April 26, 2010 § 3 Comments
Designing scientific posters and preparing photos and figures for journal publication, I have learnt this: people, and scientists in particular, don’t understand image resolution. So here’s a quick crash course.
One big source of resolution confusion is the requirements issued by the journals and congresses themselves. Often written with too much information and insider terminology it can be hard to understand what they mean, even if you work with this every day. Here’s an example from one journal’s submission guidelines:
Image Size: Minimum electronic image width of 5 in (depth is not important) at a minimum resolution of 300 ppi.
You get it? No? Let’s start with the basics:
All digital images are made up of a number of pixels, or picture elements. Each of the pixels have a tonal value, and the combination of these determines the content of the image. In this posts I’ll use an image that is 300 pixels in both width and height as an example.
A pixel is square, but have no other inherit physical dimensions. How large a digital image is represented on a computer screen is dependent on the resolution and the physical size of the screen. Here are the relative sizes of the same 300×300 pixels image with screen resolution at two different settings:
If you have a computer screen with a resolution of 1280×768, a 300×300 image will occupy 1/4 of your screen and maybe be of a suitable size for your powerpoint presentation. If you want to use the same image on a printed scientific poster however, it will be far too small.
Pixels Per Inch (PPI)
The journal guideline quoted above mentioned “300 ppi”. PPI means Pixels Per Inch and is a value that indicates the printing size of a digital image. The higher the pixel density per inch, the higher the printing quality, but 300 ppi is sufficient for high quality printing. If I print my 300×300 image at 300 ppi it will measure 1×1 inch on the paper:
That is a very small image on a large scientific poster, but what happens if we print the image at 200% – 2×2 inches? What happens is that the effective PPI is halved to 150 and as the pixel density is now lower, so is the image quality. Each pixel is printed twice as large as at 300 ppi. The image looks blurred and you can almost see the pixels in the needle:
2×2 inches is still too small for a poster. An 8 inch wide image is not uncommon. At that size, the effective PPI of the 300×300 image is only 38, and the image quality is consequently useless:
So what does this mean?
Pixels matter, forget the rest
The journal guidelines stated: “Minimal resolution: at least 300 ppi at 5 in wide”. This is just a complicated way of saying that the submitted images should be a minimum of 1500 pixels wide, which is 5 inches times 300 pixels per inch.
When it comes to making or choosing images for print, the only thing that matters is the number of pixels the image is made up of.
The larger the number of pixels, the larger can the image be printed at 300 ppi. If you want an image on a poster to be 8 inches wide, it should be at least 2400 pixels wide (8 inches times 300 ppi). It is not possible to make a small image larger and keep the image quality. But as you have seen above, 300 ppi is not an absolute requirement. The image can be good enough at 150 ppi, especially when seen from distance, as most posters are.
The golden rule is therefore: Always use images with as many pixels as possible.
If you’re using photos, use the original file from the camera. If you’re exporting images from a computer program (radiograms, statistical graphs etc.) use the export setting with the highest possible pixel count. Pixels are even more important than file type. If your image is 300×300 pixels it doesn’t matter if it’s a JPEG or a TIFF, it’s still small.
How to find out
In Windows you can check the image size of a file by choosing View->Tiles in Windows Explorer. The width and height will appear below the file name, as shown in the screen shot below:
On Mac you mark the file and press Command+i to open the File Info window. The image size are listed as “dimension” under “More info”:
Hope that cleared things up a bit. If not, I’d love to hear your questions in the comment section.
And, by the way, the image at the top of this post is a low resolution version of this: