Prepare photos for printing
It’s not the dark art it once was, but take our tips for better output
Whether you’re printing photos on your inkjet, sending them to a photo printing service, or including them in a printed poster, or brochure, a few quick checks will help them look their best.
Color is handled well by default these days, from taking a picture with automatic white balance to sharing image files with embedded ICC color profiles. The best way to keep everything right is usually to leave it as it is. If you have a non–Apple display, its profile should be installed in ~/Library/ColorSync/ Profiles, and selected in Apple > System Preferences > Displays > Color.
If a pic looks washed–out, lacking contrast, try the one–click fix options in photo–editing software. In Photoshop, apply Auto Tone (Shift+Cmd+L); in Affinity Photo, Filters > Colors > Auto Levels. Haze removal may also help: find this in Affinity Photo in the Filters menu, and in Photoshop as a slider in Camera Raw, which opens when you import a raw photo, or can be accessed by pressing Shift+Cmd+A. In Lightroom, similar options are in Develop or Quick Develop.
Only manual shooters are likely to find pics under– or overexposed, but to retrieve detail lost in shadow, increase the Shadows slider; to rescue blown highlights (where detail is lost), decrease Highlights. This is most effective during Camera Raw import, or developing raw images in Lightroom, but can be accessed from the Filters menu in Affinity Photo or Image > Adjustments in Photoshop.
up the resolution
When it comes to resolution, everyone knows images for printing need to be 300dpi, right? Well… It’s really 300 pixels, not dots per inch. (Don’t worry, everyone says 300dpi.) The number comes from halftone printing, where images were traditionally rephotographed through a screen to break up continuous tone into discs of three primary colors
— cyan, magenta and yellow — plus black. For magazine-quality printing, the halftone screen would have 150 to 175 lines per inch.
Today a computer–to–plate system at the press generates the halftone patterns, but the principle is the same. So you’d probably guess images would need 150–175 pixels per inch. In fact, we need 1.5 to 2x the resolution to allow for any misalignment between the images and the halftone screen — hence 300ppi.
Laser printers (and toner–based digital presses) do their own halftoning, but inkjets work differently. Instead of regularly spaced discs that bloat from points into squares the more ink is required, they print tiny dots of a fixed size scattered at random.
This frequency modulated (FM) rather than amplitude modulated (AM) method is known as “stochastic” screening, from the Greek for guessing. That’s why inkjet printers work at ridiculously high resolutions, like 5760x1440dpi. Within each pixel, they print lots of dots to create the right color. The image itself just needs enough pixels so that the picture looks clear and detailed, not fuzzy or blocky. It turns out that, for normal eyesight at
handheld viewing distance, about 240 pixels per inch is enough. Obviously, though, you could still aim for 300ppi.
Dedicated photo printing services often use another technology: dye sublimation. Here, the color ingredients are heated and diffused into the paper, with no halftoning or stochastic screening; in effect, every pixel is printed directly in its own color. For this method, images should be supplied at the printer’s native resolution, usually 300dpi.
There’s no such thing as a “300dpi photo”, because an image file doesn’t have a physical size. If it’s, say, 1800x1200 pixels, then if you print it at 6x4 inches it’ll be 300dpi, but at 12x8 inches it’ll be 150dpi. To check the effective resolution of an image file, open it in your photoediting software and go to the Image Size dialog (Cmd+Alt+I in Photoshop or Affinity Photo). Turn off Resample: now any changes won’t affect the image itself, so the number of pixels will stay the same. Then change the units from pixels to inches, and enter a value to see the resolution; or change the resolution to 240 or 300ppi and see what size you get.
If the resolution is too low, turn on Resample and increase the size, using Bicubic interpolation (or try any other option except Nearest Neighbor), and judge the result for yourself, at 100 per cent zoom. You’re not adding any detail, but up to 120 per cent or so, it may still look fairly sharp. If the resolution is too high, on the other hand, don’t reduce it except to make a smaller file (perhaps to meet an upload limit) or to use Photoshop’s Bicubic Sharper mode to increase the impression of detail in a pic you’re going to print very small. Save the edited file separately.