What Exactly is “Resolution” Anway?

It’s best to think of a digital image’s resolution as “resolution at a certain size.” This makes it much easier to relate to elements involved in defining an image’s properties.
Consider a stamp-sized image (see above), the dimensions of which are 1 inch by 1 inch. If this image is at 300 DPI (dots per inch)* it means that it will be 300 pixels wide and 300 pixels high. This will give us a total pixel count of 90,000 pixels (300 x 300). A megapixel is 1,000,000 pixels, so this 300 DPI image, as “high-resolution” as it is, has a fraction of the pixels that even the cheapest digital camera captures with each embarrassing picture it snaps at your office party.
Similarly, an 8 by 10 inch picture at 300 PPI will be 3000 pixels wide and 2400 pixels high; this will give us a total pixel count of 7.2 million pixels (7.2 megapixels).
What Resolution Do you Need for your Project?
When printing to paper, we should be asking ourselves how many total pixels we’ll need in order to fill a given physical paper size. Let’s see what this means given that our standard high-resolution file is 5000 pixels in the widest dimension.
In the magazine world, for instance, the standard resolution is 300 DPI. Well, if your file is 5000 pixels by 5000 pixels (25 megapixels), at 300 DPI (meaning it takes 300 pixels to print an inch of the image) the maximum print out size will have physical dimensions of 16.67 by 16.67 inches (5000/300=16.67).
Meanwhile, a standard resolution for posters is 150 DPI, and at that resolution the same digital file can print a poster at 33.3 x 33.3 inches (5000/150=33.3). Moving on, you could print out a billboard ad just over 9¼ by 9¼ feet at 45 DPI, a perfectly acceptable resolution for this application.
Resizing
There are a couple ways to increase the size of an image (by which we mean increase the total number of pixels) so it will print to a larger area than the maximum defined above.
One is to increase it through a “resampling” procedure in Photoshop. This method uses various algorithms to best create new neighbouring pixels. We increase the size in 10% increments so as to minimize the amount of new information generated in each step, thereby resulting in less pixelation (a term denoting the point where individual pixels become visible) and a higher quality print. This method has proven to be just as effective as using custom resizing software.
The second is to produce a high quality print with continuous tone colour similar to photos developed on traditional photo paper. This print is in turn scanned at the desired final output resolution (ie. 12 feet wide at 45 DPI). The inks in the printing process blend together a bit at their edges, effectively creating new neighbouring pixels (as above) in a manner that also reduces pixelation.
The first resizing method is free, but, depending on how big your large-format target medium is, it can, in some instances, result in some pixelation. For billboards this isn’t an issue because the viewer isn’t standing close to the image; that is, when you’re 10 meters away from an image you simply won’t see a difference. On larger posters this is also more than adequate.
The latter procedure costs money, but might be the better choice for certain applications such as very large interior murals, since in such cases the viewer can be right next to the image.
Why 5000 pixels?
You might wonder why we chose 5000 pixels as our maximum dimension. There’s no single answer to this question, there being many factors at play.
For one, notice that doubling the height and width of an image results in a file with quadruple the amount of information; the image of the stamp above, if doubled to 600 pixels across, would contain four times as much information (360,000 pixels vs 90,000 pixels). A 25 megapixel image becomes a 100 megapixel image! Not only would this dramatically increase rendering times, but working with files of this size would tax even the most powerful workstations—and we’re already running computers with 16 processing cores!
Larger files means slower everything, including opening files, lighting, populating, and doing those final subtle tweaks that turn a good rendering into something stunning. 5000 pixels is a size that balances the needs of the majority of our clients with what can realistically be accomplished in the time frames of the average project.
What Does all this Mean?
For the vast majority of projects 5000 pixels should be more than enough to get you a crisp image sharp enough for just about every application. If you’ve got something else in mind let us know: we’ll be happy to help figure out the best way to get that rendering on the side of an airplane!
*For our purposes we will equate DPI to PPI (pixels per inch) in order to simplify the relationship between pixels and print as far as the concept of resolution is concerned.