Does Downsampling Reduce Image Quality?

Ken Hall wrote:

: I understand that upsampling can result in a poor quality image, but I
: have always been under the impression that downsampling does not
: reduce the image quality.

: It's a little difficult to be sure this question is clear, so I'm
: going to use the following hypothetical scenario.

: Assume my printer cannot take advantage of resolutions greater than
: 300 ppi and that I have an image which is 1200 ppi.

: If I downsample from 1200 ppi to 600 ppi and print it, will the image
: be lower quality than if I had printed it at 1200 ppi?

: If I downsample from 1200 ppi to 300 ppi and print it, will the image
: be lower quality than if I had printed it at 1200 ppi?

The answer is "it depends".

For example if the image is an image that would print at 8 x 10 inches at
1200 dpi (or ppi) and you use a printer that is limited to no more than
300 dpi, the final resolution would be an 8 x 10 at 300 dpi. And thus, yes
there would be a reduction in resolution. But if you have an image at 1200
dpi that would have a print size of 1/4 of the available page size (2 x
2.5) and print it to a 300 dpi at 8 x 10 size, the resolution would
remain the same (just spread out over a larger area).

Since an image that is 1200 dpi at 8 x 10 inches would be an image that is
about 115 meg this is probably not what you have (at least not from a
normal Dcam). So don't just look at the dpi but also at the print size. If
you multiply the dpi by the print size dimension and then compare that to
the printer print dimensions at the max dpi (similar multiplication) to
see if the image resolution is being changed in any way. Small percentages
of change is probably likely but not normally noticeable.

Randy

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Randy Berbaum
Champaign, IL