Imatest Documentation

Imatest Documentation
Left of
center
Corners
Inside (L,R)
of corners N/A
Mirror
Notes on the charts:
Log F-Contrast chart, printed 30 cm (11.81 inches) high on 13x19 inch
paper
SVG Squares 2x3 (Same orientation). Two of the charts must be
printed as mirror images to obtain approximately radial and tangential
edges: in Inkscape, select the chart, then click on Object , Flip
Horizontal before printing.
Log F-Contrast chart, printed 20 cm
(7.87 inches) high on US letter-size paper
1x2 SVG SFR chart, 20:1 contrast. See
note on left about printing mirror images.
1x2 SVG SFR chart, 20:1 and 2:1
contrast, trimmed to remove step chart.
See note on left about printing mirror
images.
Mount a mirror, centered horizontally on the target, above or below the center charts. Automotive mirror replacement
glass (easy to find in automotive supply shops) or hobby/cosmetic mirrors should do. Mark the center of the chart
(with a sticker, bright pen, etc.). Use the "rope trick" described below to center the camera horizontally.
MTF measurements for the two charts were compared using a 39-megapixel medium
format camera with an excellent lens. MTF was approximately 10% higher when
measured on the 40x60 inch target. That is the basis of the above recommendation that
the larger chart be used for cameras with over 13 megapixels.
The mirrors are difficult to see in the above images because they are reflecting a
darkened room on a dark background. The mirror above the center of the 40x60 inch
target is illustrated on the right.
The 20:1 slanted edges are used as the standard edges for SFR sharpness measurements.
They are arranged so sharpness can be measured near the image center, half-way
between the center and the four corners, and near the corners. This is important
because lens response may be asymmetrical in lenses that are decentered due to sloppy
manufacturing. This contrast is lower than the minimum specified by the ISO standard
(40:1), which is now understood to result in large errors if gamma is poorly estimated or
if the tonal response is nonlinear (clips or deviates from a straight gamma curve). Much
lower contrast will be recommended in the revised ISO standard.
The 2:1 slanted edges measure MTF in regions where contrasty edges are absent. In Mirror, showing red dot • stickers
such regions there may be less software sharpening; software noise reduction (lowpass marking the horizontal target center
filtering; the opposite of sharpening) may be employed. Signal processing that differs in
different parts of the image is called nonlinear.
Aligning the camera with the target
To obtain the most accurate and consistent results from Imatest's spatial measurements (sharpness, chromatic aberration, and
distortion), the camera and target must be aligned presicely. This means that
The axis of the lens should be normal (perpendicular) to the surface of the target,
The edges of the image should be parallel with the edges of the charts in the target,
In most cases, the center of the image corresponds to the center of the target.
Accurate alignment is easy to obtain with an optical bench, but optical benches large enough for Imatest testing are extremely
expensive. For this reason users need to develop a collection of tricks to facilitate alignment for different cameras, lenses, or focal
lengths.
Leveling (horizontal, vertical (front/back))
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