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Resolution

 

Resolution

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.
 

Photographic film

Line chart
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.

 

File Resolution

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 horizontally.
 

Print resolution

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.

Note

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.

 

Note

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.

 

Monitor resolution

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.
 

High resolution

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

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.