© Alex – Fotolia.com
The mysteries of print resolution and ppi uncovered
“For the print version of our journal, production-quality figures are required. Can you please update your files according to our standards”… Does this sound familiar?
Many research scientists get confused and sometimes even frustrated when it comes to the technical aspects of their figures for publication. In most cases there is an easy fix, sometimes it is problematic, but in all cases it is poor understanding that is causing the problem. I’ll try to demystify some of these technical aspects and give you some good practices that will avoid problems down the road.
There is one rule (when applied correctly, as you will read later in this article) that will save you all the trouble and that will assure that your figures will always meet the journal’s standards.
Prepare all your figures at the size they will appear in your publication.
Alright, this sounds like a no-brainer and not too complicated. However, you’ll need to know a few things to nail it every time again.
Size in this rule means physical size, a dimension measured in cm or inch. That is an important distinction with the size in pixels that is used in the digital world. Remember, this information will help you prepare publication-ready figures for print. Most journals have 2 or 3 size options for figures. Single column and double column width and sometimes 1.5-column width. You should be able to find this information in the author guidelines of every journal. As a rule of thumb you can use 9 cm (or 3.5 inch) for single column width and 18.5 cm (or 7.3 inch) for double column width.
This sounds very easy, but there is a huge pitfall. All journal guidelines will always instruct you to use 300 ppi (pixels per inch) for color figures and often even 600 ppi or 1200 ppi for greyscale and line-art figures. This information causes all the trouble and confusion. Mostly because many of the heavily used software packages to design figures in the academic world – I think particularly about presentation software such as Microsoft Powerpoint – don’t allow to specify this metric. If you export your Powerpoint slide as a TIFF file it typically results in an image of 72 ppi. You understand that it doesn’t meet the standards of the journal, but what does it mean?
Pixels per inch (ppi) defines the amount of pixels that will be printed in 1 inch (2.54 cm). For optimal print quality of color figures you need 300 ppi. In other words, for a one column figure (3,5 inch width) we need a figure of 300 ppi x 3.5 inch = 1050 pixels wide. If we look back to the exported Powerpoint slide and do the math, we only have 72 ppi x 3.5 inch = 252 pixels. This will result in poor quality prints.
Presentation software such as Microsoft Powerpoint is not designed for print design and image editing. It is presentation software after all and as long as the slides stay on screen, there will be no issues, but as soon as you export the slides to an image format, problems might arise. There are better alternatives. True graphics software such as Adobe Photoshop or the free alternative Gimp allow you to set up your file in the correct physical dimensions with the desired ppi (= print resolution).
However, with the right knowledge, you can make publication ready figures with Powerpoint too. Imagine you are designing a one column figure. From our calculations above we know that we need a slide of 1050 pixels wide to meet the standards. We know that Powerpoint exports at 72 ppi. So, we have to setup our slide at 1050 pixels / 72 ppi = 14.58 inch. Cumbersome, but this workaround also results in the required quality.
My advice to avoid problems is to setup a document in Adobe Photoshop (or similar software) with the correct resolution (see screenshot), before you start . You could even save different documents as templates for one and double column width figures. If you work with such a document and you import figures from other sources, you will immediately notice that these figures are resized to the right print resolution. After you finish your figure, you simply export as a TIFF file and you are good to go. This figure will never be rejected based on technical requirements. Another one down, good job!
If you like what you read, feel free to subscribe to our newsletter and mark the box “Tips and Tricks to ‘Visually’ Succeed in Scientific Publishing”. This section of our newsletter is especially meant for publishing authors, PhD students, postdoc and research scientists.