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Misunderstandings about dpi  
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Misverständnisse über dpi

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Digital Images

Before we can discuss digital image resolution, we must clarify a few things about digital images.

Digital images are made up of pixels. Pixels (short for picture elements) are the small sections of color and/or tone that together form a digital image. Pixels form an image like pieces of a mosaic. A digital image is a grid of pixels. When the pixels are viewed together in proper registration, the image is formed. When there are enough pixels and they are small enough so as not to be individually discernible, the digital image can achieve photo quality. Increased magnification of any digital image will eventually show the individual pixels, as in the enlarged section in the image at right.

The Two Forms or States of a Digital Image

Digital images take two distinct forms:
1) informational records or "files", and
2) physical images.

A digital image as it is created by a digital camera or scanner, and as it resides on digital storage medium like a hard drive or a flash card, is simply an informational record of a grid of pixels - a map of specific tones at specific locations (bitmap). It is common to refer to computer records as files, and thus digital images in this state, and generally, are commonly referred to as "digital image files," "image files," or even just "files." The image has no physical size. While if stored in certain file formats the image may have a specific physical size designated as part of the informational record, this can easily be set or later adjusted to any size.

When an image is displayed or output it takes on physical form as an image. Whether being displayed on a monitor or printed on paper, the physical image has spatial dimensions - width and height. The same digital image can assume many different physical sizes.

Two Types of Image Resolution

Digital image resolution designations fall into two basic categories. Much of the confusion over digital image resolution can be settled by understanding the difference between Pixel Count Resolution and Spatial Resolution.

PIXEL COUNT RESOLUTION

 SPATIAL RESOLUTION

Pixel Count Resolution is simply the amount of pixels a digital image contains, or is made up of. It is expressed in either megapixels or pixel dimensions.

The term Megapixels simply means millions of pixels. One megapixel equals 1 million pixels. A six megapixel image is an image made up of 6 million pixels, or some rough approximation thereof.

Pixel Dimensions represent another, more descriptive method of designating pixel count resolution. By stating the pixel dimensions of a digital image, the pixel count is inferred. In addition, the aspect ratio of the image is revealed, as are the exact dimensions.

Pixel Count Resolution is a fixed property of an image. Unless the image is resampled or cropped the image remains the same number of pixels.

Spatial Resolution relates to the number of pixels  in a spatial measurement of a physical image - pixels per inch. Spatial resolution does not apply to an image file (except as a temporary/variable specification thereof), only to a physical image. An image literally can not have a spatial resolution if it doesn't take a physical form - it can't have any given number of pixels per inch if it doesn't have physical dimensions.

The spatial resolution of an image is commonly referred to in terms of "dpi." What is being specified is pixels per inch, however "dots" per inch has gained a foothold in common terminology.

Spatial Resolution is a variable property of an image file - it only becomes a fixed property of an image once it is output in permanent form, i.e. printed. As this resolution is conditional upon output, this resolution is commonly called output resolution or print resolution.

Pixel Count Resolution and Spatial Resolution are related in that pixel dimensions and pixels per inch are the two factors that determine output image size - pixel dimensions divided by pixels per inch yields output (print) size, but the two are independent of one another. A given megapixel resolution or pixel dimensions does not infer any specific pixels per inch output resolution.

An image in its informational record or image file state (an image on a disk, drive, or card) can be printed or displayed at varying output resolutions - the dpi of an image file is a variable. As stated above, spatial resolution does not apply to an image in this state. An image generated by a digital camera doesn't have a dpi resolution - it can be output at any dpi resolution - the dpi resolution is variable. The only resolution category that applies emphatically to an image in this state is Pixel Count Resolution. If you are trying to figure out what dpi the image from your 6 megapixel camera is, stop. The answer is, What dpi do you want it to be? If a camera salesperson is quoting you the dpi of images produced by a certain camera, or inferred by a certain megapixel designation, run away. This person is confused.

An image in any output state (a print or displayed on a monitor) has the same Pixel Count Resolution it had in its file state (unless it has been resampled or cropped), and also has Spatial Resolution that is directly related to its physical size and pixel dimensions (pixel count resolution). The pixel dimensions and related physical dimensions determine the spatial resolution (pixels per inch). Likewise, the pixels per inch and pixel dimensions determine the physical size (dimensions in inches).

The Most Confusing Thing - dpi

By far the most confusing aspect of digital image resolution is the common use of the term "dpi" to refer to distinctly different aspects of digital image resolution. As discussed above, dpi refers to dots per inch.

  • dpi that should be ppi
    dpi is commonly used to refer to ppi - pixels per inch. As dpi is also used to refer to other, different resolution aspects as detailed below, ppi should arguably be used to refer to pixels per inch.
     
  • dpi properly used - for printer resolution
    dpi is also used to refer to dots per inch in terms of printer resolution. Inkjet printers printers list resolutions like 2400 x 1220 dpi. This printer resolution is independent of and unrelated to image resolution as described above. The "dpi" in such inkjet printer specifications refers to ink dots, not pixels. Inkjet printers can lay down different amounts of ink dots per inch, irrespective of how many pixels they are reproducing per inch. This is the dpi referred to in inkjet printer specifications. "ppi" or "dpi" as associated with image resolution refers to pixels and is a conditional property (it only applies to the printing) of the image being sent to the printer. So, a 1440 dpi inkjet printer printing a digital image at 300dpi(ppi) is using 1440 ink dots per inch to render 300 pixels per inch.
     
  • dpi used to refer to the spatial input resolution of scanners
    Scanners capture digital images of prints or images on film. When they scan the image on print or film, they break it down into pixels. This process has an associated spatial resolution that is related to the original surface being scanned. If the scanner breaks each linear inch into 1000 pixels, then the scan is being conducted at 1000 pixels per inch. This resolution is sometimes also referred to as the sampling rate. Most commonly though it is referred to in terms of dpi, even though ppi or spi are far more appropriate terms. This resolution is further characterized as input resolution or scan resolution, whereas the dpi/ppi of a print made from the resulting digital image is referred to as output resolution or print resolution. Note that the scan resolution has no bearing on the print resolution, unless one wishes to match the print size to the size of the original image scanned, in which case the print resolution should be set to match the original scan resolution.
     
  • dpi used to refer to illegitimate concepts
    This has been and continues to be such a confusing topic for some, that gross misinformation has come from assumed reputable sources. The list of erroneous and ridiculous concepts involving ambiguous misapplications of the term dpi is very long. The confusion has been so bad that many labs go to great lengths to avoid the topic with customers, relying on a standard file format and bit depth to infer resolution from file size related to a given print size.

Technically there is nothing wrong with referring to pixels as dots. However making a clear delineation between image resolution and printer resolution and using correspondingly more appropriate terms helps to clarify what can be a confusing subject. Think of dpi as a property of the printer independent of the ppi resolution (print resolution) of the image you send to the printer.