I have discovered the majority of photographers and people in the graphics world really don’t quite comprehend the term PPI which stands for “Pixels Per Inch” or sometimes shown as “pixels/inch.”
This is not a new term to me because I have worked in the world of digital imaging and computer graphics for decades. But I find a lot of people really miss the point about what PPI means and how that relates to image resolution.
First, I should be clear about what a “pixel” is. The term “pixel” is short for “picture element.” A pixel is a single dot of color in your photograph, which is represented by numeric values of measurement of red, green, and blue light which are the primary colors of light. You can mix varying amounts of red, green, and blue to create any color for that single pixel in the image.
Lots of people talk about “resolution” and “PPI” and confuse the terminology. PPI really has nothing to do with the native image resolution of the digital image you create with your camera. The image resolution is determined by the sensor recording the image. For example, the Canon EOS 5D Mark IV creates images that are 6720 x 4480 pixels.
If you compare that Canon 5D Mark IV with its 6720 x 4480 pixel images, to a Canon Rebel T3i which creates 5184 x 3456 pixel images, you would see that the T3i creates slightly lower resolution images.
When we are talking about the image resolution created by the cameras, the only thing that matters is the number of pixels (width and height) created by the sensor. The only time the term “PPI” is relevant is when you choose to print a photograph. The number of Pixels Per Inch determines how large or small to print the photograph. The PPI specifies how many pixels are to be printed in each inch. For the sake of simplicity, let’s say we had a sensor that created images that are 1000 pixels wide. If you printed those images at 100 PPI, the print would be 10 inches wide (100 pixels in every inch, over the span of 10 inches = 1000 pixels). If you printed that 1000 pixel wide image at 1000 PPI, then it would only print 1 inch wide.
SO, PPI is just really only relevant when we are talking about printing. For most printing purposes either 240 PPI or 300PPI is quite good and sufficient. If you print at any higher PPI, you would have a hard time discerning the difference.
Lots of people talk about creating digital images that are to be displayed on a computer screen with specifications that the image needs to be 72PPI. (I see this ALL the time.) When displaying a digital image on the computer screen, really ALL that matters is the number of pixels in the image. PPI is really not relevant. A 1000 pixel wide image displayed on your computer will appear as 1000 pixels wide, regardless of what PPI setting might be embedded in the file The PPI value is only useful for printing to know how to scale the print.
But where did this spec come from where people claim a digital image needs to be 72PPI for display on a computer? That just used to be the scale to which images appeared on computers a VERY long time ago, back in the 1980’s because Apple made displays that rendered images on their screens which only had 72 pixels per inch on the screen, and why did they choose 72? 72 divided evenly into the print resolution of their printers at that time which printed 144 dots per inch. Computer screen resolutions have improved so much since the 1980’s but people still believe this myth that digital images need to be set to 72PPI for display on computers. That is just not true any more. Even so, the myth persists to this day.
If all this has confused you as well, don’t worry. You are not stupid. You are with the majority of people who just don’t quite understand what PPI is and there is so much misinformation out there, it just keeps people totally confused.
Here’s an example I encounter frequently. Let’s say someone says they need me to provide a high resolution image for some project. Since they specifically are asking for a high resolution image, then I of course need to ask just how “high resolution” do they need. The response is usually that it just needs to be a 300PPI image. Actually that response is meaningless, but even so that is virtually ALWAYS the response I get. They say they just need it to be a 300PPI image. That is NOT a specification for a digital image resolution which would be measured in pixels. A PPI specification just would say how to print that digital image and how many pixels will print in each inch. If they say they want a 300PPI image, I could provide them an image that is only 300×300 pixels and if they print it to be one inch x one inch, that would indeed be 300PPI. But, is 300×300 a “high resolution” image? No! Not at all! So, if you say you need an image that is 300PPI you need to also tell me how large you intend to print that image, in inches, in order for me to know how much resolution the image needs to be.
If you say you want an image that will print at 300PPI and say that you need it to be printed at 20 inches wide, then that would tell me how much resolution that is needed. 300PPI x 20 inches = 6000 pixels. So I would need an image that is at least 6000 pixels wide. I simply MUST know how you are going to use the image and how big you are wanting to print it. Just telling me a PPI number is really insufficient.
I hope that makes sense. Lots of people are confused about this, including even people in the graphics design world.
Oh and one more thing. There is another term out there called “DPI” which stands for Dots Per Inch. People often say “DPI” when they technically mean “PPI”. The term DPI refers to how many dots are printed by a printer, such as an inkjet printer, in order to print a photograph. If your printer has maybe 8 color ink cartridges, it splatters microscopic dots of ink to intermix to render the color for every pixel in the image. So it has to print a lot of colored dots to produce each pixel. Therefore, your printer prints way more dots per inch than it does pixels per inch. For example, an Epson XP-620 prints 5760 x 1440 dots per inch. So when you are printing a photograph at 300 pixels per inch on that printer, horizontally, it is using 5760 tiny dots of color horizontally to render those 300 pixels in that inch.
I hope this helps clarify some of the mystery and myths of PPI and DPI.
Happy Picture Taking!
Kevin Gourley
