What Is MTF Testing and Why Does It Matter for Camera Lenses

MTF means modulation transfer function. This test shows how well a camera lens shows details and contrast from a subject. MTF testing lets photographers compare sharpness and image quality between camera lenses. Makers and buyers use MTF charts to see real differences in how lenses work, not just what the product says. Lens MTF Testing uses numbers and facts, so people can pick a camera lens with confidence. Knowing about MTF helps people make better choices for their photography needs.

Key Takeaways

• MTF testing checks how well a lens keeps contrast and details. This helps photographers compare how sharp and clear lenses are. High MTF values mean the lens makes sharp, clear photos. These photos have strong contrast and show fine textures. Low values mean the lens loses detail and looks blurry. MTF charts help you see how a lens works from the center to the edge. They show if the lens keeps good image quality all over the picture. MTF testing can find lens problems like tilt or decentering. These problems can make photos less sharp. This testing helps make sure the lens is good before you buy it. MTF testing is helpful, but real-world things matter too. Lighting, focus, and how you use the lens also change photo quality. So, use MTF data with hands-on tests for the best results.

Modulation Transfer Function

What MTF Measures

The modulation transfer function, or MTF, tells us how well a lens shows contrast and detail from a subject in a photo. Scientists use MTF to check how much contrast from the object stays in the image as the details get smaller. Spatial frequency means how many lines or patterns fit in a small space, measured in line pairs per millimeter. When spatial frequency goes up, the details get smaller and harder to see. MTF shows how much contrast is left as these tiny details appear.

MTF is important for checking how good a lens is. It gives a fair way to compare how different lenses handle contrast and resolution. The MTF chart has contrast on the Y-axis and spatial frequency on the X-axis. This chart lets people see how well a lens keeps image quality as details get smaller. The MTF value starts at 100% for big, simple patterns and drops as the patterns get finer.

In labs, experts measure MTF with special test targets. They shine light through slits or pinholes and take a picture with the lens. Then, they use a math process called a Fourier transform to study the image. This shows how the lens handles different levels of detail and contrast. Modern machines can test many directions at once, so we get a full look at lens performance.

Tip: MTF gives a fair way to judge lens quality, so it is easier to compare different models.

Contrast and Resolution

MTF checks two main things in a lens: contrast and resolution. Contrast shows how well the lens keeps the difference between light and dark. Resolution shows how well the lens can show small details. Both are needed for sharp, clear photos.

• Contrast: At low spatial frequencies, MTF shows how well the lens keeps strong differences between light and dark. High contrast here means the image looks bold and clear.

• Resolution: At high spatial frequencies, MTF shows how well the lens can show tiny details. High MTF here means the lens can show fine patterns, which is important for camera resolution.

MTF charts use thick lines for low frequencies and thin lines for high frequencies. This helps people see how the lens does with both contrast and resolution. For example, a lens with high MTF at low frequencies makes images that stand out, while high MTF at high frequencies means the lens can show fine textures and edges.

How well a lens works depends on how it balances contrast and resolution. A lens with high MTF at both low and high frequencies will make sharp, detailed images. If the MTF drops fast as spatial frequency goes up, the lens may lose fine detail, even if it keeps good contrast in bigger shapes.

MTF also helps explain why some lenses look sharper than others. The MTF50 value is where the lens keeps half the original contrast, and this matches how people see sharpness. Our eyes notice certain spatial frequencies most, so MTF charts can help guess how sharp a photo will look.

• MTF values at higher spatial frequencies show if the lens can show fine details, which is important for camera resolution.

• Differences between lines on the MTF chart can show lens problems like astigmatism, which affects sharpness in different directions.

A good lens keeps high MTF values at many frequencies. This means it can give both strong contrast and high camera resolution, so photos look better overall.

Lens MTF Testing

Testing Process

Lens mtf testing uses special tools to check image quality. The test starts when a worker shines a small light through the lens. The light goes through the lens and lands on a sensor or screen. The worker moves the light and image very carefully. This helps make sure the results are correct.

The main goal is to see how much contrast the lens keeps at different detail levels. These levels are called “spatial frequencies” and are measured in line pairs per millimeter. When there are more line pairs, the details get smaller and harder to see. The mtf curve shows how well the lens keeps contrast as details get finer. When spatial frequency is very high, the lens cannot keep much contrast. The image then looks gray.

Here are the main steps in lens mtf testing:

1. The worker shines a small light through the lens.

2. The light and image are set in the right place.

3. The system checks how the lens changes the light and records the impulse response.

4. The impulse response helps make the mtf curve, which shows contrast at different spatial frequencies.

5. The testing room is kept clean and controlled to stop outside light or mistakes.

6. Sometimes, a slanted-edge target is used. This is a sharp black-and-white edge put at a small angle to the sensor.

7. The system checks how the edge looks after going through the lens. It uses math to turn this into an mtf curve.

8. Some tests use a reverse way, shining a test pattern from the image side back through the lens.

9. Through-focus mtf testing checks the lens at different focus points to see how focus changes image quality.

Note: The mtf curve usually drops as spatial frequency goes up. This means the lens has a harder time keeping contrast with very fine details.

Professionals use special machines for lens mtf testing. Some common systems are LensCheck™ and OpTest® Lens Measurement System. These tools help measure mtf fast and correctly. Software like OpTest® 7 and ImageMaster® MTF Lab Software helps experts look at the results and compare lenses.

Detecting Aberrations

Lens mtf testing does more than check sharpness. It also helps find problems in the lens, like tilt and decentering. These problems can make photos blurry or uneven.

If a lens is not lined up right, it can cause tilt or decentering. Tilt means the lens is at an angle to the sensor. Decentering means the lens is not in the center. Both problems lower the mtf values, especially at higher spatial frequencies. This means the lens cannot keep contrast in small details.

MTF testing can find these problems by checking how sharpness changes across the image. If one side of the photo is sharp and the other is blurry, the lens might have tilt. Decentering can cause strange shapes or patterns in the image, like coma or uneven blur. These problems show up as drops in the mtf curve at some points.

• MTF numbers, like sharpness uniformity, change if the lens has tilt or decentering.

• The optical center, which is the brightest spot in the image, can move if the lens is tilted.

• If the mtf curve drops more on one side, this can mean the lens is not set up right.

• Some problems, like keystone distortion, happen when the test target is not lined up with the sensor.

Tip: MTF testing can find lens problems that are hard to see with your eyes. It helps lens makers and users check if the lens works as it should.

By using lens mtf testing, experts can find and fix problems before the lens is sold. This means better photos and more trust in the lens.

Reading MTF Charts

Axes and Lines

An mtf chart gives a clear picture of how a lens performs from the center to the edge of an image. The horizontal axis on an mtf chart shows the distance from the center of the image to the edge, measured in millimeters. The vertical axis displays the mtf value, which ranges from 0 to 1, or 0% to 100%. This value tells how well the lens keeps contrast and detail as you move across the frame.

Each mtf chart uses several lines to show different details. Thick lines stand for lower spatial frequencies, like 10 line pairs per millimeter, which means coarser detail and strong contrast. Thin lines show higher spatial frequencies, such as 30 line pairs per millimeter, which means finer detail and higher resolution. Solid lines on the mtf chart represent sagittal measurements, which follow a line from the center outwards. Dashed lines show meridional measurements, which go in a direction perpendicular to the sagittal lines. Sagittal lines often stay higher on the mtf chart, especially near the edges, while meridional lines can drop more quickly. The closeness of these lines also hints at the lens’s bokeh quality. When the lines stay close together, the lens can create smoother out-of-focus areas.

Tip: When reading an mtf chart, look for lines that stay high and close together. This means the lens keeps good contrast and resolution across the whole image.

High vs. Low Values

The mtf chart helps photographers judge lens quality by showing high and low mtf values. High mtf values mean the lens keeps strong contrast and fine detail, so images look sharp and clear. Low mtf values mean the lens loses contrast and detail, which leads to blurry or dull photos.

• High mtf values show better image quality, with more detail and less blur.

• Low mtf values mean the lens cannot keep up with fine details, so images lose sharpness.

• A lens with high mtf at high spatial frequencies can show tiny patterns and textures.

• If the mtf chart shows high values at low frequencies but drops at high frequencies, the lens may look sharp in simple scenes but miss fine details.

Photographers use the mtf chart to compare lenses. They check how the lines behave at different points on the chart. For example, a lens with high, flat lines across the mtf chart will likely give sharp images from the center to the edge. The mtf chart also helps spot problems like astigmatism, where sagittal and meridional lines separate. This separation can affect sharpness and bokeh.

The mtf chart is a powerful tool for lens selection. It gives real numbers for contrast and resolution, helping photographers choose the best lens for their needs.

MTF in Real-World Use

Sharpness and Contrast

MTF plays a big role in how sharp and clear a photo looks. Sharpness comes from both resolution and contrast. MTF charts show how well a lens keeps contrast at different detail levels. High MTF values mean the lens keeps fine details and strong contrast, which leads to sharp images. Low MTF values mean the lens loses detail and contrast, so photos look blurry or dull.

• MTF charts measure contrast at different spatial frequencies, but most tests use infinity focus. Real scenes often use closer focus, so results can change.

• Field curvature can make the corners look less sharp on MTF charts, but in real photos, corners may look better if the subject is not flat.

• Some qualities, like “3D pop” or “clinical” sharpness, do not show up on MTF charts. These depend on how people see the image.

• The whole imaging chain, including the sensor and processing, affects sharpness. Sharpening can make a photo look sharper, but it does not add real detail.

Note: MTF charts help compare lens sharpness, but they do not tell the whole story about how a photo will look.

Image Quality Factors

MTF gives important facts about lens performance, but it does not cover every part of image quality. Photographers use MTF to see how a lens handles resolution and contrast. This helps them pick lenses that fit their needs.

1. MTF shows how well a lens resolves fine detail and keeps contrast, which is key for sharpness.

2. Lenses usually perform best in the center. MTF charts show how performance drops toward the edges.

3. Stopping down the lens (using a smaller aperture) often improves both contrast and resolution, as seen in higher MTF values.

4. MTF results help compare lenses from the same brand and set real expectations for sharpness and contrast.

5. MTF does not measure distortion, color, vignetting, or flare. These also affect image quality.

6. Real-world performance can change due to light, focus distance, and even small differences between lens copies.

MTF, along with sensor quality, shapes the final image performance. High MTF values mean better image quality, but other factors matter too. Photographers should use MTF as one tool to judge lenses, not the only one.

MTF Limitations

What MTF Misses

MTF testing tells us about lens sharpness and contrast, but it does not show everything. This test checks one spot at a time, so it takes longer than tests that check many spots together. MTF testing happens in a lab, not outside. Lab lighting and shooting are different from real life. Because of this, the results might not match how a lens works outside.

Things like humidity, temperature, and dust can change how a lens works. MTF testing does not look at these things, so it can miss problems that happen in daily use. For example, if it is very humid or if you use a lens for a long time, the lens might not work as well. Real-world use often makes lenses act differently than in the lab.

The tools for mtf testing cost a lot and need careful use. If someone sets up the test wrong, the results can be wrong. Some mtf tests, like slanted-edge testing, are very sensitive to light and can make big mistakes with fine details. The way the camera measures angles can also make some mtf results hard to understand.

Note: MTF testing shows lens sharpness, but it cannot find every real-world problem.

Other Evaluation Factors

Photographers use other tests with mtf to learn more about lens quality. Reverse projection checks many spots at once and is fast, but it cannot check contrast and depends on what people see. Slanted-edge mtf testing checks several spots and uses software to measure the whole system, like the camera and light. But it can be sensitive to light and how the camera is set up. Camera tests show how the lens works in real photos, but they are hard to compare and do not have much outside help.

• Humidity, temperature, and dust can change how a lens works in ways mtf does not show.

• Real-world tests help find problems that lab tests do not catch.

• Field curvature testing checks how a lens works in three dimensions. This test finds problems like field tilt or decentering that mtf might not show. It takes longer but gives a better idea of how a lens works in real scenes.

Photographers should use mtf and other tests to pick the best lens for what they need.

Understanding MTF testing helps photographers make smart lens choices. MTF charts give clear data about sharpness and contrast, but other factors matter too. Experts suggest balancing MTF with real-world needs, cost, and lens features.

1. Learn how to read MTF charts for your lens type.

2. Compare lenses with similar focal lengths and apertures.

3. Think about how you will use the lens in real life.

4. Use MTF data with reviews and hands-on tests for the best results.

MTF testing gives valuable insight, but the best lens fits both your technical and creative goals.

FAQ

What does a high MTF value mean for a lens?

A high MTF value means the lens keeps strong contrast and fine detail. Photographers can expect sharper and clearer images. High values across the chart show the lens performs well from the center to the edge.

Can MTF charts predict real-world photo quality?

MTF charts help predict sharpness and contrast, but they do not show everything. Real-world factors like lighting, focus, and lens handling also affect photo quality. Photographers should use MTF charts with hands-on tests and reviews.

Do all lens makers use the same MTF testing methods?

Lens makers often use different testing setups and standards. Some use computer simulations, while others use real lenses. Comparing MTF charts from different brands may not always give a fair result.

Where can photographers find MTF charts for lenses?

Most lens makers post MTF charts on their official websites. Review sites like DPReview and LensTip also share MTF data and comparisons for many popular lenses.