Interpreting MTF Charts

Navigation: Pillar Guide | Lens Design | Interpreting MTF Charts

Review status: pending. [Reviewer name and credential to be confirmed]. No reviewer has yet approved this page.

What an MTF chart shows

The modulation transfer function (MTF) describes how much contrast a lens transmits at each level of detail. The horizontal axis is spatial frequency, usually in line pairs per millimetre. The vertical axis is the contrast retained, from 0 to 1. A high curve means the lens keeps fine detail at high contrast.

The chart describes the lens, not the whole camera. The sensor, processing and focus accuracy all affect the final image.

Reading the curves

Charts usually show several curves. Solid and dashed lines often represent different orientations of the detail, such as radial and tangential. Each curve is shown at several points across the frame, usually the centre, the middle and the edge. Comparing curves at the same aperture is the fair comparison.

A curve that drops quickly at high frequency shows that fine detail is lost. A curve that stays high to a higher frequency shows a sharper lens under those conditions.

Linking the chart to the sensor

Compare the frequency range of the chart with the Nyquist frequency of the sensor, which is 1 divided by twice the pixel pitch. Detail beyond the Nyquist frequency can cause aliasing. A lens that transmits detail well beyond the Nyquist frequency of a given sensor is not necessarily a better choice for that sensor.

Checking charts against real tests

Manufacturer charts are measured under particular conditions. For a camera-level test, ISO 12233 describes methods for measuring resolution and spatial frequency response of digital still cameras, using standard test targets. Independent tests using that method show how the whole system performs.

Common mistakes

Comparing charts measured at different apertures or wavelengths. Ignoring the frame position of the curve. Treating an MTF chart as a guarantee of sharpness in every shooting condition.

Sagittal and tangential orientations

Lens designers usually report MTF for two orientations of the test pattern. Sagittal detail runs radially from the centre of the frame, and tangential detail runs around the centre, at right angles to it. A lens can perform differently in these two orientations, particularly towards the edges. A large gap between the two curves can indicate astigmatism or other asymmetric aberrations.

Using MTF to compare lenses fairly

Fair comparison requires the same aperture, the same focus distance and the same frequency range. Charts measured at f/2.8 cannot be compared with charts measured at f/8, because the curves change with aperture. The frequency at which each lens drops to a given contrast, often 50 percent, is a useful summary figure, provided the measurement conditions are stated.

Limits of the chart

A chart describes the optical quality of the lens at the measurement conditions. It does not describe flare, ghosting, focus accuracy, image stabilisation, sensor noise or the processing applied to the raw file. A lens with an excellent chart can still produce a poor image in backlit or low-contrast conditions. The chart is one tool among several.

Spatial frequency and line pairs

Spatial frequency is the number of line pairs per unit length, where one line pair is a dark line and an adjacent light line. A frequency of 100 line pairs per millimetre means that the pattern repeats every 10 micrometres. Charts show MTF against this frequency, so the curve describes how well the lens keeps contrast at each pattern spacing.

Contrast values and what they mean

MTF values run from 0 to 1. A value of 1 means the contrast is fully preserved, and 0 means the detail has disappeared. Many photographers look at the contrast at 10, 30 and 50 line pairs per millimetre on the sensor plane, because these correspond roughly to coarse, medium and fine detail. The exact frequencies used depend on the sensor and on the chart conventions.

The effect of focus

MTF depends on focus. A chart measured at best focus shows the lens at its best. A chart measured slightly away from best focus shows lower values, and the drop can be steep. When comparing charts, check whether the focus was optimised for each lens or set by a standard procedure.

A worked comparison of two lenses

Suppose lens A has a 50 percent contrast frequency of 60 line pairs per millimetre in the centre at f/4, and lens B has 45 line pairs per millimetre at the same settings. Lens A keeps more fine detail at this aperture, so it would be the better choice for fine texture if the sensor can sample that frequency. If the sensor has a Nyquist frequency of 40 line pairs per millimetre, the two lenses may produce similar images, because both exceed the sensor limit.

Matching the chart to your use

A landscape photographer who prints large may care most about fine detail at mid-frame apertures. A portrait photographer may care more about contrast at the centre wide open and about smooth falloff at the edges. The same chart can support different choices depending on which part of the curve matters.

Common confusions

A high value at low frequency does not mean fine detail is good; it describes coarse contrast. Different manufacturers may use different test targets or conventions, so compare charts only from sources that state their method.

Going further

A chart is most useful when it is read alongside a real test. The chart shows what the lens should do under the stated conditions, and the test shows what it does with your camera, your focus and your subject.

Summary

The key point of this article is the one stated at its start. Check the formula, use the stated assumptions, and confirm any result with a test before relying on it in practice.

Sources

Eugene Hecht, Optics, Pearson (any current edition).
Max Born and Emil Wolf, Principles of Optics, Cambridge University Press (any current edition).
ISO 12233: Photography, Electronic still picture imaging, Resolution and spatial frequency responses.

Next step

Return to the pillar guide or the Lens Design summary to read the related articles.

Who should review this: This technical documentation should be reviewed by a credentialed expert. Reviewer: [name and credential to be confirmed]. Last reviewed: [date to be added at review].

General information only. This page explains optical principles and does not replace lens design work, lens testing or advice from a qualified optical engineer.