Analysis of Log Frequency-Contrast charts
| New in Imatest 4.0 Automatic region is available with the revised version of the chart, which includes registration marks in the corners. Sharpness and Texture Analysis using Log F‑Contrast from Imaging-Resource compares the the effects of sharpening and noise reduction in several cameras using images downloaded from Imaging-Resource.com. |
Revised Log F-Contrast chart: works with automatic region detection
Introduction
Log F-Contrast (short for Log Frequency-Contrast; not in Imatest Studio) measures the effects of signal processing— noise reduction and sharpening— on imaging system performance using a chart that varies in spatial frequency on the horizontal axis (log frequency increases with x) and in modulation ((max-min)/(max+min) signal, sometimes called Michelson contrast) on the vertical axis. Modulation is proportional to (y/h)2 for image height h. Because the Log F-Contrast chart is sinusoidal (rather than a bar pattern) its modulation is equivalent to SFR or MTF.
Log F-Contrast can be run through the Rescharts interface or as a fixed module (capable of analyzing batches of images).
| The Log F-Contrast chart is very similar to the Campbell-Robson CSF (Contrast Sensitivity Function) chart, which was first published in 1968. The (vertical) contrast variation is quite different for the two charts. |
This module is particularly valuable for measuring the loss of fine, low-contrast detail (texture) due to software noise reduction, which can result in a “plastic” appearance in areas where fine texture is missing. This type of nonuniform image processing, which is common in consumer cameras, especially those with tiny pixels (< 2 microns) such as camera phones, is often implemented as a bilateral filter, which can be simulated and analyzed in the Imatest Image Processing module. The chart can be purchased at the Imatest store or created by Test Charts and printed on a high quality inkjet printer. An image of a Log Frequency-Contrast chart, acquired with a Canon EOS-20D camera, is shown below, slightly reduced. The chart design was modified in July 2014 to work with automatic region detection in Imatest 4.0+.
Log F-Contrast Image, Canon EOS-20D camera, 24-70mm f/2.8L lens set at 42mm, f/5.6, ISO 100
(pre-Imatest 4.0 version). Click on image to load full-size image.
| Modulation and Contrast The modulation ((maximum-minimum)/maximum+minimum) level of the sine pattern, represented linearly — also called Michelson contrast) varies from 1 at the top of the chart to 0 at the bottom. It does not vary linearly because the eye is highly nonlinear: if modulation were to decrease linearly the chart would have high visual contrast over most of its area. The dropoff in visual contrast is much more uniform when modulation is proportional to (y/h)2 for image height h. The contrast ratio is the maximum/minimum modulation. It would be infinity at the top of the chart (where modulation = 1), but it’s limited in practice by the printing process: it’s about 100:1 for semigloss (luster, etc.) charts and about 40:1 for matte charts. Contrast ratio drops off rapidly: in the middle of the chart, where modulation is 0.52 = 0.25, the contrast ratio is (0.5 + 0.25/2)/(0.5 – 0.25/2) = 0.625/0.375 = 1.667. It illustrates the eye’s extreme nonlinearity: the contrast ratio in the middle appears to be around half the ratio at the top of the chart, which is 25-60 times higher. |
Log F-Contrast divides the chart into twenty-five zones (numbered from top to bottom) for analyzing contrast (MTF), which can be displayed in several ways. Second-order interpolation and smoothing are used to increase the accuracy and readability of the MTF calculations. Spatial frequency is derived from the pattern.
