Low versus high resolution

Bitmap images, as opposed to vector-only images, depend on the number of pixels in the image to
describe the image’s detail. In Photoshop, image resolution is measured in pixels per inch (ppi)
and can be seen and changed in the Image Size dialog box (choose Image ➪ Image Size), shown in
Figure 1.18. If an image does not have enough resolution for its purpose, it may look blurry or
pixilated.
Generally, images destined for print will have enough pixels if the resolution is 300 ppi at the final
size in inches. Images destined for on-screen display should be 72 ppi at the final size in pixels.
An adequate number of pixels needed to describe an image’s detail should be captured initially, for
example, at the time a digital photo is taken or a photographic print is scanned. If the resolution is
increased or the size of the image is enlarged after importing it into Photoshop, there may not be
enough pixels in the image to adequately describe the image’s detail at a larger resolution or size.
Even though you can increase the resolution or size in Photoshop, no new pixel information that
describes detail is added.

The Image Size dialog box

Photoshop may simply guess at what kind of pixels to add, which is called resampling up or interpolation.
This may result in an image that looks blurry or pixilated.
To avoid resampling up, uncheck the Resample Image check box in the Image Size dialog box when
you increase size or resolution numbers. You can check the Resample Image check box when you
are decreasing an image’s size; this will just throw away unneeded pixels by resampling down.
To double-check that there is no resampling up happening, enter the desired numbers in the
Image Size dialog box and before you click OK, check the total file size at the top of the dialog box
to the right of Pixel Dimensions and make sure it does not display a new larger size, along with a
smaller previous size. A new larger size would indicate resampling up.