Lack of sharpness is usually already compensated for by the camera or the integrated recording software. In addition, the Sharpen function is also used as a creative option in image processing programs.
ZePrA uses the “Unsharp Masking” algorithm for sharpening, which is also used by Adobe Photoshop. Sharpening images involves intensifying differences in brightness or color between two adjacent pixels. This can lead to lines between image areas with different brightness/color when sharpening strongly.
With ZePrA’s Sharpen function, images can be sharpened after color conversion. In media production, there are several reasons for using sharpening:
- To compensate for digitalization shortcomings (digital photo or scan)
- Sharpen as a creative option
- To compensate for loss of detail after changing the image resolution
- To compensate for shortcomings in the printing process (e.g. screening)
Sharpening of image data can be applied to individual color formats (CMYK, RGB, Multicolor, Gray, Lab) or all color formats simultaneously.
Sharpening generally takes place after color conversion. In certain situations, for example when a transparency reduction is to be performed at the same time, images are sharpened before conversion.
Sharpening Presets
Three presets are available for sharpening – in addition to the option not to sharpen. Custom settings are also possible.
Note: Files that were created in a controlled, in-house working environment can usually be sharpened stronger.
Preset
Moderate Sharpening of all Images: Recommended presetting. Compensates for typical anomalies in the printing process.
Strong Sharpening of RGB and Gray Images: For media-neutral workflows that frequently use high-resolution RGB and Gray images.
Moderate Sharpening of downsampled Images: Compensates for minor sharpening losses that can occur, for example, when downscaling.
Custom: Allows all sharpening parameters to be defined individually. An Amount of 80% and a Threshold of 8 usually provide stable results.
The following parameters can be set individually:
Radius: The wider the radius, the wider the line created during sharpening. The choice of the correct radius depends on several factors, such as the usual viewing distance or the resolution of the printing process used. For more information see the section on the relationship between scaling and image resolution below.
Amount: Indicates the intensity of sharpening in relation to the selected radius and should take into account the sharpness of the original image and the anomalies of the printing process.
Threshold: Describes the difference in color or brightness at which the sharpness filter will apply. The lower the threshold, the more image areas will be sharpened and the greater the risk that unwanted image artifacts will also be sharpened. The usual values here are 2 to 10.
Tip: Too much sharpening due to an excessively high Radius and too much Amount can lead to an unnatural image display. Depending on the Sharpening settings, artifacts that were previously not visible can now be visible and over-emphasized. This applies in particular to the square patterns of JPEG compression or the image noise in dark areas of digital photos.
To learn more about the special work processes/flattened transparencies and sharpening, see Working with Transparencies.
Image-based vs Output-based Sharpening Radius
The sharpening Radius can be defined in two different ways: image-based or output-based.
Image-based sharpening: The Radius is specified in pixels. With a radius of 2 pixels, the effective width of the sharpening is correspondingly 2 pixels. Corresponds to the method of the “Unsharp Mask” filter in Photoshop.
If a sharpened image is placed in a layout program, the visually perceived effective width or radius of the sharpening depends on the image resolution in pixels per inch (ppi) and the scaling in the layout program.
Example: If you specify a radius of 2 pixels for an image with 288 ppi resolution and place it in a layout program with a 100% scale, the visually perceived radius of sharpening is 0.5 points (0.18 mm). This results from the fact that 72 points represent one inch. For an image with 288 pixels per inch, one pixel is 0.25 points (0.09 mm) wide.
Output-based sharpening: The sharpening radius (in points or mm) remains the same for images with different resolutions or scaling in the layout program. Since output-based sharpening mainly compensates for detail losses of the output system (e.g. due to rasterization), it is ensured that the sharpening radius of all images in the document counteracts the detail loss of the output in a comparable way.
Correlations Between Scaling and Image Resolution
If you reduce the image from the previous example with 288 ppi resolution to 50% in the layout program, the image resolution changes to 576 ppi. An output-based sharpening with 0.5 points results in a doubled radius of 4 pixels.
Typical image-based sharpening radii of 300 dpi images with 100% scaling in the layout program correspond to the following values of output-based sharpening in points:
0.8 pixels = 0.19 points
0.9 pixels = 0.22 points
1.0 pixels = 0.24 points
1.1 pixels = 0.26 points
1.2 pixels = 0.29 points
1.3 pixels = 0.31 points
1.4 pixels = 0.34 points
1.5 pixels = 0.36 points
1.6 pixels = 0.38 points
Converting the Width of an Image Pixel of Any Resolution into Points
The following correlation applies:
Radius (in points) = 72 / Image resolution (in ppi)
Note: Adobe Photoshop uses the term dpi (dots per inch) instead of ppi (pixel per inch).
The radius of the sharpness should not be larger than the raster width in the print. The following table shows the raster width and the radius of the sharpness (in mm) in a ratio of 1:1:
60 l/cm= 0.17 mm (0.48 points)
70 l/cm = 0.14 mm (0.40 points)
80 l/cm = 0.13 mm (0.37 points)
90 l/cm = 0.11 mm (0.31 points)
100 l/cm = 0.10 mm (0.28 points)
110 l/cm = 0.09 mm (0.26 points)
120 l/cm = 0.08 mm (0.23 points)
200 l/cm = 0.05 mm (0.14 points)
l/cm = lines per centimetre
If you are printing with a 70 l/cm screen ruling, the radius should be 0.14 mm or less.
The sharpening radius (in mm) corresponding to a given screen ruling can be calculated using the following formula:
Radius (in mm) = 10 / screen ruling (in lines per centimeter)
Sharpening and Image Resolution in Relation to Viewing Distance, Printing Process and Screen Ruling
The radius of the sharpening can be determined by taking the viewing distance, the printing process and the screen ruling into account.
The maximum resolution of the eye – depending on the contrast of the structures and the position in the field of view – is approximately 0.2 mm (127 pixels per inch) at a viewing distance of 1 m or approximately 0.05 mm (508 pixels per inch) at a viewing distance of 25 cm.
The image resolution and effective width (or radius of sharpening) should only be adjusted to the print resolution if the selected printing method is capable of reproducing details in the resolution of the eye for the respective viewing distance. To be on the safe side, the image resolution should be approximately 1.5 times higher than the print resolution or the assumed viewing distance.
Example 1: An offset print with a screen ruling of 70 l/cm (178 lpi) can reliably reproduce details up to 0.14 mm wide, which also defines the maximum radius of sharpening. With a 1.5-fold reserve, the image resolution should be at least 267 ppi.
Example 2: At a trade fair stand viewed from a distance of 1 m, the normal eye can detect details down to approximately 0.2 mm, which determines the radius of sharpening. With a 1.5-fold reserve, an image resolution of 191 ppi is sufficient.
