Understanding Image Resolution
Understanding image resolution, and conversion from inches or millimeters to pixels can sometimes appear confusing, so we will take a quick look at the various aspects and terms to know.
Understanding Pixels?
For example, pixels are used as the unit of measurement for monitors and screens. A “pixel” is a mash-up of the words “picture” and “element,” and you can think in terms of pixels being the building blocks of graphical information.
All of the information that you view on a screen is comprised of pixels. If you are old enough to remember the basic table tennis game on ancient screens, or even the first space invaders games, then you could actually see the squares that made up the screen. These were pixels. On older screens, these pixels were more visible because they were less dense.
However, newer computers and mobile devices have the ability to display so many more pixels in a smaller physical space, resulting in a crisper, smooth appearance.
Understanding Resolution?
Simply put, the true definition of the term "resolution" is the relationship between the dimensions of an image and the number of pixels contained within those dimensions. Therefore it follows that the more pixels, the higher the resolution. For example, a 2000 x 2000 pixel image could be considered a high resolution image for digital purposes, as this would appear large on most standard monitors with a screen size often 1980 x 1080.
If you were to print this at the same resolution, this density of pixels would be sufficient for a high-quality print of some 169 x 169 mm or 6.6 x 6.6 inches. However, if you wanted a significantly larger print size (like 15 x 15 inches), you would encounter image quality issues because the density of the pixels would be much lower.
Understanding Pixel Density (DPI or PPI)?
In printing terms, DPI stands for “dots per inch” and refers to the number of dots of ink in one inch of a printed image. PPI is the digital equivalent, meaning “pixels per inch,” and refers to the number of pixels within one inch on a digital screen.
While these terms don’t mean exactly the same thing, they’re often taken as the same in general image resolution terms. They are the important determining factors for print quality. As a general rule, you would want a DPI of 300 for the best printing results, and this is the norm in most commercial printing.
For general on-screen use, you can get away with smaller pixel dimensions and still have a good quality image. This is because printing generally requires a higher DPI for better results.
For example, on most monitors and screens, 72 PPI has been the standard for digital use for years, but the actual pixel dimensions are what’s most important because display resolution can change across different devices.
Websites and digital platforms will list dimension requirements in pixels because monitor and phone displays are measured in pixels. So, if you’re prepping images to use on the web, you want to set the units to pixels. On the other hand, print requirements will be in inches or millimeters, depending on the system you use where you are (Imperial or Metric).
You can check the DPI/PPI of any image in Adobe Photoshop by simply opening your image and go to "Image Size". If you don't have Photoshop, don't worry as there are plenty of free "Photoshop" like software and online editors available such as GIMP, Pixlr Editor or Paint.NET.
Since dimensions and resolution are relative, the best time to check the effective DPI of your image is when you place it in your design at the size you desire.
Changing an Image Size
When you alter or resize an image, you change the dimensions and density of pixels that define its quality. The relationship between image resolution and dimension is directly connected. If you reduce the number of pixels per square inch you will effectively increase the dimensions. The dimensions will determine how big the image appears on screen.
It is important to note that you can always make an image smaller without losing quality, but you can’t make an image much bigger before noticing a drastic reduction in quality.
In a digital image, the number of pixels are represented by the DPI (or PPI) and the width x height dimensions. For instance, a 2000 x 2000 pixels image at 72 DPI has 4,000,000 total pixels. To make the image smaller, say 1000 x 1000 pixels, I can simply reduce it in size and it’ll retain the same level of detail, just in a smaller image.
If I want to make that same image bigger than the original size, pixels would have to be created. That means the computer multiplies the pixel count to fit the new dimensions, creating distortion and other effects by using the information in the image to guess at what should be used for the enlargement.
This is due to artificially creating pixels from other pixels, instead of capturing them from the original information.
As a general rule of thumb, consider beforehand whether you’ll be using your designs for the web or for print.
For web-based images, choose 72 DPI.
For high resolution print images, choose 300 DPI (however, on lower resolution print images, you may be able to go as low as 150 DPI at which point you will simply losetoo much quality).
Remember, uploading an image with the correct dimensions ensures no loss of quality, so it’s important that you choose the right dimensions for the image’s end use.
