Image size and resolution

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Dimensions are the total number of pixels along the width and height of an image.

Resolution is the number of image pixels assigned to each inch when an image is printed - measured in pixels per inch (ppi). Thus, the more pixels an image has per inch, the greater will be its resolution. And, a high-resolution image will produce a better quality printed output. 

When changing the Dimensions or Resolution, remember that the image data remains constant until you resample it. If you change the resolution, the width and height will change accordingly to maintain the same amount of image data.

Note the relationship between Image Size and Resolution in the Image Size dialog box. 

To navigate to the Image Size dialog box, follow these steps: 

To learn more about Resample option checkbox, go to its detailed description. You can also go through the following table:

You can adjust the Width and Height of your image in two ways - either in pixels for images to be used online or in inches (or centimeters) for images to be printed.

Click the link icon to highlight it and preserve proportions, which will help you automatically adjust the height when changing the width. If you do not click the link to preserve proportions, you will get a tall, thin or short, wide image that looks stretched when changing one dimension.

Learn more about the different interpolation methods by referring to Resampling.

Choose the Automatic option, which will help you with default interpolation. For more refined control, you can opt for the other options as well. Each one of these options is designed for specific image enlargement or reduction workflows. 

Monitor resolution

Monitor resolution is measured in pixels. If the resolution of the monitor and pixel dimensions of the image are the same size, the image will fill the screen when viewed at 100%. 

Factors that decide how large an image appears on screen

• Pixel dimensions of your image
• Size and resolution settings of your monitor

In Photoshop, you can change the onscreen image magnification, so you can easily work with images of any pixel dimensions.

When preparing images for onscreen viewing, you should consider the lowest monitor resolution that your photo is likely to be viewed on.

File size

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 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.

Printer resolution

Printer resolution is measured in dots per inch (dpi). The higher the dpi, the finer the printed output you’ll get. Most inkjet printers have a resolution of approximately 720 to 2880 dpi.

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 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 about 1.5 to 2 times the screen frequency.

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.

Resolution specs for printing images

A resolution of 300 pixels/inch is the industry standard for high-quality prints. This resolution will ensure that your image looks sharp and detailed when printed. 

A resolution of 300 pixels/inch is perfect for viewing small prints from up close, but you can also opt for lower resolutions for large prints if they are intended to be viewed from far away. For example, if you are printing a billboard to be erected off a highway, you can print it at a lower resolution without compromising on the quality, because high resolution becomes less important as you move farther from the image.

Default resolution in printers

Usually, printers have their default print resolution at 300 pixels/inch, and if you print an image with a lower resolution, they will adjust their image settings to print your image at the default resolution.  

This means, you cannot print an image at less than the printer's default resolution, and if you don't enlarge the image, your printer will.

View the print size onscreen

You can do either of the following to view the print size onscreen - either go to View > Print Size. Or, 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 onscreen print size.

Resampling

Resampling is changing the amount of image data as you change either the pixel dimensions or the resolution of an image.

Downsampling is decreasing the number of pixels - when you downsample, information is deleted from the image.

Upsampling is increasing the number of pixels - when you upsample, new pixels are added.

You specify an interpolation method to determine how pixels are added or deleted.

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 onscreen 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 your method 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.
• 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.

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 recommended to change the dimensions and resolution first, without resampling. Then resample only as necessary.

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 take more time to edit and print.

Make sure your files are not too large - for large files, 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.