Imatest Documentation

Imatest Documentation
Spatial Frequency Response (MTF)
Lower left: Frequency domain (MTF) plot
Black line
(bold)
Red line
(dashed,bold)
Right column
text
(results)
Input data
Spatial Frequency Response (MTF) for the
luminance (Y) channel. Unprocessed.
Spatial Frequency Response (MTF) for the
luminance (Y) channel with Standardized sharpening.
[Std Sharpening checked]
MTF50 (50% contrast spatial frequency) for the original uncorrected image in cycles/pixel and line widths per
picture height (LW/PH). Its relationship to print quality is discussed in Interpretation of MTF50.
MTF50 (with standardized sharpening; corrected) in cycles/pixel and LW/PH. Radius R used for setting
standardized sharpening is displayed. Defaults to 2 (larger for unusually broad transitions (poor MTF response)). It may
be adjusted in the input dialog box. [Std Sharpening checked]
Equivalent "ideal megapixels" in brackets [n mpxls ideal]: a number based on the concept that the "ideal" pixel
would have an MTF of 1 up to the Nyquist frequency (0.5 C/P), and 0 above, and hence, MTF50 = 0.5 C/P. The
number of "ideal pixels" that would yield the identical sharpness (MTF50) is,
4 * MTF50(C/P horizontal) * horizontal pixels * MTF50(C/P vertical) * vertical pixels
The number displayed assumes that MTF50 is the same in horizontal and vertical directions— usually, but not
always, a good assumption— there are exceptions, such as the Nikon D70. You should not expect the "ideal
megapixel" count to equal the total megapixel count of the camera. In the real world, performance is
excellent if it is half the actual megapixels. In cameras with weak anti-aliasing filters or a high degree of
sharpening, MTF may be greater than 0.5 at the Nyquist frequency. In this case, [> n mpxls ideal] is displayed,
where n is the total number of pixels in the camera, is displayed. Larger numbers are meaningless. Aliasing issues
such as moire patterns may be present, but the simple slanted edge pattern cannot be used to evaluate the
seriousness of aliasing.
MTF50P (spatial frequency where contrast is 50% of its peak value) for the original uncorrected image
in cycles/pixel and line widths per picture height (LW/PH). This is the same as MTF50 for slightly to moderately sharpened
edges, but smaller for oversharpened edges. It may be a better indicator than MTF50 of the perceived sharpness of
oversharpened images. [Std Sharpening off]
Oversharpening or undersharpening. The amount the camera is over- or undersharpened with respect to standardized
sharpening. 100% ( MTF( feql ) - 1) where feql is the frequency where MTF is set to 1 by standardized sharpening. [Std
Sharpening checked]
MTF30 (30% contrast spatial frequency for the original uncorrected image) is the default value of the Secondary
Readout, which can be set in the SFR input dialog window. You can select MTFxx, the spatial frequency for xx%
contrast, or the MTF at a spatial frequency specified in cycles/pixel, pixels/mm, or pixels/inch.
MTF at the Nyquist frequency. May indicate of the effectiveness of the anti-aliasing filter and likelihood of aliasing
effects such as Moire patterns. But it not an unambiguous indicator because aliasing is related to sensor response, and
MTF at Nyquist is the product of sensor response, the de-mosaicing algorithm, and sharpening, which can boost
response at Nyquist for radii less than 1. Aliasing effects may become serious over 0.3. There is a tradeoff: the more
effective the sensor anti-aliasing, the worse the sharpness. Foveon sensors (used in the Sigma SD9 and SD10) are more
tolerant of aliasing than Bayer sensors, which are used in most digital cameras.
Nyquist frequency: Half the sampling rate = 1/(2*pixel spacing). Always 0.5 cycles/pixel. Indicated by the vertical blue
line.
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