Methods for Measuring Resolution
and 50% MTF
Resolution
Resolution tests were run at 28.4 inches from the target. An
Edmund Scientific lens
resolution chart was illuminated with a monolight flash adjusted
with a flash meter at each aperture exposure. The camera and lenses were
mounted on a Bogen 3033/Arca Swiss B1 tripod/head combination. Three
exposures at 1/250th second were taken at each aperture with an EOS-1v via
cable shutter release on Kodak Technical Pan film rated at ISO 25 and
developed in Technidol. The central autofocus point was centered over the
center pattern for each exposure. No focus bracketing was used. In
previous tests, it was determined that camera autofocus function yielded
consistently higher line pairs per mm scores than trying to use manual
focus and bracketing with an eyepiece magnifier. The lens was manually
defocused, and then refocused using autofocus for each exposure. Line
patterns were read under a compound brightfield microscope at a
magnification of 40X.
Measurement were made at the center pattern, a middle pattern, and the
far upper right pattern on the four exposures. The highest resolution
score for each aperture was recorded for data in the center-weighted
resolution graph. Center-weighted resolution was calculated (60% center;
30% middle; 10% edge). Resolutions 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
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 under tungsten bulb lighting. A
Canon EOS-1Ds Mark II was set at ISO 100, tungsten bulb white balance and
shot with + 1.3 EV using evaluative metering with the camera set for
mirror lockup. Raw files were converted to 300 dpi tifs with Capture One
Pro v. 3.6. Measures of 50% MTF were calculated using the central chart
and edge chart. Calculated values were center weighted (60% center; 30%
edge.)
The imaged sine patterns were analyzed withand
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.
Details of calculating 50% MTF:
1. The 5mm Koren 2003 lens test chart designed to be printed at 25 cm
long (50X magnification) was downloaded from the Koren website and printed
on semi-gloss paper with a Epson 1270 printer at 1440 dpi. Charts are
trimmed and mounted on the Edmund Scientific Test Chart as shown:
2.
The chart is photographed at a working distance that is 1/2 the
recommended distance so that the entire Edmund Scientific chart can be
photographed for resolution and determination of 50% MTF. It is also
possible to detect lateral chromatic and other aberrations from the same
test images. 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. The reduction of distance by 1/2 requires that
lp/mm figures read off the chart be adjusted by 1/2.
3.
Photographic RAW files are converted to tifs with Capture One Pro. The tif
files are opened in image analysis software to analyze the sine patterns
on the chart (top band). I used
ImageJ
software, public domain software off the NIH site.
Click on "File" and then "Open" to select and open the
tif of interest.
4. Click on the "magnifying cursor symbol"
to fill the window with the Koren chart image and click on the "hand"
icon to move the chart image into the middle of the window.
5.
Click on the line icon and draw a straight line through the upper sine
pattern bar on the Koren chart.
6.
Click on the "Analyze" menu and select "set scale" and
enter "known distance" as "25" and "units"
as "cm".
7. Click on "Analyze" again and
select "Plot Profile."
8. A sine wave pattern will be
generated and displayed.
9.
The full amplitude of the sine wave on my computer screen has a 7 cm
sweep. I just take a rule and run it down the plot towards 25cm until the
amplitude is 50% (3.5 cm). In the example, 50% amplitude is at 17 cm on
the chart. This corresponds on a plot of cm of chart versus a log plot of
spatial frequency below to 147 lp/mm.
10.
Because the working distance was decreased by 1/2 when photographing the
chart, the lp/mm value is divided by 2 to generate the 50% MTF value.
