What exactly is DPI?

Every week someone inadvertently asks us to “give them an AO rendered image at 300 DPI, please” without understanding the implications of the request. Rather than try to create a NASA-sized 140 megapixel image each time, we try to explain that preparing 3d images for web-print means planning in pixels, not DPI, as resolution is not the driving force in determining the final image size.

What exactly is DPI?

DPI is an acronym for dots-per-inch, which is a measurement of resolution. Basically how many dots can be placed along a single inch length. (That’s 25.4 mm for those of us in metric-land). It is a common term that is used by the printing industry to describe the resolution that an image is printed at. Higher DPI images produce clearer and more detailed results while lower DPI equals lower resolution images with less detail.

108x43 pixels

216x86 pixels

1080x430 pixels

Resolution

Resolution only depends on what the final printed application is. Magazines, for instance, are printed at around 300 DPI while newspapers and posters are normally set to lower resolutions like 170 DPI. If a poster is especially big then the resolution can drop down to as low as 100 DPI. Billboards, which are designed to be viewed from far distances, can be as low as 20 DPI! It all depends on viewing distance. Generally, the rule of thumb is ‘the larger the final print size the lower the resolution’.

It all depends on viewing distance.
The larger the print size, the lower the resolution.

What are megapixels then?

Megapixels refer to the amount of pixels that make up the image, i.e. the length of pixels multiplied by the height in millions of pixels. Modern smartphones can take a photo of around 10 megapixels while the latest DSLR cameras are pushing into the range of 50 megapixels. Typically, buildmedia produces final rendered images around the same size as photos produced by professional DSLR cameras.

Final rendered
images are approximately the same size as photos produced by professional DSLR cameras.

Compared to TV screens

We like to compare the concept of resolution to TV screens. Full high-definition TVs have a pixel size resolution of 1920×1080 pixels. It doesn’t matter if it’s 35″ or 60″, there are still the same number of pixels that make up the image. It’s just that the image scales up in size and effectively the number of pixels per inch decreases. The larger the TV, the larger the actual LED pixels are. Typically the bigger the TV the further away you will be viewing it and therefore the lower the resolution.

Computer-generated 3d imagery

Typically when CG images are produced they are rendered at a default of 72 DPI – exactly the same as digital photos from your digital camera.

Originally 72 DPI was a standard monitor resolution in the desktop publishing world and it has sort of stuck. This doesn’t mean that they are small images, rather that the DPI resolution default has not yet been set. It could be a low DPI image to be used on a billboard or a high DPI image in a brochure. Either way, we normally don’t consider DPI, rather we only focus on what the actual number of pixels are that make up the image. Preparing images for the web or print means planning in pixel dimensions not in DPI.

For more information refer to this brilliant article that dates back to 2007 – How Big Can You Print?

Images that can be printed in a magazine at
300 DPI are more than suitable for printing to a billboard.

Testimonial content text goes hereGareth has been working with 3d visualisation since the days of floppy disks. He now spends his time wrangling a specialised team of creatives at Buildmedia.

Testimonial Gareth Ross
Gareth Ross
Creative Director