#2. Defining image and figure resolution
This is the second installment of a weekly series about image screening. Go to this page for all published posts this far.
To understand and apply our image standards, it is necessary to understand what image size and resolution are.
- Image size can be defined as the total number of pixels in an image and is therefore routinely given in pixel dimensions: pixel width x pixel height. Size can also be given in inches, mm, cm, picas, etc.
- Image resolution refers to the level of detail of an image, or in other words, pixel density. For digital files, it is commonly measured in dpi (dots per inch: the number of dots per inch of a printed image) or ppi (pixels per inch).
As indicated by these units, resolution depends on size. Opening an image in Photoshop or any Adobe software will allow you to discover what its size and resolution are by clicking on “Image size” in the “Image” menu; the information will be displayed in various units.
Our image screening focuses on both image data (think western blot images or micrographs) and entire figure files themselves, and JCB and Rockefeller Press journals require that:
- the size of the figure should be comparable to that of the published, printed figure (about 7 inches high x 9 inches wide);
- the figure file itself should be at a resolution of 600 dpi (this is higher than the images that go in it so that their resolution is preserved and it is necessary to meet printing requirements);
- each individual image copied or imported into the figure file should also originally be at a resolution of 300 dpi at least (more on how to obtain 300-dpi image data in our next post).
For example, here is a quick video showing how to set up a figure file using Adobe Illustrator,
Beware: your program’s default setting for a new document may be only 72 dpi! When creating a new figure file in Photoshop, Illustrator, or any other software, you must first check its resolution in a program such as Photoshop and set it at 600 dpi. If you’re using a “vector” program, such as Illustrator, you can instead set the Profile to “Print quality” or rasterizing effects to “High.” Can’t find these settings in PowerPoint? This is one of the main reasons we discourage its use -- it is not designed to preserve print quality and high resolution, and we’ll explore the differences in software in a future post in this series.
Setting the resolution initially is very important as a lower resolution figure file means losing some of the quality of the pieces of image data you are placing into the file and displaying in the figure. It is essential to pay attention to resolution from the start as, unfortunately, “resampling” in Photoshop does not yield true high-resolution images -- in other words, manually changing the resolution of a 72-dpi file to 300 dpi will not make it of acceptable resolution and quality, and the figure will most likely fail a routine production screening.
How can you deal with images that have been acquired or stored at less than 300 dpi? Come back next week to find out!