An MTF chart is a graph that shows how well a lens preserves contrast and fine detail from the centre of the image to the corners. Higher lines mean better contrast, and lines that stay close together mean more even performance. Charts are useful for comparing lenses, but they have limits that are worth knowing.
What the Axes Show
The horizontal axis is the distance from the image centre, usually in millimetres, running from the middle on the left to the corner on the right. The vertical axis shows contrast transfer, from zero to one, where one means perfect contrast. A line near the top means the lens reproduces the pattern with strong contrast at that position.
Understanding the Line Types
- Thick lines usually show lower detail frequency, often 10 line pairs per millimetre, which relates to overall contrast.
- Thin lines usually show higher detail frequency, often 30 line pairs per millimetre, which relates to fine detail.
- Solid lines commonly represent sagittal patterns, which run along the radius.
- Dashed lines commonly represent meridional patterns, which run across it.
Makers can use their own conventions, so always read the notes beside the chart. Also check whether the chart is for the lens wide open or stopped down.
Reading a Chart Step by Step
- Check the conditions: aperture, focal length and the maker’s note.
- Look at thick lines first. High and flat lines suggest good overall contrast across the frame.
- Look at thin lines. Higher values mean finer detail is recorded.
- Check how far the lines drop toward the right. A big drop means softer corners.
- See whether the solid and dashed lines separate. A wide gap hints at astigmatism, which affects how out-of-focus areas and edges look.
Quick Interpretation Guide
| What you see | What it suggests |
|---|---|
| High, flat lines | Consistent contrast across the image |
| Lines falling at the edge | Softer corners |
| Wide gap between line types | Uneven rendering of edges |
| Low thin lines, high thick lines | Good contrast, less fine detail |
Why Aperture Changes the Chart
A lens usually looks different wide open than stopped down. At the widest aperture, aberrations are stronger, so the lines may sit lower, especially in the corners. At moderate apertures they commonly rise and flatten. When comparing two lenses, make sure you look at the same aperture and focal length, or the comparison tells you little.
FAQ
Can I compare charts from different makers?
Be careful. Makers may calculate charts in different ways, and some use design data instead of measured data. Comparing within one maker is more reliable.
Do MTF charts tell me everything about a lens?
No. They do not show flare, colour rendering, focus speed, distortion or sample variation. Real photos still matter.
The Short Version
MTF charts plot contrast and detail from centre to corner. Read the conditions, compare thick and thin lines, and watch for drops at the edge. Use them as a guide, together with real sample images.
