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Resolution
Resolution is an important thing, but not the only thing, in digital photography.
Digital cameras are usually referred to in terms of their resolution (1 megapixel,
2 megapixel, 3 megapixel, etc.). A greater number of pixels make pictures
look more realistic, although color and
image compression are also critical elements in the quality of a photograph.
Resolution can be reduced in a photograph, but it can never be improved after a
picture is taken. A digital photograph is information about light energy striking
a chip in a digital camera. The image that is created is unique, and contains
a unique set of pixel values (information) that will never occur again. You can
never increase the amount of original information in the picture after it is taken;
you can only decrease it. For this reason you should never edit the original
copy of a digital photograph - always work with a copy of the original. The
best situation is to edit pictures from a Recordable CD, on which the images are
stored in read-only format. It is easy to make a mistake and save changes
to an original image. Most digital cameras support a variety of resolutions.
For comparison here are four unaltered digital photos taken of the same subject
with a Nikon 990:
2048x1536 dpi (946 KB)
1024x768 dpi (282 KB)
640x480 dpi (97 KB)
2048x1360 dpi (819 KB)
There is an additional uncompressed setting at 2048x1536 that we have not included
here because it would be 9 MB download.
For a Nikon 990, for example, in fine resolution there are 1536 rows of pixels and
2048 columns of pixels, with a total of ~3.4 million pixels (megapixels).
This is the size of the image you have to work with in the best case. This
is the base unit of measurement in any digital image, since a pixel is the essential
atomic element of a digital image. Some image editing programs will try to
make this basic fact "easier" by obscuring it with measurements of the picture in
inches or centimeters. The problem with this approach is that the number of
pixels per inch depends on what is being done with an image. For example,
my computer monitor is about 14 inches wide, and the resolution is 1024x768, so
there are 1024 pixels displayed in that 14 inches (73 pixels (dots) per inch, dpi).
If, however, I crank up the resolution on the same monitor to 1600x1200, then there
are 114 pixels per inch (dpi). If I switch the monitor to 640x480, it becomes
about 46 pixels per inch (dpi). In my experience, image editing software does
not figure this out, but rather uses a constant value for all monitors.
This picture size issue becomes even more complex when printers become involved
- some printers print at 300x300 dpi, some at 1200x1200 dpi, and some at 720x1440
dpi. These numbers are much higher pixel per inch than is true of a computer
monitor. You can generally take a digital image that is too large to display
on a computer monitor and print it in the corner of a piece of paper. So if
your image editing program says an image is 4 inches by 6 inches, what does that
mean? Not very much. The image editing program could force the picture
to be 4x6 on whatever device you are displaying or printing the image on.
To do this, however, the image editing program has to effect quality on both ends
of the display range. If you are displaying on a computer monitor at 4x6,
you will display a lot fewer pixels than on a printer, so the only a fraction of
the pixels in the image will be displayed. The image is still only one size,
but printing it on a 1200x1200 dpi printer will print 16 times as many pixels as
on a 300x300 dpi printer. Some quick math indicates that 4x6 inch image at
1200x1200 dpi is an image 4800x7200 dpi - or 34,560,000 pixels (34.5 megapixels).
These extra pixels have to come from somewhere, and the image editing program had
to add them.
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