Graphics Programs Reference
In-Depth Information
Pixels per Inch (ppi)
When you view your images on a computer monitor, you are see-
ing pixels displayed on your screen. Computer monitors use the
concept of logical inches. Originally, the Mac OS most commonly
used 72
pixels per inch (ppi)
to match the concept of the printing
idea of 72 points per real inch of paper. The Windows OS has
traditionally used 96 ppi.
As computer monitors and portable devices have evolved, they've
advanced to support variable resolution settings. As such, the
actual ppi for a screen can vary greatly depending on the physical
size of the screen and the resolution being used by the computer's
graphics card. For example, modern laptops often use resolu-
tions between 100 ppi and 140 ppi, and devices like an iPhone
can jump all the way up to 326 ppi to make images crisper on the
small screen.
Modern computer monitors support
various screen resolutions. Changing
the monitor resolution results in a
different amount of pixels per inch
displayed on your monitor. Do not run
Photoshop at a screen resolution of
less than 1024 x 768, or it will cause
user interface problems (1280 x 800 or
higher is recommended).
Wor r y less about the rat io of pixels per inch on your screen and
simply accept that the standard measurement of resolution in
Photoshop (and most computer programs) is ppi. When talking
about displayed graphics, its ppi, not dpi.
Samples per Inch (spi)
Although scanners are less common than they used to be, many
professionals still use them to load sketches, photos, and original
negatives. Manufacturers often tout the dpi capabilities of their
scanner. This is inaccurate. Scanners don't use dots, they use
samples. A
sample
is when a scanner captures part of an image.
Samples per inch (spi)
is a measurement of how many samples
are captured in the space of one inch. In general, an increase in
sampling leads to a file that is truer to its analog original. How-
ever, there is a threshold: Once a certain amount of information
is surpassed, human senses (and electronic output devices) cannot
tell the difference.
Consumer-level scanners can capture optical resolution ranging
between 300 spi and 4800 spi. Professional devices can capture
significantly higher optical resolution. If you're working with a
large image, a lower number of samples is fine. If you're enlarg-
ing a very small image, a large number of samples is crucial.
More samples per inch translates into more information avail-
able as pixels, which can then be harnessed in output when they
