About printer resolution

Printer resolution is measured in ink dots per inch, also known as dpi. Generally, the more dots per inch, the finer the printed output you’ll get. Most inkjet printers have a resolution of approximately 720 to 2880 dpi. (Technically, inkjet printers produce a microscopic spray of ink, not actual dots like imagesetters or laser printers.) Printer resolution is different from, but related to image resolution. To print a high quality photo on an inkjet printer, an image resolution of at least 220 ppi should provide good results. Screen frequency is the number of printer dots or halftone cells per inch used to print grayscale images or color separations. Also known as screen ruling or line screen, screen frequency is measured in lines per inch (lpi)—or lines of cells per inch in a halftone screen. The higher the resolution of the output device, the finer (higher) a screen ruling you can use. The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results. To determine your printer’s screen frequency, check your printer documentation or consult your service provider. Note: Some imagesetters and 600-dpi laser printers use screening technologies other than halftoning. If you are printing an image on a nonhalftone printer, consult your service provider or your printer documentation for the recommended image resolutions.

Screen frequency examples
A. 65 lpi: Coarse screen typically used to print newsletters and grocery coupons B. 85 lpi: Average screen typically used to print newspapers C. 133 lpi: High-quality screen typically used to print four-color magazines D. 177 lpi: Very fine screen typically used for annual reports and images in art books

Determine a suggested resolution for an image
If you plan to print your image using a halftone screen, the range of suitable image resolutions depends on the screen frequency of your output
device. Photoshop can determine a recommended image resolution based on the screen frequency of your output device.
Note: If your image resolution is more than 2.5 times the screen ruling, an alert message appears when you try to print the image. This means
that the image resolution is higher than necessary for the printer. Save a copy of the file, and then reduce the resolution.
1. Choose Image > Image Size.
2. Click Auto.
3. For Screen, enter the screen frequency for the output device. If necessary, choose a different unit of    measurement. Note that the screen value is used only to calculate the image resolution, not to set the screen for printing.
4. For Quality, select an option:

Draft Produces a resolution that is the same as the screen frequency (no lower than 72 pixels per inch).
Good Produces a resolution 1.5 times the screen frequency.
Best Produces a resolution 2 times the screen frequency.

View the print size on-screen | CS5 and CS6
Do one of the following:
Choose View > Print Size.
Select the Hand tool or Zoom tool, and click Print Size in the options bar.
The image is redisplayed in its approximate printed size, as specified in the Document Size area of the Image Size dialog box. The size and resolution of your monitor affect the on-screen print size. Note: The Print Size command is not available in the Creative Cloud version. Resampling Resampling is changing the amount of image data as you change either the pixel dimensions or the resolution of an image. When you downsample (decrease the number of pixels), information is deleted from the image. When you resample up (increase the number of pixels, or upsample), new pixels are added. You specify an interpolation method to determine how pixels are added or deleted.

Resampling pixels
A. Downsampled B. Original C. Resampled up (selected pixels displayed for each set of images)
Keep in mind that resampling can result in poorer image quality. For example, when you resample an image to larger pixel dimensions, the image
loses some detail and sharpness. Applying the Unsharp Mask filter to a resampled image can help refocus the image details.
You can avoid the need for resampling by scanning or creating the image at a sufficiently high resolution. If you want to preview the effects of
changing pixel dimensions on-screen or to print proofs at different resolutions, resample a duplicate of your file.
Photoshop resamples images using an interpolation method to assign color values to any new pixels based on the color values of existing pixels.
You can choose which method to use in the Image Size dialog box.
Nearest Neighbor A fast but less precise method that replicates the pixels in an image. This method is for use with illustrations containing edges
that are not anti-aliased, to preserve hard edges and produce a smaller file. However, this method can produce jagged effects, which become
apparent when you distort or scale an image or perform multiple manipulations on a selection.
Bilinear A method that adds pixels by averaging the color values of surrounding pixels. It produces medium-quality results.
Bicubic A slower but more precise method based on an examination of the values of surrounding pixels. Using more complex calculations,
Bicubic produces smoother tonal gradations than Nearest Neighbor or Bilinear.
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Bicubic Smoother A good method for enlarging images based on Bicubic interpolation but designed to produce smoother results.
Bicubic Sharper A good method for reducing the size of an image based on Bicubic interpolation with enhanced sharpening. This method
maintains the detail in a resampled image. If Bicubic Sharper oversharpens some areas of an image, try using Bicubic.
You can specify a default interpolation method to use whenever Photoshop resamples image data. Choose Edit > Preferences > General
(Windows) or Photoshop > Preferences > General (Mac OS), and then choose a method from the Image Interpolation Methods menu.

Change pixel dimensions of an image

Changing an image’s pixel dimensions affects not only its on-screen size but also its image quality and its printed characteristics—either its printed
dimensions or its image resolution.
1. Choose Image > Image Size.
2. To maintain the current ratio of pixel width to pixel height, select Constrain Proportions. This option automatically updates the width as you
change the height, and vice versa.
3. Under Pixel Dimensions, enter values for Width and Height. To enter values as percentages of the current dimensions, choose Percent as
the unit of measurement. The new file size for the image appears at the top of the Image Size dialog box, with the old file size in
parentheses.
4. Make sure that Resample Image is selected, and choose an interpolation method.
5. If your image has layers with styles applied to them, select Scale Styles to scale the effects in the resized image. This option is available
only if you selected Constrain Proportions.
6. When you finish setting options, click OK.
For best results when you produce a smaller image, downsample and apply the Unsharp Mask filter. To produce a larger image, rescan
the image at a higher resolution.
Change the print dimensions and resolution
When creating an image for print media, it’s useful to specify image size in terms of the printed dimensions and the image resolution. These two
measurements, referred to as the document size, determine the total pixel count and therefore the file size of the image; document size also
determines the base size at which an image is placed into another application. You can further manipulate the scale of the printed image using the
Print command; however, changes you make using the Print command affect only the printed image, not the document size of the image file.
If you turn on resampling for the image, you can change print dimensions and resolution independently (and change the total number of pixels in
the image). If you turn off resampling, you can change either the dimensions or the resolution—Photoshop adjusts the other value automatically to
preserve the total pixel count. For the highest print quality, it’s generally best to change the dimensions and resolution first, without resampling.
Then resample only as necessary.
1. Choose Image > Image Size.
2. Change the print dimensions, image resolution, or both:
To change only the print dimensions or only the resolution and adjust the total number of pixels in the image proportionately, select
Resample Image and then choose an interpolation method.
To change the print dimensions and resolution without changing the total number of pixels in the image, deselect Resample Image.
3. To maintain the current ratio of image width to image height, select Constrain Proportions. This option automatically changes the width as
you change the height, and vice versa.
4. Under Document Size, enter new values for the height and width. If desired, choose a new unit of measurement. Note that for Width, the
Columns option uses the width and gutter sizes specified in the Units & Rulers preferences.
5. For Resolution, enter a new value. If desired, choose a new unit of measurement.
To restore the initial values displayed in the Image Size dialog box, hold down Alt (Windows) or Option (Mac OS), and click Reset.

What affects file size?

File size depends on the pixel dimensions of an image and the number of layers it contains. Images with more pixels may produce more detail
when printed, but they require more disk space to store and may be slower to edit and print. You should keep track of your file sizes to make sure
the files are not becoming too large for your purposes. If the file is becoming too large, reduce the number of layers in the image or change the
image size.
You can view the file size information for an image at the bottom of the application window.
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