Good Quality Digital Prints

Camera

Today I got in a discussion about the resolution that a print should be for it to be considered "good quality."  As a computer scientist, I fell back on the definition of photo quality prints and research based on what the human eye can see.  The other position was a little more subjective (i.e. "good quality" is defined by one person's interpretation of what looks "good").

Photo quality prints are between 240 pixels per inch (PPI) and 300 PPI depending on the information source.  Beyond 250 PPI you will be hard pressed to find any human who can determine differences in print quality.  In other words, a 250 PPI print will
appear to be the same quality (even under fairly heavy scrutiny) as a 500 PPI print.  However, this is not so for prints under 250 PPI.  For example, a 150 PPI print would be noticeably lower quality than that of a 250 PPI print.

However, I think few people would argue that a 150 PPI print is "low" quality.  It is certainly not photo quality.  But only when compared to a print of substantially higher quality would someone likely consider it "low" quality.

Another consideration is the practicality of taking an image that can be printed at the desired size with high pixel density.  For example, let's say you wanted to print a 14'x18' picture.  At 240 PPI, the required image resolution is 3360x4320.  That's almost 14 megapixels!  Such an image is certainly attainable with current digital cameras, but not everyone walks around with a 14 megapixel camera in their pocket.

Now consider a 14'x18' picture at 150 PPI.  The required resolution of the image to be printed is 2100x2700.  A 6 megapixel camera can easily do the job.  And there are a lot more people walking around with 6 megapixel cameras than there are people walking around with 14 megapixel cameras.

If you put your 150 PPI print next to a 240 PPI print then will you see the difference? Definitely.  The contrasts on the 240 PPI print will look richer and clearer.  But take the 240 PPI print away and the 150 PPI print doesn't look too shabby.

So, what is a "good" quality print?  Some people might say that a 150 PPI print is a "good" quality print.  And it may very well be good enough for what you need.  But if you can print at 240 PPI to 300 PPI then you not only have a "good" quality print, you have the best quality print.