Changing
the pixel dimensions of an image is called
resampling.
Resampling affects not only the size of an image on‑screen, but
also its image quality and its printed output—either its printed
dimensions or its image resolution. Resampling can degrade image
quality. When you
downsample, meaning that you decrease
the number of pixels in your image, information is removed from
the image. When you
upsample, or increase the number
of pixels in your image, new pixels are added based on the color
values of existing pixels, and the image loses some detail and sharpness.

Resampling an image
- A.
- Image downsampled
- B.
- Original
image
- C.
- Image upsampled
To
avoid the need for upsampling, scan or create the image at the resolution required
for your printer or output device. If you want to preview the effects
of changing pixel dimensions on‑screen or print proofs at different
resolutions, resample a duplicate of your file.