How is sharpness measure

The response of photographic components film, lenses, scanners, etc. These components can be thought of as low-pass filters that pass low frequencies and attenuate high frequencies.

Figure 6. Sine and bar patterns, amplitude plot, and Contrast MTF plot. Figure 6 consists of upper, middle, and lower plots and are described as follows:. The equation for MTF is derived from the sine pattern contrast C f at spatial frequency f, where. To normalize MTF at low spatial frequencies, a test chart must have a low-frequency reference. The low-frequency reference is satisfied by large light and dark areas in slanted edges and by features in most patterns used by Imatest, but not satisfied by lines and grids.

For systems where sharpening can be controlled, the recommended primary MTF calculation is the slanted-edge, which uses a mathematical operation known as the Fourier transform. In other words, MTF is the Fourier transform of the impulse response i. Note : Imatest recommends the star or web patterns if the sharpening of the image signal processor cannot be disabled.

Because the vanishing resolution measurement is the spatial frequency where image information disappears, it is strongly dependent on observer bias and is a poor indicator of image sharpness. Note : The USAF chart is poorly suited for computer analysis because it uses space inefficiently and its bar triplets lack a low frequency reference. Furthermore, small change s in the chart position sampling phase can cause the chart bars to appear or disappear. The chart also is prone to aliasing where small changes of position can cause the appearance of its bars to change when they shift from being in phase or out of phase with the pixel array.

Figure 7. USAF chart; not supported by Imatest. Although MTF can be estimated directly from images of sine patterns using Rescharts, Log Frequency , Log F-Contrast , and Star Chart , a sophisticated technique based on the ISO provides more accurate and repeatable results and uses space more efficiently.

See Slanted-edge versus Siemens Star for more details. Figure 8. Most readers will be familiar with temporal frequency. The frequencies of radio transmissions measured in kilohertz, megahertz, and gigahertz are also familiar.

Spatial frequency is similar because it is measured in cycles or line pairs per distance instead of time. Spatial frequency response is closely analogous to temporal e.

The more extended the response, the more detail can be conveyed. See Figure 8 and is the measurement intended to determine how much detail a camera can reproduce or how well the pixels are utilized.

Yet, digital sensor sizes vary widely—from under 5mm diagonal in camera phones to 43mm diagonal for full-frame DSLRs to an even larger diagonal for medium format backs. There is no need to use actual distances millimeters or inches with digital cameras, although such measurements are available Table 1.

Table 1. Summary of spacial frequency units with equations that refer to MTF in selected frequency units. Angular frequencies. Pixel spacing or pitch must be entered. Note : Different units scale differently with image sensor and pixel size.

Several summary metrics are derived from MTF curves to characterize overall performance. Several Imatest modules measure MTF using the slanted-edge technique and include:. MTF results for pure vertical or horizontal edges are highly dependent on sampling phase the relationship between the edge and the pixel locations , and hence can vary from one run to the next depending on the precise sub-pixel edge position.

The edge is slanted so MTF is calculated from the average of many sampling phases, which makes results much more stable and robust Figure 9. Figure 9. Clipped high-contrast vertical edge results are not valid. Edge Contrast should be limited to at the most, and a edge contrast is generally recommended. The slanted-edge method has several advantages and disadvantages:. Measures MTF and other image quality parameters from Imatest SFRplus chart recommended or created using Imatest Test Charts a wide-body printer, advanced printing skills, and knowledge of color management required.

Offers numerous advantages over the old ISO test chart: automatic feature detection, lower contrast for improved accuracy, more edges less wasted space for a detailed map of MTF over the image surface. Measurements are ISO-compliant; includes automatic region detection. Since the default value of gamma in Imatest is 0. We recommend leaving this button unchecked because the Imatest calculations re more accurate— definitely superior in the presence of noise and optical distortion.

Note that Additional calculation details can be found in the Peter Burns links below. It works by smoothing the Line Spread Function LSF; the derivative of the edge at a distance from the edge center, but not near the center. Because it has little effect on average MTF, it should be kept on unless the result needs to be strictly ISO-compliant.

Click on the button below for the full description. Show the Modified Apodization noise reduction method. Modified apodization is applied when the MTF noise reduction modified apodization checkbox is checked in the Settings windows for any of the slanted-edge modules or in the Rescharts More settings window.

Note : Imatest recommends keeping noise reduction modified apodization on. LaVeigne, Stephen D. The fundamental assumption is that all important detail at least for high spatial frequencies is close to the edge Figure 1.

The original technique involves setting the Line Spread Function LSF to zero beyond a specified distance from the edge. The modified technique strongly smooths low-pass filters the LSF instead, which has much less effect on low-frequency response than the original technique and allows tighter boundaries to be set for better noise reduction. The Line Spread Function LSF; derivative of the average edge response; the green curve at the bottom of the figure on the right is smoothed lowpass filtered to create the blue curve in the middle.

Smoothing is accomplished by taking the 9-point moving average the average of 9 adjacent points. Note : These samples are 4x oversampled as a result of the binning algorithm , so they correspond to approximately two samples in the original image.

The smoothing eliminates most response above the Nyquist frequency 0. The benefits of modified apodization noise reduction are shown on the right for an image with strong simulated white noise.

Several related techniques affect sharpness results, including:. Show More. Documentation — Current v Spatial Frequency Units Figure 8. Comparing sharpness in different cameras recommends spatial frequency units based on one of two broad types of application: Image-centric such as landscape photography, where detail on the image sensor is important : Line Widths or Pairs per Picture Height is recommended.

Object-centric for medical, machine vision, etc. Summary Metrics Several summary metrics are derived from MTF curves to characterize overall performance. The most common summary metric; correlates well with perceived sharpness. Much less sensitive to software sharpening than MTF50 as shown in a paper we presented at Electronic Imaging All in all, a better metric. A particularly interesting new metric because it closely tracks MTF50 for little or no sharpening, but does not increase for strong oversharpening; i.

Still relatively unfamiliar. Details on measuring monitor TV lines are found here. Fast calculations. Relatively insensitive to noise more immune if noise reduction is applied.

The best pattern for manufacturing testing. May give optimistic results in systems with strong sharpening and noise reduction i. Gives inconsistent results in systems with extreme aliasing strong energy above the Nyquist frequency , especially with small regions. This is the primary MTF measurement in Imatest.

Log frequency Calculated from first principles. Displays color moire. Sensitive to noise. Inefficient use of space. Primarily used as a check on other methods, which are not calculated from first principles.

Log f-Contrast Best pattern for illustrating the effects of nonuniform image processing. Strong sensitivity to sharpening near the high contrast top of the image and noise reduction near the low contrast bottom, with a gradual transition in-between. Shows loss of fine detail due to software noise reduction. Siemens star Included in the ISO standard.

Relatively insensitive to noise. Provides directional MTF information. Slow, inefficient use of space. Limited low frequency information at outer radius makes MTF normalization difficult.

Moderate sensitivity to sharpening and noise reduction. Compared with the slanted-edge in Slanted-edge versus Siemens Star. Pattern statistics are similar to typical images.

Moderate sensitivity to sharpening and strong sensitivity to noise reduction make it usable for an overall texture sharpness metric that correlates well with subjective observations. Consists of stacked randomly-sized circles. Random scale-invariant Reveals how well fine detail texture is rendered: system response to software noise reduction. Not suitable as a primary MTF measurement.

Sensitive to sharpening. The effects of noise and low Signal-to-Noise Ratio — SNR can be greatly reduced by acquiring and signal-averaging multiple images. Two useful regions from the old ISO chart are indicated by red and blue arrows see top, far-right image in the Examples column. A typical region a crop of a vertical edge slanted about 5. Includes automatic region detection. Checkerboard Sensitive to framing, making it ideal for through-focus tests.

Provides precise distortion calculations. Burns Contains an image of the low-contrast slanted-edge test chart proposed for the revised ISO standard. Nasse of Carl Zeiss. Excellent, thorough introduction. Accurate sharpness measurement of a blade edge is an essential part of knife quality control; this instrument gives you that knowledge CATRA has a 53 year history of sharpness measurement in all fields of sharp edge cutting technology and has been designing, building and using sharpness testing machines coupled with a very detailed working knowledge of sharpening techniques.

The test media holder unit is static and manually loaded. The blade holder and slide are mounted on top of the frame with the motor drive system and controller underneath. The digital cutting force display and the motor controls are mounted on top of the frame. The rubber test media is manually loaded around a self gripping former with quick action index mechanism, allowing the operator to accurately and repeatedly present a fresh surface towards the blade.

Alternative test medias and holders are available for certain unusual blade or needle formats The blade mounting unit consists of a flat plate which by a novel tilting mechanism can quickly align the blades, thereby enabling both flat and taper ground blades to be tested. On this plate are fitted moveable clamps to accommodate various blade cross sections or blade shapes, thereby giving accurate and repeatable blade positioning.

The blade unit is mounted to a linear low friction bearing slide, which is coupled, to a high accuracy and resolution force measuring cell. The linear slide has have a stroke length of 1. Within the control panel surface a digital display provides immediate readout of the peak cutting force in Newtons. Optional computer interface for data logging within an Access database complete with graphical display.

The access database allows report generation and test comparisons. Optional media carrier is available for membranes for needle and other sharp point testing.