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PGY 4440C PHOTO COMPUTER IMAGING
Tips on Using Resolution
Resolution determines the level of detail recorded by the scanner, and is measured in dots per inch (dpi). The greater the dpi number, the higher the resolution and the resulting file size. Image quality improves with higher resolution, but only up to a point, after which in most cases, increasing resolution simply makes file sizes unmanageable without yielding any visible improvement to the image.
When dealing with resolution, it's important to distinguish between optical -- or true -- resolution, from interpolated resolution, which is resolution enhanced through software. Optical resolution is the key factor in determining the sharpness and clarity of an image. Interpolated resolution is useful for certain tasks, such as scanning line art.
Choosing the best resolution setting
For most users who are scanning images to print on their home inkjet printers,
there's no need to use overly high resolutions. In fact, the best resolutions
to use fall between 266 to 360 dpi, with many preferring to use 300 dpi
as a median of sorts.
If you use a resolution lower than 266 dpi, jaggedness will likely crop up in your images, making them look coarse and unrefined. If you use a resolution higher than 360 dpi, however, you're not really obtaining any more visible advantage to your image but are simply creating larger files than necessary. This is true even if your printer advertises itself as being able to print at 1,440 dpi.
For those who scan images intended to be output on commercial printers and services, be aware that these services may use the LPI -- or lines per inch -- specification (instead of dpi) to measure the resolution of printed images.
In this case, an easy way to determine the best resolution is to find out the lines-per-inch capability of your output device, and then multiply it by 1.5 to 2.0.
For instance, to tailor your scanned image to a typical magazine printing press that prints at 133 lines per inch, multiply 133 x 1.5 or 2.0, which gives 199.5 or 266. Thus, the optimal resolution setting for your image would be 200 or 266 dpi (depending on how high the output quality will be). Anything higher would be useless, as the printer will be unable to process the additional image information anyway.
Lpi varies, depending on the quality of the printing job. A newspaper uses approximately 85 lpi, magazines from 133 to 150 lpi, and fine art books may go as high as 200 to 300 lpi.
When to use high resolution
High resolution is important if you're processing an image through a high-end
color system that carries continuous tone data from the scanner through
the final film output. This is because high resolution can improve the
sharpness and clarity of the dots that make up the image.
High resolution also comes into play when you are enlarging small originals and do not want to lose image clarity or sharpness. For instance, if you have a 2"x2" photograph being scanned at 300 dpi and wanted to enlarge the scan to 4"x4" (double the size), double your resolution to 600 dpi. When the image is printed at the enlarged size, there will be no loss of detail.
When to use interpolated resolution
Interpolated resolution is useful for scanning line art. Line art is defined
as black-and-white art or one-color art (typical of logos, blueprints,
or mechanical drawings).
To make the most of line art images, set the resolution equal to that of your output device. For instance, if you're producing line art to be printed by a 1200-dpi printer, set the resolution to up to 1200 dpi for superior results. This will produce smoother lines and eliminate some of the jaggedness characteristic of line art scans.
Photo Printing Resolution Calculator
Number of pixels = Edge length x Print resolution (lpi)
For example, in order to obtain a 13 x 18 cm print, the image should ideally have:
(13 / 2.54) * 300 = 1535 pixels
(18 / 2.54) * 300 = 2126 pixels
Consequently, of course, you can also calculate how large a print you can produce using a specific data volume and a specific printing process:
Edge length = Number of pixels / Print resolution
The result for the above example of the 1535 x 2126 pixel image would be:
(1535 / 300) * 2.54 = 13 cm
(2126 / 300) * 2.54 = 18 cm
So where do you actually define the size of an image for printing in practice? This is generally done by setting the image resolution in your image-editing program. It determines how the existing pixels are distributed on the paper later on during printing. When determining the so-called output size, you should again really use the term ppi (pixels per inch) and not dpi (dots per inch).
In many cases, such as with dye sublimation printers, you will obtain good results if the image resolution (ppi) corresponds to the print resolution or the screen ruling of the printer (lpi). In other words, in order to print a 1280 x 968 pixel image on a 300 dpi (which you know should really be lpi!) dye sublimation printer, you should assign a resolution of 300 ppi to the image. Remember that the change in image resolution usually doesn't change the number of pixels. You can think of it as a message for the printer embedded in the file which says, for example: "distribute 300 of the pixels sent over every inch of paper". In order to actually influence the number of pixels, you have to use the commands referred to as "recalculate" or "resample".
In the examples above, the ratio of pixels to printer dots was assumed to be 1:1. However, you can obtain slightly better results, particularly in halftone printing, if the image resolution is somewhat higher than the print resolution. A value of 1.4 to 1.6 has become established for this so-called quality or sampling factor for images with photographic content. However, depending on the motif, impressive prints can also be produced with a sampling factor of 1.
Next: Scan Resolution and Scaling
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