What Is Sharpness？
What Is Sharpness？
Sharpness is arguably the most important single image quality factor: it determines the amount of detail an image can convey. The image on the upper right illustrates the effects of reduced sharpness (from running Image Processing with one of the Gaussian filters set to 0.7 sigma).
Device or system sharpness is measured as a Spatial Frequency Response (SFR), also called Modulation Transfer Function (MTF). MTF is the contrast at a given spatial frequency (measured in cycles or line pairs per distance) relative to low frequencies. The 50% MTF frequency correlates well with perceived sharpness— much better than the old vanishing resolution measurement, which indicated where the detail wasn't.
Original | Oversharpened
Several alternative patterns, which cause cameras to apply differing amounts of sharpening and noise reduction, can be used for measuring MTF. All require more real estate than the slanted-edge. They include
Log Frequency, which uses a sine pattern chart that increases in frequency logarithmically. It provides a check on the slanted-edge method. More direct but less accurate,
Log F-Contrast, excellent for examining loss of detail due to noise reduction,
Star Chart, a multi-directional sinusoidal pattern,
Random/Dead Leaves, which measures texture sharpness. The Scale-invariant random pattern minimizes sharpening and maximizes noise reduction. The Dead Leaves pattern is more representative of typical images.
The MTF Measurement Matrix compares the different methods.
