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! 🙂 

[Lumigraphics]

Don't convert print files to CMYK. That just opens up a whole problematic mess. Leave them as sRGB for both print and digital.

[Xander36210402r7ns]

Cool, thanks a bunch for the heads up. I think RGB looks better anyway on m monitor, but I don't know anything. I'll try everything out as RGB first unless I run into color that's just like...what?

[Conrad C]

I do know that you need to set print files to CMYK, not RGB

By @Xander36210402r7ns

There are times when that is still true, but less so every year. It depends on the print workflow. Converting to CMYK was more standard years ago when usually, a job only ever went to one press, and if it was ever run again, would probably be on that same press. So you could convert to that press’s CMYK numbers and it wouldn’t matter much that the color space of CMYK presses at other companies or regions was different.

But in more recent years where an image could end up on a range of materials and presses in different regions, each with different CMYK characteristics, the inconvenience of CMYK being device-specific caused everyone to rethink that whole thing. Now, in many workflows, jobs are submitted as RGB (for example, using a PDF export preset that meets one of the PDF/X industry handoff standards) and the conversion to CMYK happens at the RIP receiving the job. Or, RGB images are laid out in the software and the software does the CMYK conversion at export. So the same RGB job can be sent to different devices and they each get an appropriate device-specific RGB-to-CMYK conversion at the destination.

Will your print-on-demand workflow requires leaving the images in RGB, or converting to CMYK? That will depend on how the company you work with does things, let them tell you what they need.

For example I just looked at the Blurb help files, and they support both. They basically say, if you’re a consumer using their app, keep everything in sRGB, lay them out in their app, and just send it in (the conversion to CMYK will be on their end). If you know what you are doing, laying out the book using pro tools like InDesign, you can convert to CMYK as long as you do it using their specific CMYK profile.

If you want to read more about RGB image workflows for print, here are a couple of links:

https://creativepro.com/import-rgb-images-indesign-convert-cmyk-export/

https://creativepro.com/whats-difference-between-rgb-cmyk-printing/

[Xander36210402r7ns]

Sweet, thanks for the links. I'll check this out for sure. I need to re-visit POD principles and options. One thing that stood out the most to me, I don't plan on doing shirts like a lot of people, but what made an impression was the process they used to get good 'blacks'. That seems to be a key thing to look for when selecting vendors and people who procss stuff. Depending on the medium it can be important that the printer, if going direct to surface, needs to put down a primer layer/coat for the color to absorb into otherwise things can seem washed dependin on the color of your surface.

I 'll mos def check out those links this evening and leave CMYK transfer to the pros, thanks again!

[DClark064]

The PPI and DPI specs on an image file are largely obsolete stuff that still hangs around.  Ignore it.

The only time you may want to consider PPI is when preparing a file to be printed.  In order to know what you should do to prepare a file you need to know a bit about the printer you are going to be using and how it renders the file for printing.  If you are sending the file to a service, review their instructions and guidelines.  My own rule of thumb is that I want 150 PPI or more.  If you are printing to an inkjet printer the printer driver will scale the image PPI and half-tone the RGB image file for CMYK printing using all the ink colors available.  Unless, you have some expertise let the driver make any PPI conversions required.  If you have some expertise (your question indicates you do not), you may want to control scaling the file resolution yourself to match the way the printer half-tones.  I spent many years developing digital printing systems so I usually do my own scaling, but most of the time the printer driver does as well as I can.  My basic advice is don't worry about it.

[Xander36210402r7ns]

Okay, cool. My biggest concern is handing things off that are not to spec and somebody puts a file in a printer without minding it and because I had something set to 72 instead of 300 (even though it's like 4200 pixels) the thing comes out like EGA graphics. I've worked in advertising for years, even at big poster houses like BLT and mOcean. I've worked on poster mechanicals, but just for digtal ad purposes. I don't know squat about printing or ink. What's crazy nowadays is that I'm doing my graphic edits on lower rez generations and then upscaling which is bizarre, but for my process/results it actually makes sense rather than in-painting/doctoring/batching at higher resolution.