Pixels are an integral part of electronic publishing and being as such, a clearer understanding as to how they affect the printed product is beneficial to anyone who is creating files for print publication.

First, we must define pixels.

When you make a scan, take a digital photo, or even convert a file to a bitmap mode you are capturing or creating that image as pixels. The amount of pixels you capture or create at a specified image size relates directly to its resolution.

The discussion of pixels can become very confusing when discussing pixels per inch (ppi) versus dots per inch (dpi). They seem interchangeable but are not. Expressing resolution as dpi should only occur when referring to a printed piece while expressing resolution as ppi should only occur when refering to an electronic image. Yet these distinctions rarely seem to happen in the real electronic publishing environment. In fact, most scanner software will indicate that you are scanning in dpi not ppi, yet when the same image is opened up in Adobe Photoshop and the image size is checked the resolution is represented as ppi and not dpi. As professional publishers and designers we should make the distinction between the two. For the sake of discussion though I will refer to any measure of resolution as ppi.

How do pixels and resolution come to play in the electronic publishing world? If you are scanning a continous tone image (b/w or color photo) for professional publishing (the product is going to be output from a high resolution imagesetter for films or direct to plate) then your scans should be 266 to 300 ppi at a 1:1 ratio. (Reminder: When scanning line art, it is recommended the the scan be at 800 to 1200 ppi for 1:1 ratio.) Where does that 300 ppi number come from? The publishing "rule of thumb" is to make your resolution twice the intended lpi screen of the finished product. Following that line of thinking, an offset press produces about 133 to 150 lpi, yielding our range of 266 to 300 dpi for images. The 1:1 ratio is important because so many times an image is scanned at one size and then reduced or enlarged by the designer at a later time.

Enlargement is the worst of the two scenarios. For instance, if you scan a 3" x 5" image at 100% for 300 ppi (which gives you a 900 pixel x 1500 pixel image) and you intend for the final image size to print at 6" x10", what happens is that the 900 pixels that originally were spread across the 3" dimension must now stretch the entire 6" dimension giving you only 150 ppi! The same is true for the other dimension. 1500 pixels now must spread across 10" leaving you only 150 ppi. When reducing the image the opposite is true. Resolution (ppi) increases for your entire image, but that does not necessarily mean your image reproduces better. Since 300 ppi is the optimum setting for resolution at a 150 lpi screen you get diminishing returns when your resolution goes above 300 ppi. To explain further: an image with more ppi has a larger file size which results in longer RIP times and therefore increased costs. Your return on that investment of time and money is an image that appears the same as the optimal 300 ppi image.

In the ePUB Section we see many image files that customers have pulled from low resolution sources like the web. We always recommend that the customer find the image from a high-res source for good quality. Invariably we get the question: Why not just add to the ppi reading in Adobe Photoshop or similar software package? While you can add resolution, the process (interpolation) does not accomplish the desired effect of adding detail. Great looking printed images are achieved by scanning an image with the proper settings for high resolution. When interpolating the resolution, the software identifies single pixels in the image and creates additional pixels of varying color/density in the amount necessary to achieve the newly requested resolution. For example, if you were to add resolution through interpolation to a 72 ppi scan to make it 300 ppi the software is going to create an additional 228 pixels per inch of that image. These additional pixels only serve to make the image appear with less detail and sharpness, in other words the image actually becomes slightly worse than the original. This happens because the pixels are simply not there for 300 ppi. In the left hand column we have the ePUB Puppy Adoption Test of PPI Quality. Pick your favorite puppy to see which resolution, interpolated 300 ppi or true 300 ppi, is for you!

So what have we learned? A better understanding of what a pixel is and how it relates to your digital image files. Always remember that there is no substitute for a properly scanned or created image. Good luck living in the hi-resolution world!

 

This article pertains to:

Who Would You
Take Home?

Stinky the Pixelated Pooch
This little guy lives life
at interpolated 300 ppi.

OR


Laddie the High-Res Pup!
This power pup is on
the edge at true 300 ppi.