An MTF (Modulation Transfer Function) chart maps how well a lens reproduces contrast and detail from the center of the image to the edges. Once you know what each element represents, you can use these charts to compare lenses and predict how they’ll actually perform in your photographs.
What the Axes Tell You
The Y-axis shows contrast transfer, displayed as a value from 0 to 1.0 (or 0% to 100%). A value of 1.0 means the lens perfectly reproduces the contrast of the original scene. A value of 0 means contrast is completely lost. In practice, no lens hits 1.0 across the board, and values above 0.8 are considered very good.
The X-axis shows the distance from the center of the image sensor, measured in millimeters. Zero on the left represents the dead center of the frame. The far right represents the corner. For a full-frame sensor, that outer edge sits around 21.6mm from center. So the chart reads left to right as center to corner, letting you see exactly where a lens starts losing performance.
Line Pairs: Contrast vs. Resolution
MTF charts typically plot two spatial frequencies: 10 line pairs per millimeter (lp/mm) and 30 lp/mm. These are drawn as thick and thin lines, respectively. A “line pair” is one black line and one white line side by side, so 10 lp/mm actually means 20 individual lines packed into each millimeter, and 30 lp/mm means 60.
The 10 lp/mm lines (thick) measure overall contrast. These are relatively large details that any decent lens can resolve, so the question isn’t whether the lens can see them but how much punch it preserves. A lens with high 10 lp/mm values across the frame will produce images that look crisp and three-dimensional. Things that drag down this measurement tend to be general contrast killers: internal reflections, flare, and impure glass surfaces.
The 30 lp/mm lines (thin) measure resolving power, the lens’s ability to separate very fine details. These are tiny structures (each element just 0.033mm wide), and this is where lenses start to struggle. A high 30 lp/mm score means the lens renders fine textures, like individual strands of hair or fabric weave, with real clarity. Keep in mind that modern sensors can resolve far beyond 30 lp/mm. An 18-megapixel APS-C sensor, for instance, can resolve up to about 80 lp/mm in full color. The 30 lp/mm benchmark is a useful standard, but it doesn’t represent the absolute ceiling of what today’s gear can do.
Sagittal and Meridional Lines
Each spatial frequency is plotted twice: once for sagittal patterns and once for meridional (also called tangential) patterns. Sagittal lines are usually drawn solid, meridional lines dashed. This distinction exists because lenses don’t always reproduce detail equally in all orientations.
Sagittal patterns are stripe patterns that radiate outward from the center of the image, like spokes on a wheel. Meridional patterns run perpendicular to those, like the rings of a bullseye. The sagittal direction typically shows better contrast transfer than the meridional direction.
The gap between these two lines matters. When the solid and dashed lines at the same frequency sit close together, the lens handles detail uniformly regardless of orientation. When there’s a large gap, the lens has astigmatism: it focuses one orientation of detail at a slightly different plane than the other. This creates a directional softness that can be visible in fine textures, especially toward the edges of the frame.
That gap also predicts something about bokeh. The closer the sagittal and meridional lines are to each other, the smoother and more uniform the out-of-focus rendering tends to be. A large separation between them often produces busier, more “nervous” bokeh with visible outlining on blur circles.
Reading the Slope From Center to Edge
A perfectly flat horizontal line would mean the lens performs identically at the center and the corners. That almost never happens. What you’re looking for is how steeply the lines fall as you move right across the chart.
A gentle, gradual slope means the lens holds its performance well across the frame. A line that stays above 0.8 all the way to the edge indicates excellent consistency. A steep drop-off, especially one that plunges below 0.6 in the outer third, tells you the corners will look noticeably softer than the center. This is especially important for landscape and architecture photographers who need edge-to-edge sharpness.
When a line dips and then rises again toward the edge, that often indicates field curvature: the lens is focusing the edges on a slightly different plane than the center. Stopping down can reduce this effect, but it’s a characteristic of the optical design itself.
Wide Open vs. Stopped Down
Some MTF charts, particularly Canon’s, overlay two sets of data: one at the lens’s maximum aperture (drawn in black) and one at f/8 (drawn in blue). This comparison is extremely useful because f/8 is generally the sharpest aperture for most lenses, so the blue lines show you roughly the best the lens can do, while the black lines show you what you’re getting when you shoot wide open.
Consider a 50mm f/1.4 as an example. At f/1.4, the thick black line (10 lp/mm contrast) might start between 0.7 and 0.8 at center and fall to just above 0.2 at the edge. That’s a dramatic contrast loss in the corners. The thin black line (30 lp/mm resolution) might start at only 0.5 in the center and drop to between 0.1 and 0.2 at the edge, meaning fine detail is genuinely soft even in the middle of the frame.
Stop that same lens down to f/8 and the picture changes completely. The thick blue line might hold near 0.9 across almost the entire frame. The thin blue line might hover around 0.9 until about 17mm from center and only dip below 0.8 at the very outer 3mm. That tells you the lens is exceptionally sharp throughout when stopped down, even though it’s mediocre wide open.
When the wide-open and stopped-down lines sit close together, the lens performs nearly as well at its maximum aperture as it does at f/8. That’s the hallmark of a high-end optic and one reason fast primes from premium manufacturers command higher prices.
Comparing Lenses Across Brands
Here’s an important caveat: you cannot directly compare MTF charts between different manufacturers. Canon’s MTF charts are based on theoretical calculations from the lens design phase, while other manufacturers like Zeiss use actual optical measurements from production lenses. Both approaches have merit, but they produce different numbers for the same real-world performance.
Comparing MTF charts between lenses within the same brand’s lineup is reliable and genuinely useful for purchase decisions. Comparing a Canon chart against a Nikon or Zeiss chart is not, because the testing standards differ. Third-party reviewers who test all lenses on the same equipment provide the only true apples-to-apples comparison across brands.
Putting It All Together
When you look at an MTF chart, scan for four things in order. First, check the thick lines (10 lp/mm) for overall contrast. Values above 0.8 across the frame mean the lens produces punchy, high-contrast images. Second, check the thin lines (30 lp/mm) for fine detail. Values above 0.6 are decent; above 0.8 is excellent. Third, look at how much the lines drop from left to right. Flat is ideal, a gentle slope is normal, and a steep cliff means soft corners. Fourth, compare the solid and dashed versions of each line. A tight pairing means uniform rendering and smoother bokeh; a wide gap means astigmatism and potentially distracting background blur.
No single number on an MTF chart tells the whole story. A lens with slightly lower center sharpness but excellent edge-to-edge consistency might be a better choice for landscapes than one that peaks sharply in the center but falls apart at the margins. The chart gives you the data. What matters is matching it to how you actually shoot.