• 
• 

Tuesday, November 29, 2011

The Right Resolution

The conventional wisdom says set 300 dpi and forget it, but it turns out that there’s more to optimizing image resolution for fine-art printing

This Article Features Photo Zoom

There seems to be some confusion in the industry about how to prepare and optimize image resolution for fine-art printing from modern high-end inkjet printers. Some people say you can't really see any difference between prints made at higher or lower output resolutions. Well, I can.

Contrary to what some people may say, size really does matter (well, at least file size). By file size, I'm referring to the size of digital captures, not the size of the final print. A Canon EOS-1Ds Mark III camera has a capture size of 5616 pixels by 3744 pixels. Until processed, this capture size is not yet tagged as having a dimension or resolution. In fact, the dimension and resolution of an image are simply a ratio of size at a given pixel-per-unit designation. So, an image that's 5616x3744 pixels can have a dimension of 23.4x15.6 inches when set to a pixel-per-inch (ppi) of 240. If you set the resolution to 360 ppi, the print dimensions are reduced to 15.6x10.4 inches. Would there be a huge difference in the image quality of the two prints? No, but the print made at 360 ppi will have better image quality and finer detail. Why? Well, that's what I'm going to show. However, before I do that, I need to explain a few things.

Resolution And The Eye
My good friend and colleague, the late Bruce Fraser, wrote about human visual acuity in his Real World Image Sharpening book (which I've taken over as coauthor for him). In the book, Bruce indicated that "the generally accepted definition of normal (20/20) visual acuity is the ability to resolve a spatial pattern whose features are separated by one minute of arc, or 1⁄60 of a degree." Unfortunately, that doesn't translate to printer-output resolution very easily. It involves trigonometry and calculations, which I hate. Fortunately, Bruce already did the math as shown in the table below.

As you can see, the closer the distance, the more resolution the eye can see when expressed as dots per inch (dpi). The normal viewing distance of a print is typically between 1.5 and 2 times the diagonal of a print. So a 3.5x5-inch print normally would be viewed from between 10 and 12 inches away, and the eye could resolve between 355 dpi and 286 dpi.

Figure 1 The Sharpening and Noise Reduction settings adjusted in the Detail panel.

Now before you get too worked up about this, realize there are a lot of factors involved, such as the illumination on the print and the vision of the viewer. If a person has 20/20/20 vision (by that I mean 20/20 vision of a 20-year-old), they'll see more resolution than somebody who's 50 years old and needs reading glasses. But let's agree to agree on the general principles, okay? A printer that prints at 360 dpi such as the Epson line of printers, or 300 dpi such as the Canon line of printers, would seem to fit the needs of output resolution. Yes, well, that depends on the nature of the viewer and presupposes that they don't walk up to a print and view it from mere inches away. When you're talking about printed billboards seen from a highway, that's a safe bet, but if you're talking about people in a gallery where the prints are well lit, people will tend to walk into a print and view it closer than normal. Bruce used to say that the normal viewing distance for a photographer is limited only by the length of their nose. So, it would seem to behoove somebody who wants his or her prints to have excellent detail to print at high resolutions.

Printer Output Resolution
Today's high-end, large-format inkjet printers are capable of outputting at high resolution. The Epson pro printers' print heads have a reported output resolution of 360/720 dpi, depending on the print mode. Canon and other printers that use similar thermal head technology have a reported 300/600 dpi. What do I mean by "reported"? The print driver communicates with the operating system print pipeline and states its resolution in dots per inch. You can send any output resolution to the print head, but the print pipeline will resample the input resolution and send the printer the resolution it asks for. Now there's some question of where and how this data gets resampled.