Testing Lenses to Determine Resolution and 50% MTF with an EOS-1Ds
Resolution tests were run at variable working distances from the target as calculated from the Koren lens chart method. An Edmund Scientific lens resolution chart with several overlain Koren 2003 lens test charts at different angles was illuminated with a tungsten modeling bulb from a monolight flash. The lenses were mounted on a Bogen 3033/Arca Swiss B1 tripod/head combination with a large Kirk bean bag weighing down the tripod/head to dampen vibration. Three exposures using aperture priority exposure and +1 exposure compensation were taken at each aperture with an EOS-1Ds via cable shutter release and in mirror lockup mode at ISO 200 in RAW mode. The central autofocus point was centered over the center pattern for each exposure. The lens was manually defocused, and then refocused using autofocus for each exposure. RAW files were converted to 300 dpi tifs with Phase One dslr software and images were analyzed in Photoshop.
Measurement were made at the center pattern, a middle pattern, and the edge pattern as shown here. Both line patterns at 90° angles had to be clearly visible. The highest resolution score for each aperture was recorded to minimize the the effect of potential autofocus error. Center-weighted resolution was calculated (60% center; 30% middle; 10% edge). Resolutions (lpm) at each f/stop were calculated using the method on the chart as follows.
Image lines pairs per mm (image lpm or lp/mm) = lpm resolved on chart X (D-fo) / fo) where fo = focal length of lens and D = Distance from the chart to the middle of the lens.
50% MTF (modulation transfer function):
There is general agreement that 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 at a distance calculated from the chart at each focal length so that the MTF test charts were photographed at 25 magnification.
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).