Here's what I've learned about print size and resolution

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Charley123 Contributing Member • Posts: 757
Here's what I've learned about print size and resolution
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I obtained this information recently from a few sources on the Internet, including one of my other threads at Dpreview.

Take the diagonal length of the print and multiply that by 1.5. This tells the closest likely viewing distance based on the rather narrow FOV that the human eye has. If a person were to stand any closer they would not be able to see the entire photo at one time.

Divide 6870 by that number from above. 6870 describes the common resolution of the human eye, or some mumbo-jumbo I can't remember about resolution of the human eye. The important Point used to use 6870 / the number from above.

This tells you the resolution needed for a quality photo at the given size for likely viewing distance. This information is partly based on the FOV and resolution of the human eye, which dictates how close someone is going to want to stand, and the amount of printed resolution that they can appreciate.

For example, here are the calculations for a 20 X 30 print.

First we use the Pythagorean theorem to find the length of the diagonal.

Pythag Theorum

A^2 + B^2 = C^2

20"^2 * 30"^2 = 1300"

Sq Rt of 1300 is 36

36" * 1.5 = 54" Viewing Distance

6870 / 54" = 127 pixels per inch needed for a quality 20 x 30 print. That really means 127^2 per inch of course, though for short we just say 127 resolution.

That's why I got excellent results printing 20 x 30 at 150 PPI on an inkjet plotter with 300 DPI back in the day. I was actually well above the 127 PPI requirement. And I also had the advantage that some of pointed out or it takes several droplets of ink to make one pixel on an inkjet printer. So it's easy to see why it worked out well for me back in the day, even though I did not understand all these issues back then.

The only thing I knew back then was that my Photoshop book told me to print using resolution at half the printer DPI. Which was simple and was great advice in the inkjet printer days. I still try to follow that advice with laser printers, but now that means I have to print at 300 res on my 600 DPI laser printer, which is possible for small prints, but for large prints some additional knowledge is required (that I did not have until recently). That new knowledge is what I typed up earlier in this post.

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