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
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.
Types of Image Resolution
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
PIXEL COUNT RESOLUTION
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
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.
Resolution is a fixed property of an image. Unless the
image is resampled or cropped the image remains the same
number of pixels.
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
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.
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).
Most Confusing Thing - dpi
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.