Does an image lose quality when being reduced?

https://forums.adobe.com/people/jay+fresno  wrote I thought that an image loses quality when being either enlarged or reduced. Yet recently I read in a couple of different places that "An image will never lose quality when being reduced in either size and / or resolution." I'm confused by this and hoping someone can enlighten me.

The confusion and conflicting information is usually because most places that write about this, and most of the arguments on the Internet, don't mention whether the image is being resampled. The answer to "will you lose quality when resizing" is "Yes" if resampling is on, and "No" if resampling is off.

An image has pixel dimensions (width and height in pixels). As long as you change the physical size without changing the pixel dimensions, the original quality stays the same. As soon as you change the pixel dimensions, the quality is changed. In the Image Size dialog box in Photoshop, the pixel dimensions are only changed when you enable the "Resample" option. (Export As and Save for Web always resample.)

If you start out with a 6000 x 4000 pixel image at 300 ppi and you:

• Change the resolution to 150 ppi with Resample off, then it is still 6000 x 4000 so no change in quality.
• Change the resolution to 150 ppi with Resample on, then it is now 3000 x 2000 so 50% of the pixels are thrown out.

Everything depends on whether Resample is on. As long as Resample is off, no change you make in Image Size will change the pixel dimensions so all original pixels will still be there after the resize. In theory.

But in the real world, image resolution and quality is always relative to your final output. You can keep Resample off to "preserve quality" and then set that 6000 x 4000 pixel image to 15 percent size which creates a 2000 ppi image, but if your final output is a print at only 300 ppi, it can't use all 2000 ppi so the final output quality will be just 300 ppi.