I’ve had an Epson 7600 for a few years and always thought of it as a big machine with an appetite for ink and a producer of impressive big prints. Up to 24″ wide by any length. Boy, have I had to recalibrate that thinking! I just got through shooting a job for some interior banners that will hang at the newly remodeled downtown Dallas YMCA. They are being printed on an HP Scitex XL Jet which prints up to 5 meters wide (196″). The images will be full bleed and the size is 9ft x 18ft. I shot using a rented Nikon D3x. I’ve written about that experience here. The resolution of the D3x is roughly 4000×6000 pixels. The long dimension of the banners of 18ft, or 216 inches. Dividing the resolution (6000 pixels) by the printed dimension (216 inches) will give us the maximum pixels per inch that I would get on the print: 6000/216=28. If I crop the image I would have fewer pixels. Is 28 ppi (pixels per inch) enough? Absolutely, because of viewing distance.
When I do my own exhibition prints, I have found that anything over about 150ppi is seldom seen in the print itself. If the image is a technical or highly detailed shot involving product with sharp edges, a super high-res multi image panorama or overlayed vector art or text, then going to 200 or 250 ppi can make a difference. Those images are hung on a wall and viewed from a few feet away – unless you’re like me and always lean in, tip your head, and look to see what kind of detail there is in the print. Its sometimes an unkind thing to do, but I’m always interested in printing technique and it doesn’t get in the way of enjoying the image as art.
These large banners will hang overhead and the minimum viewing distance from a 6′ person’s eyeballs to the bottom of the banner will be 7ft, assuming they are looking straight up. To the center of the banner would be about 11ft. More realistically, the viewing distance as people move around the room will be well over 15ft. If 150ppi is great quality at a viewing distance of 3 feet, then what happens at about 15ft? Subjectively, I can tell you that they look great. I’ve seen the prints at Color Place, who is doing the printing, and was pleased with how crisp they look from a few feet away. The numbers support this. There is a simple, linear relationship here. A viewing distance of 3ft is 1/5th of a viewing distance of 15ft. Take our target high quality image of 150ppi and divide by 5 and you get, yup, 30ppi. There is a good article on this topic here.
The ink droplets from the HP Scitex XL Jet printer that Color Place uses are pretty big. Well, big in the ink jet world. They are specified at 80 picoliters. By comparison, my Epson 7600 has a minimum drop size of 4 picoliters. That’s billionths of a liter, which is small! What that means is that the image from the XL Jet, when viewed closely, has quite a bit of dithering, a kind of noise pattern that results from the low printing resolution. That is actually a very good thing in terms of percieved sharpness. At a distance, that noise actually sharpens the image. Try it yourself. Print a photo, then add noise to it in your image editor. Put them side by side and step back. Often the noisier image will appear sharper.
I visited Color Place when they were printing and finishing the banners. The printer is really big, and so is the head assembly. I complain about buying 220ml cartridges for my printer. These guys buy ink by the gallon. When the print technician saw me photographing the bottles on the floor he said “Oh, that’s just junk ink. We can’t use that. Here’s the new stuff” and then showed me the cabinet with the new gallon jugs.
Perhaps the biggest surprise is how inexpensive this kind of banner printing has gotten. It costs me about $2/square foot in ink and paper to print on my Epson printer. You can get banners made for around $3 to $4 per sq. ft. You aren’t going to hang them in a gallery, but it is a blast seeing images this size.