| My Account | Cart Contents | Checkout |
SFR: Measure sharpness
Imatest™ SFR measures image sharpness and several additional factors that contribute to image quality, including chromatic aberration, noise, and Shannon capacity. It derives these results from simple slanted-edge images, illustrated below. Starting with Imatest Master 2.7 (February 2008) Imatest SFR can analyze Bayer raw files: standard files (TIFF, etc.) that contain undemosaiced data.
The primary sharpness indicator is MTF50, the spatial frequency where contrast drops to half its low frequency value. Spatial Frequency Response (SFR), also known as Modulation Transfer Function (MTF), is introduced in What is image sharpness and how is it measured? The section on Interpretation of MTF50 links MTF50 to print quality.
Concise instructions for using Imatest SFR and interpreting its results can be found in How to test lenses with Imatest.
|
To measure sharpness,
|
Sharpness
The Figure below illustrates the primary results of running Imatest
SFR: plots
of
sharpness in two domains— spatial (the
average
edge profile) and frequency (spatial frequency response (SFR/MTF) curves). The two plots present
similar
information from two different viewpoints. Input data is shown
on
the right of the Figure: a thumbnail of the complete test image, the selected Region
of Interest (ROI), and selected EXIF data (information
recorded by the camera). The industry-standard ISO 12233 test chart is
shown, but you can print your own charts on a high
quality inkjet printer.
The upper left plot shows the cross section of an average edge. The narrower this edge, the better the performance. The black curve is the original edge for the selected channel, usually Y (luminance). The thin, light, dashed curves are the edges for the Red, Green, Blue and Luminance channels. The dashed red curve is the edge for the selected channel with standardized sharpening— the output of an algorithm that sharpens (or de-sharpens) the image so the edge overshoot is similar for different cameras. This allows cameras with different amounts of built-in sharpening to be compared fairly. Standardized sharpening tones down the original edge of this particular camera, which is strongly oversharpened.
The primary spatial domain performance indicator is the 10-90% rise distance, indicated in black for the original edge and red for the edge with standardized sharpening, labeled 10-90%(corr) for "corrected." Shorter is better.
The lower left plot shows the Spatial Frequency Response (SFR), also called modulation transfer function (MTF), i.e., the image contrast relative to contrast at low spatial frequencies. Black is for the original; dashed red is with standardized sharpening. The thin, light, dashed curves are the MTFs for the Red, Green, Blue and Luminance channels. The primary performance indicator is MTF50 (the spatial frequency where image contrast drops to 0.5 = 50%). The higher the better, though excessive response above the Nyquist frequency (0.5 Cycles/Pixel; the highest spatial frequency where information can be conveyed) may indicate aliasing problems (low frequency artifacts). "Ideal megapixels" (shown in brackets [...] ) is the number of megapixels with ideal response (MTF50 = 0.5 Cycles/Pixel) that would have equivalent sharpness. Practical digital cameras rarely reach this level; MTF50 around 0.33 Cycles/Pixel is very decent.
A secondary readout is available. It defaults to MTF30 (the spatial frequency for 30% contrast), but it can be set to any contrast level or to MTF at a specified spatial frequency in cycles/pixel, line pairs/inch or line pairs/mm. For example, you might select the half-Nyquist frequency, 0.25 cycles/pixel.
The uncorrected response (MTF50 or 10-90% rise distance) is appropriate for evaluating the performance of different lenses on a single digital camera. The response with standardized sharpening is appropriate for comparing different digital cameras.
These measurements begin to make sense when cameras are compared, as they are here. The page on Sharpness has more details on MTF. SFR Results: MTF (sharpness) plot describes this Figure in detail.
Subjective Quality Factor (SQF)
MTF is a measure of device or system sharpness, only indirectly related to the sharpness we perceive when we observe a print. To estimate perceived sharpness for a print of a given size, we need a formula that includes assumptions about viewing distance (usually proportional to the square root of the print height, but other options are available) and the human visual system (the human eye's Contrast Sensitivity Function (CSF)). Such an formula, called Subjective Quality Factor (SQF) was developed by Eastman Kodak scientists in 1972. It has been verified and used inside Kodak and Polaroid, but it has remained obscure until now because it was difficult to calculate. Its only significant public exposure has been in Popular Photography lens tests. SQF was added to Imatest in October 2006.
A portion of the Imatest SFR SQF figure is shown above, with SQF plotted as a function of print size. Viewing distance (pale blue dashes, with scale on the right) is assumed to be proportional to the square root of picture height. SQF is shown with and without standardized sharpening. SQF is extremely sensitive to sharpening, as you should expect since sharpening is applied to improve perceptual sharpness. Since this camera is rather oversharpened— enough to have significant "halos" near edges, standardized sharpening reduces the MTF peak, and hence SQF. An interpretation of SQF is give here. Generally, 90-100 is excellent, 80-90 is very good, 70-80 is good, and 60-70 is fair (these numbers may be shifted as more data becomes available). A peak over about 105 may indicate oversharpening (strong halos near edges), which can degrade image quality. SQF measurements are more valid when oversharpening is removed, which is accomplished with standardized sharpening.
Chromatic aberration
Imatest measures lateral chromatic aberration, also known as "color fringing," which is most visible on tangential boundaries near the edges of the image.
The curves for the R, G, and B color channels are shown below. The visibility of the chromatic aberration is proportional to the area between the highest amplitude transition (in this case, red) and the lowest amplitude transition (in this case, blue).
See Chromatic Aberration for more details. Chromatic Aberration, Noise, and Shannon Capacity plot describes this Figure in detail.
Multiple runs and regions
Batch runs Imatest SFR allows you do perform batch runs on several files of the same size and same framing by simply selecting several files (Pro only). This can be particularly convenient when testing a lens at several f-stops.
Repeated runs Imatest Master can automatically refine ROI (region of interest) selection during repeated runs and batch runs. This can be especially valuable when different cameras and lenses are being tested and chart alignment cannot be perfectly maintained.
Multiple ROI runs You can also select several ROIs in a single run, then display one of the two summary plots shown below.
Multi-ROI 2D summary plot, showing Center-corner distance,
MTF50, MTF20, and CA superimposed on image.
This display, introduced with Imatest 2.5.8, can be particularly useful because lenses are not always centered properly. (If they were, performance would be a simple function of the distance from image center.) You can quickly scan the summary results, then look at the detailed results for the individual region.
Multi-ROI summary plot, showing MTF50, CA, and 10-90% rise
distance as a function of distance from center to edge.
The upper plot shows MTF50 without and with standardized sharpening. The lower plot shows 10-90% rise distance and chromatic aberration (in pixels). The legend (lower right) can be moved with the mouse if it covers data points. An MTF50 (sharpness) summary is displayed on the right. In calculating the Weighted MTF50, the image center is given twice the weight of the corners. This number is an excellent indicator of overall lens or camera sharpness. A multiple ROI plot of SQF can also be produced.
Multiple ROI (Region of Interest) plot, describes this Multi-ROI figures in detail.
| Digital Image Quality Testing - Copyright © 2008 Imatest LLC |