what is the ppi to dpi conversion ?
by tellnolies on May 10, 2004 at 12:59:36 pm
This is one of those self-education questions - Normally I send files to be printed at nothing less then 100ppi but i'd like to know, is there a conversion from ppi to dpi ? For example, how do I know what xxxppi really represents when it goes to print ?
Re: what is the ppi to dpi conversion ? by Mike Gondek on May 10, 2004 at 2:38:11 pm
ppi is your line screen, also often referred to as lpi. For example a newspaper might have large dots at 65 lpi, and a magazine is usually 133 or 150 lpi.
As far a as converstion, a good rule of thumb is to try making your computer file dpi = 2xlpi.
The best way to understand this is to look at one of the most common workflows. You use a 300 dpi file from Photoshop to make film at 150 lpi using a 3386 dpi image setter. I prefrer not to use the terminology of lpi over ppi, cause it is less confusing.
Re: what is the ppi to dpi conversion ? by tellnolies on May 12, 2004 at 6:25:38 am
So Pixels Per Inch (ppi) is the same as Lines Per Inch (lpi). That makes sense considering the medium. But I want to make sure that I understand your example in my terms. Let's say I receive a standard 3000x3000 photo at camera res which is 72 ppi. This, according to your conversion is really a file with 144 dpi ? This makes me think I could print this 3000 x 3000 at it's normal size of 42 " x 42" at 72 ppi and still have a good looking poster. But how can that be ? I have always been taught that I had to have at a minimum a 100 ppi file for large poster printing. Um, are you following me ?
Re: what is the ppi to dpi conversion ? by Joe on May 14, 2004 at 9:25:10 pm
I got a better sense of this by opening a new document in Photoshop and playing around with the numbers before clicking OK, by changing resolution (72ppi-144ppi-300ppi)- Then pixels to points - pixels to inches etc.
So Pixels Per Inch (ppi) is the same as Lines Per Inch (lpi)
dpi = 2xlpi.
-144 ppi resolution
The math changes with different resolutions and depending on what you want to print so does the quality of the output.
Open a new document in photoshop and enter 3000 x 3000 pixels at 100 pixels per inch resolution then change one of the 3000's popup to left of # to inches.
The number should say 30.
30 inches by 30 inches would be the size at 100 ppi.
I haven't done anything larger than a letter sized poster in years so I usually work at 300-600ppi resolution for print.
Most of my work is for video where 72ppi is just fine.