ppi vs. dpi

[woopit@adobeforums.com]

I was under what I guess was the mistaken impression that ppi and dpi were similar values. However, today while trying to help a co-worker edit her profiles in Acrobat we did a test of a photshop tif file that was 299dpi. We imported the file into a Quark layout and made a .ps file to be distilled. We distilled the file and created the pdf. When we executed "List all images less than 300ppi" in Preflight the image showed a ppi resolution of 149.something. Does that mean that the ppi value is 50% of the dpi value? Like a 300dpi image=150ppi? After reading a bit about this on the internet today that too really doesn't seem the case, and I'm still confused. Can one of the sages here offer a simplified explanation? Also, we noticed that when a file preflights with no problem it does not list the ppi of the images in the file. Is there a way to show that?
Thanks all.

[graffiti]

In a nutshell, PPI represents the physical size of the image when viewed on screen only and has no value when printed. DPI represents the physical size of the image when printed to hard copy.

[Phil_Taz@adobeforums.com]

For every inch of expected output, the file contains 300 pixels.

at 72 ppi, (monitor output) it is expecting that 72 pixels on your monitor will equate to 1 physical inch of monitor screen space when the image is presented at 100%.

A 15" wide monitor has 1080 pixels across it @ 72 ppi so if your 15" wide monitor is set at 768 x 1024, you have slightly bigger pixels than the applications are expecting, so a file that is 72 pixels wide is displayed at slightly more than 1 inch across.

A printer set to 300 dpi or ppi will take your 72 pixels and print them at its own resolution which will result in the same 72 pixel file being about quarter of an inch wide. (300 divided by 72).

dpi refers to 'dots per inch' in a halftone screen used for conventional offset printing. If you look at an old newspaper, you can see the screen in the photos and if you want, you can count how many rows of dots in an inch of photo, which will be from 65 to about 150 depending on the paper specs. Thus 65 - 150 dpi.

It is like kids playing a game where they get to walk so many steps each turn, the taller ones go further for the same number of steps.....ppi or dpi is the number of 'steps' in an inch for any kid. Output resolution is the size of the step (big ones 72ppi for monitors, little ones, maybe 2400 for printers).

[sws@adobeforums.com]

PPI = Pixels Per Inch. This, simply stated is the image's resolution as a digital file.

DPI = Dots Per Inch. This is, as grafitti stated, the printed hard copy. For instance, as Phil said, halftone screens, or the resolution a printer will printer.

A lot of Epson printers are 1440 x 720 DPI, which means that no matter what the resolution of the digital image, this printer will make dots of ink that are that size.

The problem with going to film or plates (or the Epson printer mentioned above) with a lower than expected PPI is that you may actually start to see pixel artifacts in the final printed piece. Theoretically you would like your PPI to be double what your final output to plates or film is going to be. If you're printing a 150 line screen halftone or process image, then make sure your original digital images are at least 300 PPI. 299 is probably OK, and, you can set up Distiller's "Images" tab to adjust for this.

[woopit@adobeforums.com]

Thanks to all... especially, sws for putting it in terms that I could easily understand.

[peter kleinheider]

Hi

PPI = DPI (also photoshop the terminology of an images' resultion is dpi)

A printers screening is defined in LPI = Lines per inch (or LPCM = Lines per centimeter)
The resolution of an output device (laserprinter, imagesetter, inkjet printer) is defined in dpi.

rgds

Peter

[sws@adobeforums.com]

"PPI = DPI (also photoshop the terminology of an images' resultion is dpi)"

No. PPI is image resolution, DPI is the Dots per inch used by the imaging device to create LPI. For instance, the laser on a Scitex Dolev 200 creates 2540 laser dots in a line inch while creating a 150 line screen. (If setup accordingly). A desktop printer, like the Epsons I mentioned earlier, print 1440 dots in a horizontal line, 720 in a vertical line, when printing an image. (These types of printers do not concern themselves with LPI, only DPI which is the size of a single dot laid down by the inkjet head).

Photoshop's terminology for image resolution has always, correctly, been PPI. Up to, and including, Acrobat 5.0, image resolution in the Distiller (What was then the "Compression" tab) Settings dialog was referred to as DPI, however that bug was corrected with Acrobat 6.0.

PPI is Pixels Per Inch and your comments go on to explain this correctly. It is only your first sentence that I take issue with. This is an area I'm a little sensitive about, because people incorrectly throw DPI and PPI around all the time as though they were the same thing, and I think it confuses people. Books have been written, and, as I stated above, software has been written and released that propagates this mistake, and it is most definitely UNhelpful when folks try to do business with misinformation.