Viewing Distance and Visual Acuity
From Online Printing
Variable-sized and overlapping dots are clearly visible on an IRIS print on photo Glossy paper (40s blow-up). �Viewing Distance & Visual Acuity Another aspect of printer resolution commonly overlooked is the relationship between viewing distance and visual acuity. Viewing Distance: It matters how close you or your viewers are to your prints. Consider the ubiquitous billboard that co old be printed at a high resolution but never is. If you’ve ever seen a billboard up close, you know that the dots are huge. Yet, billboards are perfectly readable at the distance from which they are meant to be viewed across the street or driving down the road. The key point here is that you don’t need more printer resolution than you need. Normal people will stand back to view a large image, and they will get up close to a small one. This means that larger or fewer dots are more accep on big prints destined to be viewed from further back.
Viewing distance can have an impact on the choice of printer resolution. If you’re wondering how to estimate standard viewing distances, photographer Joe Butts gives this formula: 1.5 × the diagonal dimension of the art piece. To calculate the dingo 2 2 2 al, it’s a + b =c . For example, to the viewing distance for an 8 × 10 print: 8 squared plus 10 squared is 64 plus 100 equals 164 inches. The square root of 164 is 12.806 or rounding it off, 12.8 inches. Multiplying by 1.5, the viewing distance would be 19.2 inches (see 2.2). Similarly, the normal viewing distance for a large 40 × 60-inch print is about 9 feet. You won’t see many dots from there! Viewing distance, however, is only one-half the story. Visual Acuity & Maximum Resolving Power: The ability of the human eye to distinguish fine detail is called visual acuity, and it is directly related to distance. As you move farther away from the visual source, you reach a point wheter you no longer see the detail, and everything merges together. This can be determined scientifically by using alternating black-and-white lines of a specified width and then measuring the angle made from the eye to these lines at the maximum resolvable distance. It has been shown that the visual acuity of a normal eye with 20/20 vision is somewhere between 30 seconds of arc (when lighting is ideal) and one minute of arc (when the lighting is ordinary). This is the maximum visual resolution possible for most humans. From this information, all kinds of interesting formulas [c = 2 × d × tan(RADIAN ANGLE SYMBOL ÷ 2)] and conclusions can be drawn (see 2.2). One is that at any given viewing distance, you gain nothing by having higher resolution than the maximum resolving power of the eye because no finer details can be perceived. This is the upper limit, so there’s no point going beyond that. However, things are not so simple. These resolving power charts are based on high contrasting, black-and-white lines or letters (see illustration above and think of the chart at your eye doctor’s office). The images that most of us print are anything but that. We have complex patterns of dots or device pixels, overlapping dots, and all the rest. So how does 2.2’s details per inch relate to the dots per inch of ink jet printing? It is generally believed that printer resolution (dip) must exceed maximum visual resolution (deli) by a significant amount, on the order of double, triple, or more. Plus, as digital imaging writer and publisher Wayne Cos shall explains it, there are other issues like presentation. If you print on fine art or textured paper, you could get away with a lower resolution because the paper’s texture will create its own detail and somewhat fool the eye. Also, if you frame a print behind glass that lowers the contrast of the print a little, so again, you can get away with less print resolution.
2.2 Viewing Distance & Visual Acuity Print Dimensions Standard Viewing Maximum Visual Resolution Maximum Visual Resolution (inches) Distance (inches) (Ordinary)2 (details per inch) (Ideal)3 (details per inch) 4×6 10.8 318 637 8×10 19.2 179 358 13×19 34.5 100 199 30×40 75.0 46 92 1 Formula: 1.5 × diagonal of art. 2 Ordinary means reduced illumination on the target and its surroundings. 3 Ideal means bright illumination on the target and its surroundings. The formula numbers give you a place to start, but your own experience cw and your own style of printing and displaying will determine which printer resolutions will work best for you. This entire concept of viewing distance and the eye’s maximum resolving power was brought home to me in dramatic fashion when I visited well-known documentary and fine-art photographer Joel Meyerbeer at his studio in New York City. Meyerbeer had just started experimenting with in-house ink jet printing, and he wanted to see how it compared to traditional C-prints, which he was used to getting from the top photo labs in New York. He and I both analysed two 11 × 14-inch prints made of the same image he had photographed in Tuscany (see 2.10). Using a loupe (magnifier), I could see the difference between The cloudy smoothness of the C-print and the discrete dots of the HP Design jet 130 print. At first I was discouraged, but then Meyerbeer had me put the loupe away and view both prints from a normal viewing distance. Voile! The ink jet print was beautiful and actually superior. The colours qere better differentiated and richer, and there was an overall sharpness that surpassed the traditional lab print. The ink jet print is more alive, Meyerbeer enthused. It’s just plain better, and I’ve been looking at colour prints for more than 30 years. The theory worked: When viewed at a normal distance, the ink jet dots had merged into one continuous-tone image.
