The pixel dimensions (image size or height and width) of a bitmap image is a measure of the 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 image at 72‑ppi and 300‑ppi; inset zoom 200%

The combination of pixel dimension and resolution determine the amount of image data. Unless an image is resampled, the amount of image data remains the same as you change either the pixel dimension or resolution. If you change the resolution of a file, its width and height change accordingly to maintain the same amount of image data. And, vice versa. For more information, see Resampling.

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 the width or the height or the 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).