Image size and resolution

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.

Same.

Same image at 72-ppi and 300-ppi; inset zoom 200%

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.

Pixel dimensions equal document (output) size times resolution.

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).

Quickly display the current image size If you want to quickly display a document’s current image size, use the information box at the bottom of the document window. Position the pointer over the file information box, and hold down the mouse button. 

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, and PNG 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.