How to Read MTF Curves: Contrast, Detail, and Sensor Sampling
Read the axes and test conditions first. Then check the detail and image positions your camera needs to capture.
The modulation transfer function (MTF) describes how much contrast a lens preserves at different levels of detail. An MTF plot may show contrast versus spatial frequency or versus position across the image. Read both the axes and the test conditions before comparing lenses. Sensor pixel pitch is a necessary sampling reference, but the application determines what contrast is good enough.
What MTF Measures
Imagine a pattern whose brightness changes smoothly from light to dark and back again. As its stripes get finer, a lens usually reproduces them with less contrast. MTF describes this response to sinusoidal patterns, as explained in MIT's Fourier-optics lecture.
MTF is dimensionless. A value of 0.4 means 40% of the input modulation remains at that frequency. It does not mean 40% light transmission. Normalized optical MTF is 1 at zero spatial frequency.
Spatial frequency is often given in line pairs per millimeter (lp/mm) at the sensor. One pair contains one light stripe and one dark stripe. A pattern with a 20μm period has 50 pairs per millimeter; an isolated edge or defect contains many frequencies.
Read the Axes and Conditions
| Plot | Horizontal Axis | What to Compare |
|---|---|---|
| MTF versus frequency | Detail fineness, often in lp/mm | Contrast at the same frequency and image position. |
| MTF対フィールド | Image height or field angle | Contrast across the image at a stated frequency. |
The vertical axis is contrast transfer, usually 0–1 or 0–100%. Check the aperture, wavelength band, object distance, and focus setting. Curves measured under different conditions answer different questions; Edmund Optics' guide explains these dependencies.
Read the legend for every line. A solid line does not have the same meaning in every vendor's plot, and a curve need not fall smoothly at every frequency.
Sagittal and Tangential Curves
Sagittal and tangential curves describe two perpendicular directions at an image point. In the usual optical-design convention for a point on the vertical field axis, tangential MTF measures brightness variation along that axis; sagittal MTF measures variation across it. Ansys documents this convention.
Stripe direction is perpendicular to the direction in which brightness varies. Radial stripes can therefore correspond to sagittal MTF even though tangential frequency runs radially. Check whether the legend describes stripes or frequency direction.
A gap between the curves means contrast depends on orientation. Astigmatism can contribute, but the gap alone does not identify it. Compare several focus settings and check alignment. If your task needs both orientations, judge both curves.
Check the Image Area You Use
Image height is the distance from the image center. For a centered sensor, the corner lies at half its active diagonal. Use dimensions from the sensor drawing; an inch-format name is not an exact size.
A normalized field coordinate needs a stated reference. The endpoint of a plotted curve marks the tested or calculated field, not necessarily the lens's illumination boundary. A steep drop calls for checks of focus, alignment, and aberrations before you reject the lens.
Use Pixel Pitch as a Sampling Reference
For equally spaced pixels, the row or column Nyquist frequency is half the sampling frequency. It marks a sampling boundary, not a required lens MTF value.
At exactly Nyquist, the measured contrast depends strongly on where the pattern falls relative to the pixels. Higher frequencies can appear as false lower-frequency patterns, called aliasing. Pixel area, color filters, and processing also affect the recorded image.
Choose the frequencies and contrast limits from the feature you need to detect. There is no universal 0.3 MTF target or fixed fraction of Nyquist that suits every application. ISO 12233's introduction distinguishes pixel count from measured resolution and spatial frequency response.
Magnification connects object and image detail. At a magnification of 0.1, a 10lp/mm object pattern becomes 100lp/mm in the image. For an isolated defect, test the actual feature rather than assigning it one frequency from its width.
Design MTF, Lens Measurements, and Camera Tests
Design MTF comes from an optical model. A nominal model uses the specified surfaces and positions; a tolerance analysis explores manufacturing variation. Measured MTF comes from a physical lens or camera and includes the test setup's effects and uncertainty.
A lens bench can isolate component performance. A slanted-edge camera test measures the lens, sensor, and processing together. ISO 12233:2024 covers digital-camera resolution and spatial frequency response (SFR), including edge-based measurements. Sharpening can make camera SFR exceed 1, so it should not be read as bare-lens MTF.
Compare data at matching aperture, spectrum, distance, and focus conditions. A through-focus measurement shows how contrast changes as focus moves and helps separate field curvature, astigmatism, and alignment effects. Optikos' measurement guide describes these methods.
Request Data for Your Operating Conditions
Send the sensor model, pixel pitch, object-distance range, illumination band, and required image positions. Include the detection or measurement criterion. Ask whether the results describe a nominal design, a sample lens, or a production test.
Commonlands offers MTF testing. Confirm the current measurement scope and test conditions with engineering before using a report as an acceptance specification.
よくある質問
レンズにおけるMTFとは何ですか？
MTF is the ratio of image modulation to object modulation for a sinusoidal pattern at a stated spatial frequency. It describes contrast transfer. It is dimensionless and must be read with the image position and test conditions.
MTFチャートの読み方は？
Identify whether the horizontal axis shows frequency or field position. Then read contrast at the frequencies and positions relevant to your task. Compare curves only after checking aperture, spectrum, object distance, and focus conditions.
What does sagittal versus tangential mean?
The curves describe perpendicular directions of brightness variation. A stripe runs perpendicular to that variation, so stripe labels and frequency-direction labels can be confusing. Check the vendor’s convention and evaluate both orientations where your task needs them.
Does a lens need a particular MTF at Nyquist?
No universal pass value applies. Pixel pitch sets the sampling reference, while feature contrast, noise, processing, and the measurement task determine acceptable performance. Test representative images across the required field.
Does a camera test measure only the lens?
No. A camera test includes the sensor and processing as well as the lens. A component test uses a calibrated optical bench to characterize the lens separately. Both are useful, but they answer different questions.
References and Further Reading
- MIT OpenCourseWare, Optics, Lecture 22. MTF and incoherent imaging. Further reading: Joseph W. Goodman, Introduction to Fourier Optics, 3rd ed., Chapter 6.
- Optikos, How to Measure MTF and Other Properties of Lenses. Sinusoidal response, lens testing, and through-focus measurements.
- ISO 12233:2024. Digital-camera resolution and spatial frequency response; the linked record describes the scope.
- Ansys OpticStudio, Sagittal and Tangential. Axis conventions for optical analysis.
- Edmund Optics, Introduction to Modulation Transfer Function. Reading plots and comparing operating conditions.
Need MTF Data for a Lens?
Send your sensor, pixel pitch, working distance, and required image positions. Commonlands engineering can review available data and the test conditions needed to compare lenses.