That's because upsampling requires that Photoshop guess which pixels to add. Removal of data from an image is typically preferable to the addition of data. Whenever data is removed from or added to the image, the image quality degrades to some extent. When you decrease the number of pixels ( downsampling), the application removes data. When you increase the number of pixels in this part of the dialog box ( upsampling), the application adds data to the image. Resampling changes the total number of pixels in the image, which are displayed as Width and Height in pixels in the Image Size dialog. To resample your image, ensure that Resample is selected at the bottom of the Image Size dialog. When you resize and resample an image, you change the amount of data in that file. Pressing these keys changes the Cancel button to a Reset button. Tip:To reset the Image Size dialog to its original state, press Alt (Windows) or Option (macOS). You may have to do some extra work, such as using the Unsharp Mask filter to sharpen your image, to compensate for the resampling. When you resample, the image quality can degrade to some extent. Or, you can maintain the same amount of data in the image ( resizing without resampling). You can increase or decrease the amount of data in the image ( resampling).
The Image Size dialog allows you to resize your images in two ways. Pixel dimensions = physical size / resolution.Resolution = physical size / pixel dimensions.Physical size = resolution x pixel dimensions.Photoshop calculates the physical size, resolution, and pixel dimensions of an image as follows: Image resolution when printed: This value appears in pixels per inch or pixels per centimeter.Document size: Physical size of the image when printed, including a width and height.Image size when it's open in Photoshop: This value appears at the top of the dialog box.Pixel dimensions: The width and height of the image.When you use the Image Size dialog to resize your images (choose Image > Image Size), four aspects of your image can change:
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