Test Procedure for Obtaining Resolution Results
Note: This testing procedure was based on the one of Modern Photography
magazine
and was just modified by Bozhidar Dimitrov and Frederick Wasti.
- Only test lenses in excellent optical and working
mechanical condition. Do not submit tests for lenses that
appear to have been disassembled, or that have optical
blemishes. Follow the testing procedure as accurately as
possible.
- Find a suitable testing location:
- A sturdy non-vibrating floor is a must. Wooden or
carpeted floors, floors in buildings that vibrate because of
outside traffic or use of heavy machinery are not
acceptable.
- The test location must be large enough to support the
separation of camera and targets by the following multiples
of the lens focal length:
- 55 for lenses with focal length 135 mm or less,
- 27 for longer lenses.
- Obtain 5 copies of the United States Air Force (USAF)
lens test target. The chart contains groups of six
line pairs. Each line pair contains three horizontal and
three vertical lines. Groups are laid out vertically, and are
numbered by a large number printed above them. You can purchase
USAF charts from Edmund Scientific (1-800-728-6999, ask for
"USAF Optical Test Pattern Resolving Power Chart", catalog
number U83,001). Alternatively, print this file on a 600 dpi laser printer. The
PDF file contains groups -4 and larger (-4 and -3 are not
numbered), and on my printout the quality of both the horizontal
and vertical lines of pair 4 of group 1 is impeccable. At 1:55
magnification this corresponds to 156 lpm, probably quite
sufficient for all lenses that we will ever own. At 1:55 line
pair 1 of group 1 corresponds to about 100 lpm, and that should
be the minimum printing tolerance. Print the file with Adobe
Acrobat, and make sure that in the print dialog "Shrink to Fit"
is not checked. The side of the large square in the top-center
part of the page should be 35.5 millimeters long.
- Load the camera with Kodak Royal Gold 100 print
film. We limit the tests to a particular film so that the
results from different people are more comparable.
- Mount the resolution targets. Imagine the rectangle
part of the wall that is in the field of view of the film.
Attach one test target at each corner of this rectangle and one
target in the center. The center target should be oriented in a
right-side-up fashion. For lenses with focal length 135 mm or
less, the four corner targets should be mounted such that their
centers are 56 cm (22") above and below and 81 cm (32") to the
right and left of the center of the center target. Position the
corner targets so that the bottom edge of each target faces
towards the center target. Use this
diagram as a guide. For lenses between 135 mm and 400 mm
use 28 cm (11") and 41 cm (16"). For lenses longer than 400 mm
use 15 cm (6") and 20 cm (8").
- Mount the camera body on a sturdy tripod.
- For lenses of focal length 135 mm or less, position the
tripod such that the front of the lens is at a distance 55 times
the focal length of the lens from the center target. For longer
lenses the distance should be 27 times the focal length.
Manipulate the tripod such that the camera's film plane is
parallel to the wall and the distance from the lens to each of
the four corner targets is equal. It is probably best to mount
the targets on a vertical wall and position the tripod on the
floor. Position the targets (and hence the camera) as low as
possible such that the tripod is extended to the smallest
possible extent. Every effort must be made to assure that
the camera body and lens will not suffer from even the slightest
vibration during the test exposures. If you have to, weigh
your tripod down by attaching a weight to the center post.
- Assure that the target area is uniformly
illuminated.
- Determine correct exposure. Care should be taken to
meter off a middle-toned subject that fills the metering area.
A gray card and spot metering work best. Overexposed or
underexposed targets will not give proper resolution
results.
- Focus carefully on the center target. Be
very careful with this, and use a magnifying finder or
refconverter if you have one. Possibly consider making a series
of full tests with refocusing between the series.
- Use the longest mechanical or electronic cable release
that you have. Or if your camera has an electronic
self-timer, you can use it instead.
- Prepare to keep careful records during the testing.
You must know the exact testing conditions for each and every
image obtained. To help out, a series of "yellow stickies" or
3"x5" cards may be used, affixed to the target area. At a
minimum, write the lens serial number and aperture on each of
these and replace them for each exposure. Also consider writing
the lens type, focal length, maximum aperture, etc.
- After framing and focusing very carefully, make a series
of exposures at the lens wide-open aperture and at each full
stop thereafter. For example, an f/1.7 lens should be tested
wide open at f/1.7, and then closed down to f/2, f/2.8, f/4,
etc., all the way to f/22). Adjust the shutter speed to
compensate for every change in aperture.
- For zooms:
- Repeat the tests at the shortest, middle, and longest
focal lengths. To allow for comparisons with prime lenses,
round the middle focal length to a "standard" one. The
Excel resolution chart contains three rows for each zoom,
and the middle focal lengths are indicated there. Note that
the M40-80/2.8-4 and 400-600/8-12 lenses have very limited
zoom ranges, so they will only be tested at the extremes.
- For testing variable aperture zooms at their middle
focal length, assume that the effective widest aperture is
half-way between the apertures at the ends. For example,
for an f/4-f/5.6 zoom, assume f/4.5 to be the middle (this
is not an arithmetic average, but a geometric one). To
conform to the test procedure, perform a test at this
"half-stop" aperture, and then at the full stops thereafter.
Take into account the fact that at because of the
variability of the aperture, the aperture ring is a half
stop behind the real aperture. So, set the aperture ring
between f/2.8 and f/4 to achieve f/4; between f/4 and f/5.6
to achieve f/4; between f/5.6 and f/8 to achieve f/8, etc.
If you have a camera body that can be set to aperture
priority with the lens aperture ring set on "A", then you do
not need to worry about this complication. Just use that
mode.
- Have the film developed by a lab using standard C-41
processing.
- Looking through a microscope or a loupe of at least 20x
magnification, find the smallest (largest numbered) line
pairs in the center target that are clearly resolved. Repeat the
same for the corner targets and choose the smallest line
pair that is clearly resolved in all four targets. Note that
the resolving power of the observing lens must not be allowed to
limit your ability to easily read small line pairs on film.
Critical focusing here is just as important as with the
focusing of the test lens.
- Convert the corner and center line pair numbers from
each test frame into lpm numbers:
Lpm values for magnification ratio of 1:27
Group | Pair 1 | Pair 2 | Pair 3 | Pair 4 | Pair 5 | Pair 6
|
---|
-2 | 7 | 8 | 9 | 10 | 11 | 12
|
---|
-1 | 14 | 15 | 17 | 19 | 22 | 24
|
---|
0 | 28 | 31 | 35 | 39 | 44 | 49
|
---|
+1 | 55 | 62 | 69 | 78 | 87 | 98
|
---|
Lpm values for magnification ratio of 1:55
Group | Pair 1 | Pair 2 | Pair 3 | Pair 4 | Pair 5 | Pair 6
|
---|
-2 | 14 | 15 | 17 | 19 | 22 | 24
|
---|
-1 | 28 | 31 | 35 | 39 | 44 | 49
|
---|
0 | 55 | 62 | 69 | 78 | 87 | 98
|
---|
+1 | 110 | 123 | 139 | 156 | 174 | 196
|
---|
- If you have followed all of the above steps as carefully as
you can, mail the results to me and feel very proud of
yourself!
Note: The numbers in step 15 are obtained by first
calculating the lpm number at the target plane, and then dividing
by the magnification ratio (i.e., multiplying by 27 for 1:27
magnification). The target plane lpm values are given by the
following table:
Lpm values (at the target plane)
Group | Pair 1 | Pair 2 | Pair 3 | Pair 4 | Pair 5 | Pair 6
|
---|
-2 | 0.250 | 0.281 | 0.315 | 0.354 | 0.397 | 0.445
|
---|
-1 | 0.500 | 0.561 | 0.629 | 0.707 | 0.794 | 0.891
|
---|
0 | 1.00 | 1.12 | 1.26 | 1.41 | 1.59 | 1.78
|
---|
+1 | 2.00 | 2.24 | 2.52 | 2.83 | 3.17 | 3.56
|
---|