How to check the resolution of an image?

It’s very likely you’ve heard the expression 'file resolution' - but do you know what it is all about? How to check a file? Or why it’s so important for print? If your answer varies from a definite 'no' to a not-very-convincing 'kind-of', don’t worry, you are not alone. Let us assure you it’s not complicated - in this blog, we will explore these three key topics in a way you will be able to change your reply to a decisive 'yeah, I do'.

What’s an image resolution?

Well, resolution is measured in dpi, which stands for ‘dots per inch’. It’s literally the number of colour dots in one square inch of paper.

Why is it so important for print?

Because it’s closely related to the sharpness of an image. Have you seen leaflets or magazines with images that look blurry or pixelated? That’s because the resolution was too low for the size they were printed. For good quality, files need to have a minimum of 300dpi – in other words, your image should have at least three hundred dots of colour in a square inch. That’s also called high resolution, or high res.

Can I change the resolution of an image?

Well… if you have a very large image and reduce its size, the colour dots that form the image will come closer together and, by default, the resolution will increase. The other way around is also true – if you enlarge the size of an image, the dots will separate, to cover the new area, and therefore, the resolution will be reduced.

The best practice, and our advice, is to select an image that is high resolution in the size (or bigger than the size) you need.

How to check the resolution of an image?

We've put together a short list of ways to find out the resolution of your image.

On a PC:

Right-click over the image and select ‘properties’ then choose the ‘details’ tab and check ‘resolution’.

On a Mac:

Open the image on Preview, then click Command+I to open the 'general info' panel.

On Linux:

Open the image in GIMP, then select ‘image properties’ from the image drop box menu and check the resolution on the pop up window.

On Photoshop:

Go to ‘image’ at the top menu, then select ‘image size’, now in the pop up window you can check the resolution under ‘document size’ – just ensure it’s set to pixels per inch.

On InDesign:

Select the image and hit F8 to open the ‘info panel’. There, you can check the resolution of the original image you imported to the software (called ‘actual ppi’) and, if you resized the image, you can see what the new resolution is (called ‘effective ppi’).

Note: ppi stands for ‘pixel per inch’. It’s marginally different from dpi, and it still refers to the resolution of an image.

What about a PDF?

A PDF is slightly different. It doesn’t really have an overall document resolution as each graphic element have their own resolution. This comes from the original file you created your artwork on, before transforming it into a PDF - it means you could have one image that is 50dpi and another with 1000dpi.

You will need to check the resolution of each individual element, either from the original file or by using the Output Preview tool in Adobe Acrobat PRO (not available on the free or standard Acrobat reader).

Be mindful when saving or downloading images from the internet…

When we talk about high resolution and 300dpi, we are referring to print. The resolution for web and screen is set at 72dpi. This means that, on the screen, images with 72dpi will look sharp and there is no need for them to be any higher than that although some may be. That’s why you need to be careful if you are planning to print images downloaded from websites – always check if they have a big enough resolution to still look good once printed.

Hey, we just wanted to let you know that we've transformed this blog into a cool presentation, check it out on SlideShare.

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