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 2024-08-31 06:01:47

The most frequent questions that arise in sharpness (MTF) testing are “What does the MTF curve mean?” and “How does MTF correlate with image appearance?”

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1. Reference 2. Sharpen 3. USM R=1 4. USM R=2 5. Blur 6. Blur More 7. Gaussian Blur R=2 8. Blur + USM R=1 9. Blur More + USM R=1 10. Gauss Blur + USM R=2

In this page we attempt to answer these questions through examples that let you quickly compare images with corresponding MTF curves by clicking on Quick links to the left of each each edge image.

In the MTF plots, the upper plot represents the average edge response, i.e., it corresponds directly to what the eye sees on edges. The lower plot contains the MTF curve (the subject of this article!), i.e., contrast as a function of spatial frequency, expressed here in units of cycles per pixel (C/P). Note that these two curves have an inverse relationship: reducing the edge rise distance (10-90% rise) extends the MTF response (measured by MTF50).

Note that the reference image is not “ideal” in any sense. It could have been made sharper, though this might have introduced some aliasing which would have made the slanted-edges more jagged. Sharpness is typical of Digital SLR cameras with good lenses and conservative amounts of sharpening, i.e. not oversharpened.

8. Blur with USM (R = 1), which improves visual sharpness. Much, but not quite all, of the original sharpness can be recovered.

The sharpness of both images is dominated by the resize operation, i.e., they are equally sharp. The MTF plots represent both the edge and the image.

1. Reference 2. Sharpen 3. USM R=1 4. USM R=2 5. Blur 6. Blur More 7. Gaussian Blur R=2 8. Blur + USM R=1 9. Blur More + USM R=1 10. Gauss Blur + USM R=2

Lens MTF

The Ranking is the same for the chart and gallery images and closely follows MTF50 (above), but the Sharpness numbers are (as expected) scaled differently. The Sharpness (gradient) ratios are 12.5/3.77 = 3.32 for the chart images and 27.8/7.04 = 3.95 for the gallery images. This compares with an MTF50 ratio (for the chart images) of 0.468/0.090 = 5.2.

The slight edge overshoot is unlikely to be objectionable. Image appearance is definitely improved, but the original sharpness cannot be recovered because there is little MTF response above 0.2 C/P.

Point spreadfunction

As you observe the images on this page, keep in mind that viewing conditions strongly affect perceived sharpness— and that these images do not represent typical viewing conditions. They are reproduced full size, i.e., one image pixel occupies one screen pixe. For most digital cameras they are are crops of very large images. For example, Dell’s 20-23 inch flat screen monitors have dot pitches in the range of 0.25 to 0.28mm (91-102 pixels per inch). My 10-Megapixel Canon EOS-40D produces 3888×2592 pixel images (quite an ordinary number these days). Assuming 0.27mm pixel pitch (94 pixels per inch), total image size would be 105x70cm (41.3×27.5in); larger than most images are ever likely to be reproduced. In most cases the visual appearance corresponding to a given MTF curve will be better than what you see on this page.

1. Reference 2. Sharpen 3. USM R=1 4. USM R=2 5. Blur 6. Blur More 7. Gaussian Blur R=2 8. Blur + USM R=1 9. Blur More + USM R=1 10. Gauss Blur + USM R=2

This section shows the effects of blurring. MTF is representative of what you might get from mediocre lenses or poor focus.

调制传递函数

9. Blur More with USM (R = 1), which improves visual sharpness. The original sharpness cannot be recovered entirely, but the perceptual improvement may make it acceptable in many cases.

1. Reference 2. Sharpen 3. USM R=1 4. USM R=2 5. Blur 6. Blur More 7. Gaussian Blur R=2 8. Blur + USM R=1 9. Blur More + USM R=1 10. Gauss Blur + USM R=2

Opticaltransfer function

The slight (7%) edge overshoot is unlikely to be objectionable under most viewing conditions. Image appearance is definitely improved.

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Depth of focus

In the sections below, the right side of the edge and the entire image are subjected to a variety of signal processing steps: blurring (similar to what might be expected from poor quality or out of focus lenses), sharpening, and combinations of the two (similar to real-world conditions, when blurry images are sharpened). Note that sharpening increases MTF at high spatial frequencies; blurring (lowpass filtering) decreases it.

Perceived sharpness of real images is dependent on image (& reproduced pixel) size, viewing distance, illumination, and the Human Visual System, whose Contrast Sensitivity function is described here.

Imatest’s Find Sharp Files module can produce sharpness rankings for the above files. It works on any set of similar images— not just test charts. Results are based on the absolute values of the gradients (directional derivatives) of the linearized pixel levels (yes, geeky stuff). The Sharpness numbers in the talbes below are completely arbitrary; they’re dependent on the image and crop area, i.e.,; they’re not a standard measurement and cannot be used to compare different images (or even different crops of the same image).

1. Reference 2. Sharpen 3. USM R=1 4. USM R=2 5. Blur 6. Blur More 7. Gaussian Blur R=2 8. Blur + USM R=1 9. Blur More + USM R=1 10. Gauss Blur + USM R=2

The edge overshoot may be somewhat objectionable in highly enlarged images. It’s unlikely to be an issue at small enlargements. This amount of overshoot is common in compact digital cameras.

10. Gaussian blur (R = 2) with USM (R = 2), which improves visual sharpness. The original sharpness cannot be recovered because there is little MTF response above 0.2 C/P.

Edge spreadfunction

Sharpness is moderately degraded— slightly more than for Blur. Noticeable at most magnifications. There is little contrast above 0.35 C/P.

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Understanding image sharpness and MTF A multi-part series by the author of Imatest, mostly written prior to Imatest’s founding. Moderately technical.

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The slight edge overshoot is unlikely to be objectionable. Image appearance is definitely improved. Much, but not all, of the original sharpness can be recovered because there is little MTF response above 0.35 C/P.

1. Reference 2. Sharpen 3. USM R=1 4. USM R=2 5. Blur 6. Blur More 7. Gaussian Blur R=2 8. Blur + USM R=1 9. Blur More + USM R=1 10. Gauss Blur + USM R=2

This section shows how sharpening (using Unsharp Mask (USM)) can recover some (but not all) of the visual degradation in blurred images.

Bob Atkins has an excellent introduction to MTF and SQF. SQF (subjective quality factor) is a measure of perceived print sharpness that incorporates the contrast sensitivity function (CSF) of the human eye. It will be added to Imatest Master in late October 2006.

mtf光学

The slight edge overshoot (11%) is unlikely to be objectionable. Image appearance is definitely improved. Most of the original sharpness is recovered.

How to Read MTF Curves by H. H. Nasse of Carl Zeiss. Excellent, thorough introduction. 33 pages long; requires patience. Has a lot of detail on the MTF curves similar to the Lens-style MTF curve in SFRplus. Even more detail in Part II.

Modulation Transfer Function (MTF) is a fundamental measure of imaging system sharpness. It is introduced in Sharpness and discussed further in Sharpening. MTF is measured by Imatest SFR, SFRplus, and by several Rescharts modules.