Testing Lenses to Determine 50% MTF with an EOS-1Ds
50% MTF (modulation transfer function):
Perceived image sharpness is more closely related to the spatial frequency (lp/mm) where MTF is 50% (i.e., where contrast has dropped by half) than to resolution alone. I used the EOS-1Ds and the Koren 2003 lens test chart developed and explained by Norman Koren to calculate 50% MTF. Printed test charts were placed on the Edmund Scientific Test Chart as in the middle and edge of the chart as shown here. The chart was photographed with an EOS-1Ds under tungsten bulb lighting so that the MTF test charts were photographed at 25 magnification. Images were shot at ISO 100, Raw mode, aperture priority, compensation +1.3, and converted with Phase One DSLR v. 3.6.
The distance from the chart was calculated at d1=(M+1)f where d1 = lens to target distance (mm) and f = lens focal length (mm) and M=25. This distance allowed the entire Edmund Scientific chart to be photographed and for resolution and 50% MTF to be measured and chromatic aberration to be detected from the same images. The imaged sine patterns were analyzed with ImageJ software and measurements were made on the resulting Plot Profile to determine line pair per mm frequency of 50% contrast as explained in detail on the Norman Koren website. Center weighted 50% MTF was calculated (70% center, 30% edge).