What is an Modulation Transfer Function chart?

A Modulation Transfer Function (MTF) chart represents basically how a lens is evaluated. You can find these charts, for example, on the Canon Web site where lens descriptions are found; simply go to the product page for one of the specific lenses, and in the overview section they generally present an MTF chart for it. These charts are created from data based on how well the lens transmits evenly spaced lines of black and white as measured precisely in line pairs per millimeter: 10, 20, or 30 line pairs per millimeter (lp/mm). The closer the lines are together, the more blurry and noisy the image will be because the black and white will average together into a grayish tone instead of being clearly defined.

Essentially, differences between black and white lines equate to contrast; this, in turns, helps define an image’s sharpness. For example, if you have ever applied an Unsharp mask in an image-editing application such as Photoshop, you are increasing the contrast between dark and light lines in the image — giving the appearance of a sharper, more focused image.

While optical glass manufacturing is very precise and exacting, no lens is perfect. Aberrations and areas of blurriness or defocus occur even in the most expensive L-series lenses. MTF charts define the amount of imperfection in a given lens by evaluating two different types of line categories. One type measures radial lines that point towards the center of a lens like spokes on a bicycle wheel; the other type are tangential lines, which measure different specific parts of the lens at a right angle to the radial lines with varying distances between them and line thicknesses (kind of like the hub or rim of a wheel).

Reading MTF charts, including the charts found on the Canon site, can be challenging and confusing for the average person (different manufacturers have different types of MTF charts). However, they’re interesting if you’re seriously researching how different but similar lenses perform — especially if you’re considering spending a considerable sum on one over the other. There are a few basics that you need to keep in mind when viewing Canon MTF charts — the lines and the numbers.

Lines. Upper lines are blue; lower lines are black.

  • The upper set of lines represents how a lens performed when measured with an aperture of f/8.0 (typically).
  • The lower lines represent how a lens performed when measured with the aperture wide open (to the maximum that the lens is capable, such as f/2.8 or f/1.4).
  • Dotted lines represent tangential measurements.
  • Solid lines represent radial measurements.

Numbers. MTF charts for zoom lenses include two graphs: one for the lens at maximum range, and one for minimum range.

  • The numbers on the horizontal axis of the chart, written at the bottom, are the distance from the center of the image frame as measured in millimeters.
  • The numbers on the vertical axis of the chart, along the left-hand side, are the MTF values, which equate to percentage of contrast: 1.0 is maximum (100-percent) contrast (good) and 0 is no contrast at all (not good).

Just as it’s easier for a doctor to read an x-ray than it is for the patient, it can be difficult to relate a specific MTF chart to a photograph unless you’re an optical engineer. But, like an x-ray, you can still interpret some things from MTF charts if you study a little. Generally speaking, you want to keep contrast even and not to have significant rises or falls as the distances increase. The lines will naturally fall off as the distance increases, but the smoothness and evenness of the fall-off is what will tip you off to a lens with less quality.

Category: Science of Lenses

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