed to inherent optical characteristics specific to these lens types. It is important to note that this does not imply that shooting exclusively with long telephoto lenses is recommended. However, MTF charts can be valuable tools for comparing lenses with similar focal lengths.
• Engineers state that a lens with contrast readings (represented by black lines on the MTF chart) of 0.6 or higher will yield entirely satisfactory image results. On the other hand, a lens with contrast ratings of 0.8 or higher is considered to be of superior quality.
• When the Sagittal and Meridional lines (solid and dashed lines) on the MTF chart are closer to each other, the resulting background blur produced by the lens tends to appear more natural.
• You can use an MTF chart to determine the bokeh effect of a lens. Bokeh is the quality of the out-of-focus areas in an image. It is influenced by the quality of the lens elements and the number of aperture blades. More aperture blades will produce a better bokeh effect.

To determine the bokeh effect of a lens using an MTF chart, look at the solid line and the dotted line. The closer the two lines are together, the softer the out-of-focus effect will be. So, if you are looking for a lens with a good bokeh effect, it is important to consider both the MTF chart and the number of aperture blades.

Limitation of MTF Chart information

While MTF charts indeed can give you some powerful insight into how a lens is likely to perform, keep the following points in mind:

• MTF tests are carried out only at wide-open apertures, so they don’t show how lenses perform at smaller apertures or at different focus distances. This may be limiting for some photographers, so it’s important to consider other factors when choosing a lens.
• MTF charts only measure contrast and resolution, so they cannot tell you how a lens will perform in real-world situations where other optical aberrations, such as distortion, Vignetting, chromatic aberration, and flare, may be present. In short, MTF charts are a useful tool, but they should not be used as the sole criterion for evaluating a lens.
• MTF charts only measure lens performance at one focus distance, typically the “normal” focus distance. However, lens performance can vary depending on the focus distance, especially at closer distances. So, while MTF charts can give you a good idea of how a lens will perform in general, it’s important to keep in mind your intended use for the lens and the fact that MTF tests can’t account for every variable.

In other words, MTF charts are a useful tool, but they’re not perfect. If you’re planning on using a lens for a specific purpose, such as macro photography, you may want to look for additional information on how the lens performs at different focus distances.

• MTF charts for zoom lenses only provide data for the widest and longest zoom settings. They do not provide data for mid-range zoom settings, which are often used by photographers. It is not always the case that the MTF performance at a mid-range zoom setting will be “between” the performance at the widest and longest zoom settings.
• MTF charts from different lens manufacturers are not always directly comparable: This is because different companies use different methods and standards to perform their MTF tests. This can include things like the type of test chart used, the way the test is conducted, and the way the results are analyzed.

As a result, it is important to be cautious when comparing MTF charts from different manufacturers. While you may be able to make general comparisons between two lenses with similar focal lengths, it is best to avoid making firm conclusions based on MTF charts alone.

• MTF charts can tell you a lot about the optical performance of a lens, but they cannot tell you how well-suited a lens will be for you personally. There are many other factors to consider when choosing a lens, such as:
  • Focal length: How important is this focal length to you? What type of photography do you do?
  • Size/weight/portability: How important is portability to you? How big and heavy are you willing to carry around?
  • Handling: How well-balanced is the lens? How easy is it to focus manually?
  • Minimum focusing distance: How close can the lens focus?
  • Zoom range: Is the zoom range ideal for your needs?
  • Maximum aperture: Is the maximum aperture fast enough for the type of photography you do and the environments you shoot in?
  • Image stabilization: Does the lens have image stabilization? Is this important to you?
  • Price: How much are you willing to spend?

It is important to weigh all of these factors when choosing a lens, not just the MTF charts.

Final thought

MTF charts are a useful tool for comparing the optical quality of different lenses. They can help you understand the strengths and weaknesses of each lens, and make more informed decisions about which lens is right for you.

MTF charts are not perfect, and they don’t tell you everything you need to know about a lens. For example, they don’t tell you about the lens’s size, weight, or handling. However, MTF charts can give you a good starting point for narrowing down your options and finding the best lens for your needs.

Here are some examples of how to use MTF charts:

• Portrait photographers: If you’re a portrait photographer, you may be interested in the MTF performance of a lens at wide apertures. This is because you’ll often be shooting with a shallow depth of field, and you’ll want to make sure that the subject of your portrait is in sharp focus.
• Landscape photographers: If you’re a landscape photographer, you’ll want to look at the MTF performance of a lens across the entire frame. This is because you’ll not often be shooting with a wide aperture, and you’ll want to make sure that all of the elements in your scene are in sharp focus.
• Event photographers: If you’re an event photographer, you may be interested in the MTF performance of a lens at different zoom settings. This is because you’ll often be moving around a lot and shooting in different conditions. You’ll want to make sure that the lens you’re using can produce sharp images at all zoom settings.

Overall, MTF charts are a valuable tool for photographers of all levels. By understanding how to read and interpret MTF charts, you can make more informed decisions about which lenses to buy and how to use them.

Thanks for reading, I hope you enjoyed the article, if you have any questions just drop them below & I will be happy to answer you.

The featured photo by Image by Rosalia Ricotta from Pixabay

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