PPI Doesn’t Matter (and stop saying DPI)
The 300 DPI Myth is still strong!
I’ve been talking about this subject for close to a decade. It started way back when I worked for a photographer and I realized that many printing companies were clueless when requesting files for print. I’d ask them for file specs they require to print a booklet, postcard or brochure and 90% of the time I would get back the same response.
“Just send files at 300 dpi”
WRONG ANSWER. The first problem is the measurement DPI.
DPI (dots per inch) refers to the output resolution of a printer or imagesetter, and PPI (pixels per inch) refers to the input resolution of a photograph or image. DPI refers to the physical dot density of an image when it is reproduced as a real physical entity, for example printed onto paper. — Wikipedia
BOTTOM LINE: you should NEVER use DPI (or PPI) when describing a digital image file.
Now, this isn’t just about using the wrong measurement term. If it was only that I’m sure I would have gotten over this issue a long long time ago. What irks me is that the uneducated printing company, photographer, or graphic designer that relies on a myth that has plagued the graphic design and photography industries ever since we started using computers to deliver files for output to print.
This myth has kept turning up like a bad penny. I was doing some research for a new client and found another article here on Medium that kept the myth going. For some reason they incorrectly showed this graphic with two images seemingly giving two different results by only setting different PPI.
Here is a real comparison. One of these images is set to 72 PPI, one at 300 PPI and one of them at 1 PPI. Can you tell the difference just by looking at them on your screen? Just in case Medium.com has done something to these images, you can download the originals here, here, and here.
I know you can’t, even on the highest resolution screen. Why? Because what really matters is their pixel dimensions. These files have identical pixel dimensions of 600x374 (width x height).
I like to put it in different terms which everyone should understand. If I ask you how long does it take to get to New York City and you reply “oh about 65 miles an hour” are you really answering my question? Not at all.
Pixel Dimensions Really Matter
PPI in a digital file is a relative measurement, just like the miles per hour in my example. When a file is printed what really matters are the pixel dimensions. Another way to think about it is the megapixels of the file. If you tried printing a file that has pixel dimensions of 500x500 at 10x10 inches you will get an unacceptable result. You need more input resolution to fill the 10x10 inches in an acceptable manner.
When you export a digital photograph from Lightroom or create a new file in Photoshop the dialog boxes ask for image sizes.
Ideally you don’t want to upres, or increase dimensions, any larger than the source file. There are exceptions, and if you start with a high quality image you should be ok to upres 10–20% with absolutely no noticeable difference in quality. You only need to do this if the output medium is very large.
You will notice in this graphic that when you uncheck resample in the Photoshop image size dialog box the dimensions do not change.
Bring it Around
So how should you be answering someone who asks for image file specifications? I always use the long edge dimension, take into account how it is being output and the final print size. If a client asks me to print a 5x7 print an ideal pixel dimension is at least 1890x1250. Therefore I would reply ‘at least 1890px on the long side’. As shown above I can change the PPI at will. If for some reason the client’s artwork is not large enough, I know I can get away with a source file size of about 1260px on the long side for a 5x7 print and it will still look fine.
That story would be different if I was printing an image much larger, say 20x30 inches. Then I would request it at least 4000px on the long side. Using my Epson printers I know that I will get a very good result at that pixel resolution because the image will not be viewed from a short distance, rather it will be seen from at least 2–3 feet away, possibly more when displayed in their living room.
Inkjet Photo Printing Caveat
I’ve had many discussions with Eddie Murphy, one of the Product Mangers from Epson Professional Imaging. Their recommendation is that when printing to one of their high quality photo prints to set the PPI in increments of 90, ideally 270 PPI. When printing to an Epson printer the image will be converted to 90, 180, or 270 PPI anyway so it never hurts to follow their recommendations. I haven’t seen a difference when failing to set the recommended PPI, this could be a due to many factors or just quality programming by Epson engineers. Pixel dimensions are still the primary factor and you want to start with a high quality original file no matter what.
Pay attention to your pixel dimensions when exporting. Don’t worry about PPI since it is relative and only a tiny 5 bit string of information in the header of the digital file, not actually a part of the image. If you get someone asking you for a “300 DPI” file send them over to this post, hopefully it will set them straight and properly educate them.
Do you agree? Feel this post is incomplete? It may be, and I will gladly update it with additional information. If you want more information on this subject try Googling “300 DPI myth”, you will find plenty of other people like this one, another, another who agree.