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THE RESOLUTION TAG
Resolution And File Size Go Hand In Hand
We have demonstrated that digital files really don’t have a size in height and width until output. However, be aware that digital
images and the resolution to which they are set need to be defined. Software that deals with files requires us to specify what the
size should be as we consider how to output the file. Now it is time to see how the specified resolution of a file plays a further role.
Files contain what could be called a resolution tag. When a scanner or digital camera creates an images it also creates a tag that is
embedded in the file. This tag is information describing the potential resolution in the file. It allows the true number of pixels
and the true resolution to work together to come up with different combinations of image/output sizes. It’s necessary for our software
printers so that both can achieve some desired output size. We saw this works in the previous tutorial where the RESAMPLE check box was
turned OFF. The tag let us change sizes without having to add or subtract pixels.
For example, we shoot an image with a digital camera that has a sensor that can produce 1200x1000 pixels. Have you ever noticed that
when you bring such files into an application like Photoshop and look at the resolution, often it will state that the file has a
resolution of 72 dpi This resolution tag was placed in this file at the capture stage. It could as easily have been set for 300 ppi
or any other figure for that matter depending on the camera. What is important is that we know the file has 1200x1000 pixels. If the
software thinks that this file has a resolution of 72 ppi, it will also believe that the size of this file should be 16.6 by 13.8 inches.
If the software is told the file which is 1200x1000 pixels is 300 ppi, it will believe the output is 4x3.3 inches.
We can go into an image editor and without adding or subtracting a single pixel, change this resolution tag which will, as far as the
software and printer are concerned, change the size of the file if output. As you may have noticed above, that 1200x1000-pixel file
was two different sizes depending on the resolution tag. The tag is simply information in the file that allows us to get some idea
of how we intend to divide up the total number of pixels along both axis resulting in a final size if output. Why is this tag important?
Many printers will examine the resolution tag in the file and attempt to output the file at 100% of the size it believe the file should
print at. If the printer sees that the file is 8x10 at 500 ppi, it will do it’s best to make an 8x10 print regardless of the printers
native resolution. Many printers have the ability to alter how they output a file in order to produce a size the user has requested at
100%. Since many printers have fixed output resolutions rather than variable resolutions, they will interpolate the data "on the fly"
and resample to produce a print at the desired size at 100%. This is similar to using Photoshop’s IMAGE SIZE dialog and interpolating
the file. The only way the printer software can determine what the correct output size should be is to look at the number of pixels in
the file and do the math necessary to produce a print at 100%. This resolution tag is only used for the purposes of creating a final
output figure. We need to have a figure the resolution that allows the pixels in the file to be divided up per inch to yield a final
print size.
Why do files commonly have 72 ppi as a resolution tag? This is due to a device that is commonly used at this output resolution,
the monitor! Those of you working with scanned images for the Internet commonly requires a file that is 640x480. Do the math and
divide those two pixel dimensions into 72 and you will see why. This provides a very nice file for the lowest common denominator,
a small 14" display.
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