Preparing files for print part 3 – Image Resolution

In part 3 of this 8 part series, we’ll explain what is meant by Image Resolution (and why you should care!).

1. Introduction
2. Card Sizes: things to consider
3. Image Resolution – you are here
4. Understanding Colour modes (CMYK vs RGB vs Pantone)
5. Bleed and Trim
6. Finishing Layers (Foil, flitter, emboss)
7. Getting Files Press Ready Checklist
8. Working with your Printer

Get to know your Dots from your Pixels

Printed images are made up of tiny dots. So are images you look at on screen, but in this case they are called pixels. If you’ve ever taken a magnifying glass to a picture in a newspaper, you’ll be able to see the individual dots.

The number of dots in an image is measured in Dots Per Inch, or dpi. For screen images, they are measured in Pixels Per Inch, or ppi. dpi and ppi are often used interchangeably, but they are in fact different – and for printed greeting cards you need to concern yourself with dpi.

dpi is a measure of how many dots will fit in a line measuring 1 inch. (note, it’s a line, dpi does not refer to a square inch). The more dots there are, the smoother the line will be.

Most printers will want you to supply your files at 300dpi @100%, The “@100%” bit is important. 300dpi on it’s own doesn’t give the full information. For example, imagine an image that is 10cm wide and has 300dpi. If you try to print that image at 20cm wide, then the dpi will drop to 150. So it’s important that you remember to supply your files at the correct dpi when the image is at the size you want it to print at.

Using photoshop to check the dpi of an image

You can use Adobe Photoshop to check the resolution of an image (other image editor tools will offer a similar function). To do this, open the image in Photoshop and go to Image > Image Size. Change the width/height units to mm or cm. This will tell you the current dpi for the size show,

Now uncheck the Resample checkbox. You can then change the resolution to 300 and size will change to show what size will achieve this resolution. Alternatively change the size to see what the resolution will be.

(Since there’s no standard dot size, it’s fine to use the Pixels/Inch figure in the Photoshop dialogue box as a close approximation to the dpi).

Ask your printer what resolution they need the files to be supplied at.

Don’t let your screen fool you!

A common novice mistake is to think just because something looks sharp on a screen, it will print sharp. A computer monitor only needs images to be 72ppi to display clearly. But the same image will become very pixelated if printed at 300dpi by your print supplier.

Don’t overdo it

If your printer has asked for 300dpi, then there’s no need to send a file at 1200dpi. The extra pixels will have no effect on the quality, but will make the file size unnecessarily large (in terms of bytes). This will make files harder to transfer, store and process.