Reading an MTF chart correctly means checking contrast at specific spatial frequencies across the image field, not just glancing at how high the lines sit on the page. The chart tells you about resolution and contrast potential under one specific, often idealized, set of test conditions, and it says nothing about coma, autofocus behavior, flare resistance, or how a lens renders out-of-focus areas. Used well, an MTF chart helps you compare closely related lenses tested the same way, spot likely corner-sharpness patterns, and decide whether you need to look at real sample images before drawing conclusions.

Identify the Axes and Legend

Most manufacturer MTF charts plot two things: the horizontal axis represents distance from the center of the frame, usually in millimeters, referred to as image height, running from the optical center out toward the corner. The vertical axis represents contrast, expressed as a value from 0 to 1 (or as a percentage), where a value near 1 means the lens is reproducing near-full contrast between fine light and dark lines at that point in the frame, and a lower value means contrast is being lost.

Charts typically show two or more curves for different spatial frequencies, measured in line pairs per millimeter. A coarser frequency, such as 10 line pairs per millimeter, reflects how well the lens preserves overall contrast and a sense of micro-contrast. A finer frequency, such as 30 or 40 line pairs per millimeter, reflects the lens’s ability to resolve genuinely fine detail. A lens can score well on the coarse line and weaker on the fine line, or the reverse, and both numbers matter for a complete picture.

Read Sagittal and Tangential Lines

Each frequency is usually plotted twice: once for lines oriented radially, running out from the center (commonly labeled sagittal, or S), and once for lines oriented tangentially, running around the center (commonly labeled meridional or tangential, T or M, depending on the manufacturer). This distinction exists because several optical aberrations, particularly astigmatism and coma, affect these two orientations differently away from the center of the frame.

When the sagittal and tangential lines for a given frequency sit close together across the frame, the lens is rendering that detail fairly evenly regardless of orientation. When they separate noticeably toward the edges, it usually signals astigmatism-type aberrations at that field position. It is tempting to treat a wide gap between the two lines as shorthand for harsh bokeh, but the gap describes behavior at the plane of focus, not the character of out-of-focus areas, so it should not be read as a complete bokeh rating on its own.

Compare Like With Like

An MTF chart is only informative when you know the conditions behind it. The aperture used matters a great deal: some charts are generated wide open, others at a fixed aperture such as f/8, and comparing a wide-open chart to a stopped-down chart from a different lens will mislead you every time. Focus distance matters too, since most manufacturer charts are calculated or measured at infinity focus, which may not represent how the lens behaves at portrait or close-up distances.

It is also worth knowing whether the chart is calculated from the optical design formula or measured from an actual production sample on a bench. Calculated charts describe the theoretical performance of a perfectly made lens and tend to look more flattering; measured charts reflect a real, specific sample and can reveal manufacturing losses the calculated version cannot show. Manufacturers are not always consistent about disclosing which method they used, or which wavelength of light the calculation assumes, so charts are most trustworthy when compared within one brand’s own consistent methodology rather than across brands.

  • Same aperture setting on both charts
  • Same focus distance assumption, usually infinity unless stated
  • Same measurement method: calculated versus bench-measured
  • Same brand or publisher, since methodology is not standardized industry-wide

Recognize Missing Evidence

An MTF chart describes contrast and resolution at the plane of focus under one set of conditions. It does not show coma, the comet-like smearing of off-axis point lights, distortion, vignetting, flare and ghosting behavior against bright light sources, color fringing, autofocus speed or accuracy, or how the lens handles in the hand. It also cannot capture sample variation between individual copies of the same lens model, since a chart typically represents either a theoretical design or a single tested unit.

Before treating a lens as definitively sharp or soft based on its chart alone, look for independently published resolution tests on real sample images, distortion grids, and backlit or night shots that reveal flare and coma behavior. The MTF chart is a useful starting filter for comparing closely matched options, not a complete verdict on a lens.