Resolution is how we measure the amount of detail a device can capture or an
object has. Resolution is used in both traditional and digital
photography. This section describes some of the areas of each where we
encounter resolution. Resolution is not the same as dimension nor is it
the same as file format. Dimension is discussed at the bottom of this
page. File format is discussed on the
File Format page.
Figure 1. A Lens and Film Line Chart
Film resolution is expressed in lines per millimeter. The more lines a
film (negative or slide) could distinguish per millimeter, the higher the film's
resolution. Generally, the higher a film's resolution, the lower its ISO.
But keep in mind that ISO is an indicator of a film's sensitivity to light, not
a measure of resolution. Figure 1 shows an example of a line chart used to
test the resolving power of camera lenses and film.
The resolution of an image file is a single number expressed as pixels per
inch (ppi) or pixels per centimeter. Pixels per inch and pixels per
centimeter refer to both the width and height of the image. For example, a
100 ppi image has 100 pixels for every inch in width and 100 pixels for every
inch in height. (Some people also use the phrase dots per inch, or dpi.
However, dpi refers to how many ink dots an inkjet printer can lay down on a
piece of paper.) When referring to image file resolution, only one number
is given since the number of pixels per inch is the same vertically and
Print resolution is expressed as dots per inch. Technically, dpi is not
the same as ppi but many people, including the experts, use them
interchangeably. Again, dpi refers to how many ink dots an inkjet printer
can lay down on a piece of paper. A printer has two dpi measures.
The number of dots per inch it can lay down horizontally across a page and the
number of dots per inch it can lay down vertically down a page. The
horizontal dpi is usually higher than the vertical. Consequently, it is
the horizontal resolution most commonly referenced in marketing literature.
A printer resolution of 2880 x 1440 dpi is the number of dots per horizontal
inch and the number of dots per vertical inch the printer can produce.
If you wish to use ppi and dpi interchangeably, that is
ok. However, do not confuse the two when setting up a file to be
printed. The file to be printed should have a ppi in a range such that
the print engine does not have to drastically resample the file before it
can process the file. The printer's dpi should be set to produce the
optimal print on paper. It is not uncommon for a file to be printed to
have a ppi in the range of 200 - 360 and the printer set to a print
resolution of 1440 dpi.
It is common to capture an image (either in a digital
camera or a scanner) at one ppi and create another version of the file at a
much lower ppi for printing.
A monitor's resolution is really its display dimensions expressed in pixels.
For example, if a monitor's screen resolution is 1280 x 1024, then it is
displaying a total of 1,280 pixels horizontally and 1,024 pixels vertically, for
a total of 1,310,720 pixels. Monitor resolution is not a per inch figure.
It is a total.
The meaning of 'high resolution' in both film and digital photography is
subjective to the application and to the person using the term. Someone
may think 300 ppi is high whereas another would think it medium.
Dimension is expressed as width x height. A digital image's dimensions
can be expressed either as a unit of measure, such as millimeters or inches, or
it can be expressed in total pixels. For example, I can have a digital
image that is 360ppi and is 1.5 inches wide and 1.0 inch high, or 540 pixels
wide by 360 pixels high.