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Photoshop CC has an updated Image Size dialog box. See Resizing images for more information.
About pixel dimensions and printed image resolution
Pixel dimensions measure the total number of pixels along an image’s width and height. Resolution is the fineness of detail in a bitmap image and is measured in pixels per inch (ppi). The more pixels per inch, the greater the resolution. Generally, an image with a higher resolution produces a better printed image quality.
Unless an image is resampled (see Resampling), the amount of image data remains constant as you change either the print dimensions or resolution. For example, if you change the resolution of a file, its width and height change accordingly to maintain the same amount of image data.
In Photoshop, you can see the relationship between image size and resolution in the Image Size dialog box (choose Image > Image Size). Deselect Resample Image, because you don’t want to change the amount of image data in your photo. Then change width, height, or resolution. As you change one value, the other two values change accordingly. With the Resample Image option selected, you can change the resolution, width, and height of the image to suit your printing or onscreen needs.
A. Original dimensions and resolution B. Decreasing the resolution without changing pixel dimensions (no resampling) C. Decreasing the resolution at same document size decreases pixel dimensions (resampling).
If you want to quickly display a document’s current image size, use the information box at the bottom of the document window.
The file size of an image is the digital size of the image file, measured in kilobytes (K), megabytes (MB), or gigabytes (GB). File size is proportional to the pixel dimensions of the image. Images with more pixels may produce more detail at a given printed size, but they require more disk space to store and may be slower to edit and print. Image resolution thus becomes a compromise between image quality (capturing all the data you need) and file size.
Another factor that affects file size is file format. Because of the varying compression methods used by GIF, JPEG, PNG, and TIFF file formats, file sizes can vary considerably for the same pixel dimensions. Similarly, color bit-depth and the number of layers and channels in an image affect file size.
Photoshop supports a maximum pixel dimension of 300,000 by 300,000 pixels per image. This restriction places limits on the print size and resolution available to an image.
About monitor resolution
Your monitor’s resolution is described in pixel dimensions. For example, if your monitor resolution and your photo’s pixel dimensions are the same size, the photo will fill the screen when viewed at 100%. How large an image appears on‑screen depends on a combination of factors—the pixel dimensions of the image, the monitor size, and the monitor resolution setting. In Photoshop, you can change the image magnification on‑screen, so you can easily work with images of any pixel dimensions.
When preparing images for viewing on‑screen, you should consider the lowest monitor resolution that your photo is likely to be viewed on.
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.
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.
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.
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.
View the print size onscreen
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.
The Print Size command is not available in the Creative Cloud version.
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.
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.
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.
A method that adds pixels by averaging the color values of surrounding pixels. It produces medium-quality results.
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.
A good method for enlarging images based on Bicubic interpolation but designed to produce smoother results.
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 onscreen size but also its image quality and its printed characteristics—either its printed dimensions or its image 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.
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.
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.