No longer can we say that a particular lens "resolves 60 lines per millimeter" or the like. This kind of measurement is affected by contrast; high-contrast film brings out blurred details. Instead, nowadays opticians measure how much blurring occurs at various distances from the center of the picture. That is, they measure the modulation transfer function (MTF).
Figure 7.2 shows how to read MTF graphs. Each graph has one or more pairs of curves for details of different sizes (such as 10, 20, and 40 lines per millimeter). Each pair consists of a solid line and a dashed line. Usually, the solid line indicates sagittal resolution (Figure 7.3) and the dashed line indicates meridional resolution.
What should the MTF of a good lens look like? My rule of thumb is that everything above 50% is sharp. I'm more concerned that the sagittal and meridional
Distance from center of field (mm)
Figure 7.2. MTF curve from a good lens (left) and from one that warrants concern (right). Star fields are very demanding targets.
curves should stay close together so that the star images are round, and that there shouldn't be a dramatic drop-off toward the edge of the picture. On that point, DSLR sensors have an advantage because they aren't as big as 35-mm film.
MTF curves do not measure vignetting or distortion. Also, most camera manufacturers publish calculated MTF curves (based on computer simulation of the lens design); the real MTF may not be as good because of manufacturing tolerances. An exception is Zeiss, which publishes measured MTF curves for all its lenses. You can also find measured MTF curves of many popular lenses on www.photodo.com.
One last note. Lens MTF curves plot contrast against distance from center, with separate curves for different spatial frequencies (lines per mm). Film MTF curves plot contrast versus spatial frequency. See Astrophotography for the Amateur (1999), p. 187. The two kinds of curves look alike but are not comparable.
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