Does PPI Matter?

[Xander36210402r7ns]

Hi Everybody,

I know this is kind of a dumb question, but honestly...If my image is 4200x3000 pixels (at 72 dpi/ppi) how does that affect the print quality if it's not 300 dpi/ppi?

I make changes in software programs an the resolution is the same (naturally).

[dj_paige · Accepted solution]

If you are printing, DPI is the printer setting, it lays down so many Dots Per Inch (DPI). Bigger numbers should print more dots which results in higher quality images, but after a certain point, its hard to see a difference. 

Your digital image has so many pixels per inch (PPI) when you print. These pixels are different than the dots printed by your printer in most cases. The more pixels per inch when you print would generate higher quality, but after a certain point (like 300 ppi), its hard to see a difference.

DPI and PPI are not the same, they are not related to one another, they do not have to move together, one tells you nothing about the other.

If your image is 4200x3000, then when you export it from Lightroom Classic, setting the resolution field (PPI) in Lightroom Classic has no impact (unless you also specify the size of the image in inches or centimeters). 

[Abambo · Accepted solution]

No and yes. 

First no: the ppi value does not matter for any operation that uses pixel measures. That is for all screen operation, including the web. A picture 1000x2000 pixels is the same, regardless if the ppi value us 72 or 300.

It matters, however, when you change to the printing world. PPI stands for Pixels Per Inch, so it translates the pixel values into a preferred inch value. I say preferred, as wirh modern DTP systems, you import the pixel file and scale the image to the size you need them. Programs like InDesign also use this value to generate the preview images for images linked in the design. So, if you set your PPI value to 72, the preview file will be huge, if you set it to something like 300 the preview file will be less big, setting it to 1200, the preview file will be tiny but the preview quality will be low. So yest, it matters here. 

The output setting when you generate your print file also matters. If you generate an output for the screen, you normally set a low value, for keeping the file size low to be able to transfer the file via e-mail, or permit a fast download. But for standard professional print, you will set it to 300 ppi, as that is what the printer usually needs to produce high quality prints. 

In Lightroom for export to JPEG, you can set that to 300 for a modern photo, and you will be good.

[Conrad C · Accepted solution]

This might sound similar to Abambo’s great reply, but anyway…

72 ppi or 300 ppi have no meaning…until you lock them to a real world dimension such as inches. Then it means a lot. The big mistake a lot of people make is stating a ppi value without also stating the real world size (in inches or cm) they want to print the image.

The key to the whole thing is the simple equation:

Pixel per inch (ppi) = pixels / inches

Using your 4200 x 3000 image as an example:

4200 pixels divided by 72 ppi = 58.33 inches.

Therefore, sure, it’s a 72 ppi image…if you intend to print it 58.33 inches long! But you probably don’t, so 72 ppi is not useful here. Why does it say 72 ppi? The file might not have a ppi value in it, and in some applications, 72 ppi is just the default value the software plugs in when the file doesn’t have a ppi value. So, it’s not meaningful.

Let‘s work out what would be meaningful.

If you want to print it at 10 inches wide (as in a 10 x 8 print) then the effective resolution of the printed image would be:

4200 pixels divided by 10 inches = 420 pixels per inch.

If you want to print it 30 inches wide (as in a 30 x 20 inch gallery print) then the effective resolution is:

4200 pixels divided by 30 inches = 140 ppi.

As a 6 x 4 inch print for a snapshot album, the effective resolution is:

4200/6 = 700 ppi

So the ppi of the same 4200 x 3000 px image is not one single number. It’s different for each print size. The ppi is one of its pixel dimensions (4200 or 3000) divided by whatever the same dimension of your print size is. Accounting for the print dimension is what tells us whether it will be 140 ppi, 700 ppi, or something else.

If instead you plug the ppi into the equation and let the print dimension be the variable, then the equation tells you the usable print sizes of any image. By doing this, we find that if we want an image to have an effective resolution of no less than 300 ppi, then:

The maximum print size of your 4200 x 3000 px image, at 300 ppi, is (4200/300) or 14 inches long.

The maximum print size of a 6000 x 4000 px image from my camera, at 300 ppi, is (6000/300) or 20 inches long.

Since you asked about actual print quality, you can account for viewing distance. In my 30 x 20 inch print example, it will be large enough that it will usually be viewed from further away than a small print, so a lower print ppi is OK because it will seem like more ppi from a further distance. So if a file for a 30 x 20 inch print works out to 150 ppi, that might be OK depending on your expected viewing distance.

So in the end, your file is not 72 ppi period.

It’s 72 ppi at 58.33 inches long.

and

It’s 300 ppi at 14 inches long.

and

It’s 700 ppi at 6 inches long.

etc.

[Xander36210402r7ns]

That helps answer a lot. PPI is for digital only. DPI is a resolution yardstick for printing. I've been developing a print-on-demand side thing that I've been working on all year, so I want to have the formats correct for both hi-rez digital downloads and consumer products (bookmarks, etc.) and not go through too much hellish QA with color testing. I do know that you need to set print files to CMYK, not RGB, but the whole PPI/DPI has always been a mystery to me since the results are the same digitally if something is 2000px etc.

Thanks for the feedback, guys! 🙂