Guide #3: How to set image size and resolution in Photoshop
Changing the size and resolution of an image seems like a simple task. However, a surprisingly large amount of people are either experiencing difficulties in doing so, or not managing to re-size their images correctly.
So, in this guide, I will explain exactly what the resolution of an image is and how it affects the print quality. I will base the guide on the most popular graphic editing software on the market – Adobe Photoshop. We use it here at theprintspace for editing and file preparation. It is probably the most advanced programme you can find, yet also very intuitional.
Have you ever downloaded an image from the Internet and printed it out? If so, you probably noticed the difference between the preview on your screen and the actual print. While on the screen the image looks nice and crisp, the print looks far worse than expected – small and blurred.
You probably tried resizing the image, but you gave up seeing that the more you increase its size, the blurrier it becomes. In order to understand how to effectively resize an image or change its resolution, it is important to quickly recap some fundamental information.
PPI (pixels per inch) – screen resolution
Images are built from pixels arranged in rows and columns. Pixel is the smallest single component of a graphic image. Resolution, in general, is a term describing the number of pixels displayed or printed within a square inch.
For example, if you have a display of 1280 x 1024, it means that it consists of 1280 pixels wide and 1024 pixels high. These are so called pixel dimensions of the screen. The resolution of that particular display would depend on its size.
Knowing the diagonal size of the screen and its pixel dimensions, we can calculate its resolution by counting the number of pixels per inch (PPI). An image that has 100×100 pixels in a 1-inch square will have a resolution of 100 PPI.
A small, 12” screen that can display 1280 x 1024 pixels has got a higher resolution than a 15” screen with the same pixel dimensions. The higher the number of pixels per inch, the more detailed the image. The average screen resolution is between 70 and 100 pixels per inch.
DPI (dots per inch) – print resolution
Now that we understand that screen resolution is measured in PPI, it is important to mention that the print resolution is measured in DPI (dots per inch). Both terms are often confused and used interchangeably, which is very misleading. The term DPI derives from ink printers, which have to spray ink dots on the paper in order to reproduce the image, thus dots per inch.
In my previous guide, I have explained the main differences between the photographic c-type prints and the ink giclee prints. While the resolution of ink printers is very likely to be given in DPI, photographic printer manufacturers tend to specify their resolution in PPI. Try not to confuse these terms as they have totally different meanings.
When an image is being printed on an ink-jet device, every pixel is made up from different coloured inks (usually 4-6 depending on the printer, such as red, green, blue and black). These inks are being mixed in order to reproduce the colour of every pixel displayed on the computer screen.
As a result, every pixel of the image is created by a series of tiny dots. Therefore, the resolution measured in DPI is about 4-6 times lower than a resolution measured in PPI, as it takes several dots to reproduce each pixel.
The vast majority of images on the Internet have the resolution of 72ppi. Images of this resolution look fine on the computer screen, however are not suitable for printing. The reason to keep Internet images at 72ppi is because the lower the resolution, the smaller the file size, and in result, the faster loading times.
If you care about the print quality of your photos, always ensure that you set the resolution to at least 240ppi, with 300ppi being recommended.
On the other hand, printing everything in a very high resolution will use more ink and take more time. For example, if you print simple text, you will probably notice hardly any difference between 150 and 300ppi while if it’s the photo, a difference of 150ppi would make a visible difference.
I have explained the difference between PPI and DPI, however there is one more important thing to mention. For a very long time everyone used DPI as the default when talking about resolution. PPI is a newer term, of which not everyone is aware. Therefore, you should assume that every time someone is talking about resolution using DPI, this person actually means PPI. The reason DPI is so often used instead of PPI is simply because not many people know what PPI is! Even on our website you can find examples of the use of DPI instead of PPI, so don’t get confused.
Unexpected quality loss – important things to remember
Have you ever noticed slow degradation in the quality of the image you’re working with? Every time you save it, you notice a slight change and you’re not entirely sure about the reason?
While working with your files, there are several principal rules that you should always remember.
First of all, keep in mind that every time you resize your image, you lose some of its quality. The difference is less visible for the images that are reduced in size, however if an image’s size is increased, you will instantly notice that the larger it gets, the blurrier it becomes.
Another thing to remember is that every time you save a file, it also loses some of its quality. Most popular image formats, such as JPG or PNG, compress the file every time you save it, regardless of whether you made any changes to it or not.
Therefore, always keep your original, unmodified image as a reference file. Set your camera to take larger photos, as it’s easier to adjust them without any quality loss. And finally, try to make all the changes in one go before you decide to save your file!
Now that we’ve gone through the basics, have a look at the screencast below. I prepared it to show you step by step how to resize your images in Photoshop.
Also, stay tuned for follow up screencasts, which will explain how to scale styles and change image resolution in Photoshop. They will be released this week!
If you have any questions, just leave them in the comments section below and I’ll do my best to get your questions answered asap!