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PPI & DPI

As you begin travelling down the digital photography rabbit hole, you’re sure to come across the terms PPI and DPI at least a few times.  But what do they mean? And what are they used for? We’re going to dig a bit deeper into exactly that so you know what you’re looking for, and what to expect when you run into those terms.

Although both terms describe resolution, one refers to the digital version, and the other describes the physical version when printed.  If you’re a designer, you won’t run into much trouble here, since you’ll mainly be dealing with PPI, working from a computer screen. If you’re a photographer though, especially if you plan to edit or enlarge your photos for print, you’ll likely run into DPI quite a bit.  

Both the printing industry and designers alike are notorious for using PPI and DPI interchangeably.  Although they refer to resolution similarly, they definitely mean different things. The good news is, even if you confuse the two, professionals are generally able to pick up on what you’re trying to convey either way.  

PPI, or pixels per inch, describe the resolution of an image on a digital screen in pixels. But that just leads to more questions; what exactly is a pixel anyways?  A pixel is the smallest physical point on a raster image; or can be described as the smallest element of the picture that can be controlled.  Each pixel an individual colour, all working together to form a much larger image, and depending on the amount of pixels per inch, can form extremely detailed images.  Ever zoom into a photo too much and it becomes blocky? Those blocks are pixels.

The thing is, pixels have become the standard measurement used for things on screen, but the pixels themselves don’t really have a standard size.  Let’s dive into this a bit further by examining screen size. Although screen size is measured diagonally from corner to corner, screen resolution is measured horizontally.  

For example, a 1080p television screen would have 1080 pixels if counted horizontally from edge to edge.  A 4K screen would have 4000 pixels when measured horizontally. Does that mean that a 10” screen with 1080p resolution would be a higher definition than a 50” screen with the same 1080p resolution?  The answer is yes! That’s why the image is clearer on a larger TV the further away you stand. So in turn, if your computer monitor is only 72 PPI, which is pretty standard for older models, even if the image you’re looking at is 1000 PPI it will still only show through your screen at 72 PPI.  When it comes to uploading images onto the internet, unless the image will be viewed on high definition screens, or downloaded to eventually be printed, 72 PPI or 96 PPI is often plenty. This will also help keep your file sizes low, and page load times fast.

Now we have a pretty good idea of what PPI is, let’s dive into DPI.  As we mentioned before, DPI is used to describe resolution of physical items, unlike the digital descriptor, PPI.  Printing companies often have their printers set to a particular DPI depending on the type of material that is being printed.  The standard for photographs on photo or fine art paper is usually 300dpi. Although depending on the type of paper and it’s texture, the resolution doesn’t always show through that way.  This is especially true with materials like canvas, where DPI as low as 150 can still have spectacular results.

All my part of the work is digital, if I’m not the one setting up the printer why do I need to know about DPI?  If you know your printer is going to use a particular DPI setting, you can set your PPI setting the same to more accurately soft proof.  Don’t worry if you aren’t sure how to set this, we can prep the file for print, and set the appropriate PPI, so you have a better idea of how things will turn out when printed.  

If you wanted to take the accuracy of your soft proofing a step further, your printer likely uses a unique ICC profile that you can install into applications like Adobe Lightroom or Photoshop. Using their profile will give you the closest representation of how your image will appear on a particular paper or canvas type, as well as that particular ink brand and spectrum.  You’ll need to be sure your screen is properly calibrated. We prefer products like ColorMunkie if you need a recommendation.  

PPI and DPI are both described in a per inch increment, so regardless, if the image is 4”x6” or 24”x36” the expression of the clarity of the image would be the same. Now, that’s not to say that a small image can always increase in size while maintaining quality or that DPI or PPI describes absolute quality. Note the difference between resolution and quality.  

When a 4x6 photo at 300 PPI is enlarged to 24x36 at 300 PPI, although the PPI has remained the same, there dimensions of the photo are now larger, and since PPI and DPI are expressed on a per inch basis, there are now more pixels in the 24x36 300 PPI image.  But where do these pixels come from? It’s called resampling. The software, Photoshop for example, will use algorithms to determine what the detail of those pixels would look like had they been there from the beginning. But what happens if we don’t resample?  The image dimensions would be enlarged, but the amount of overall pixels would remain the same, resulting in a drop in pixels per inch.

For example, a 4x6 image at 300 PPI would have pixel dimensions of 1200x1800.  If that same image were enlarged to 24x36 without resampling, the digital measurements would increase, but the amount of pixels would remain at 1200x1800, dropping the PPI of the image from 300 PPI down to 50 PPI.  The pixels would become larger, and more noticeable, resulting in a blocky representation of the image detail.

This can sound like a lot of information; I mean we’re just talking about pictures right?  No need to worry, we can take care of all of this on our end; ensuring no matter where you’re printing your photo, your mind can rest knowing that the resolution will be high, and the photo clear.  

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