Imatest Documentation

Imatest Documentation
Image Quality
Sharpness
What is it and how is it measured?
Image sharpness
Sharpness is arguably the most important photographic image quality factor: it's the factor most closely related to the amount of
detail an image can render. But it's not the only important factor. Imatest measures a great many others.
Sharpness is defined by the boundaries between zones of different tones or colors. It is illustrated by the bar pattern of increasing
spatial frequency, below. The top portion represents a target used to test a camera/lens combination. It is sharp; its boundaries are
abrupt, not gradual. The bottom portion illustrates the effect of a high quality 35mm lens on a 0.5 millimeter long image of the
pattern (on the film or digital sensor plane). It is blurred. All lenses, even the finest, blur images to some degree. Poor lenses blur
images more than fine ones.
One way to measure sharpness is to use the rise distance of the edge, for example, the distance (in pixels, millimeters, or fraction
of image height) for the pixel level to go from 10% to 90% of its final value. This is called the 10-90% rise distance. Although rise
distance is a good indicator of image sharpness, it has one limitation. It is poorly suited for calculating the sharpness of a complete
imaging system from the sharpness of its components, for example, from a lens, digital sensor, and software sharpening algorithm.
To get around this problem, measurements are made in frequency domain, where frequency is measured in cycles or line
pairs per distance (typically millimeters in film measurements, but may also be inches, pixels, or image height). Line pairs per
millimeter (lp/mm) is the most common spatial frequency unit for film, but cycles/pixel is convenient for digital sensors. The
image below is a sine wave— a pattern of pure tones— that varies from low to high spatial frequencies, in this case from 2 to 200
lp/mm, over a distance of 0.5 millimeters. The top portion is the original sine pattern. The bottom portion illustrates the effects of
the same high quality 35mm lens, which reduces pattern contrast at high spatial frequencies.
The relative contrast at a given spatial frequency (output contrast/input contrast) is called the Modulation Transfer Function
(MTF) or Spatial Frequency Response (SFR).
Illustration of Modulation Transfer Function (MTF)
(Spatial frequency response (SFR) )
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