Canon® EOS Digital Photography Photo Workshop

4.6. WHAT IS AN MTF 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; ...