Print your photos as posters
Make the most of your photos’ pixels to get optimal giant printouts
Okay, you’ve taken your personal “shot of the year,” a photo so stunningly great you want to blow it up to a giant size and display it. The trouble is, even a 12MP shot from an iPhone 7 can barely reach 16 inches across without starting to look soft. So how did Apple create those giant “real iPhone shots” for its billboard ads? The answer is simple: interpolation. The images have been processed for large-format printing by dramatically boosting the number of pixels and applying careful sharpening. Repeat after us: it’s not cheating, it’s
optimizing. Photographers and designers tweak and tune images for different kinds of output all the time. It’ll help to know a bit about image resolution and to get hold of software to do the optimization. First, though, make sure your photo really is technically good. If it’s a bit fuzzy, if the focus is off, or there’s camera shake, that’s going to be magnified along with the scene itself. Start with the best originals you have.
The secret to image resolution is simple. Never forget this fact: a digital photo is just a grid of pixels, like a sheet of graph paper with extremely fine squares. If you stretch that grid to fill a larger space, you make the squares bigger. Take this enlargement too far and that grid becomes noticeable; the image looks pixelated and we call it low resolution. Scale the same image grid down to a smaller print size so we can’t see the pixels and we call it high resolution.
You can take a photo and “interpolate” it – recalculate all the pixels using a finer grid – so it can be printed much larger without those pixels being visible. On its own this would soften the image a little, so sharpening tricks are often used to make the picture look crisp again. However, always remember that you
cannot add detail to the image; if it wasn’t in the original, it never will be.
There’s a resolution rule of thumb that says you should go no lower than 300 pixels per inch (ppi, also known as dpi). This means that an image that is 3,000 pixels wide can be printed at up to 10 inches across without going below that threshold. But that’s really for best magazine-quality printing; if you hang something on a wall you’ll be viewing it from further away, so you can scale that same image up a bit, taking the effective resolution down to around the 240ppi mark. Really giant posters are normally seen from even greater distances, so you could even swing under 200ppi.
This should help you work out roughly how many pixels wide and tall you need to interpolate to for your final print. Remember: size in inches x minimum pixels per inch = the total number of pixels your image needs to be. For example, a 36-inch-wide print at 240ppi needs to be at least 8,640 pixels across. So you’d better get interpolating!
Doing this resizing process well is much easier now than it was in the past. If you have a recent version of Photoshop to hand, you can use its Image Size command. Photoshop CC includes a “Preserve Details (enlargement)” option that does an excellent job of preserving detail and avoiding ugly pixel blockiness while upscaling images. Photoshop Elements has the slightly simpler “Bicubic Smoother” for this job.
Resizing images more easily
ON1 Resize ($79.99, free trial available at on1.com) is the latest version of what used to be called Genuine Fractals. It is dedicated to the process of resizing images for large-format printing, and it includes a number of presets. You can apply a short selection of effects to your image, then pick from a range of printerspecific paper sizes and types, or enter the dimensions and pixel resolution by hand. Printing can be done directly, or the scaled image can be saved. It also has some special tricks to tile your image into multiple parts or create a “gallery wrap” border to cover the sides of a canvas print frame. The app creates images that preserve detail well, but you get exactly the same quality level from Photoshop, and you’ll do so significantly faster.
What really surprised us is how good the humble Preview app – yes, the one that comes with macOS – is at scaling images up. It doesn’t apply the same automatic sharpening as Photoshop and ON1 Resize, but the difference is relatively slight. It’s also easy to apply sharpening yourself after you’ve resized an image. You get world-class photo editing options in Photoshop, and easy paper presets with ON1 Resize, but you needn’t buy anything to scale, sharpen, optimize and crop a photo. In our tests the differences were hard to spot!
All digital photos are captured as RGB images: they are made from different strengths of red, green, and blue light. Printing doesn’t work in the same way; it’s done using CMYK – cyan, magenta, yellow, and black. (The K stands for “key,” for the definition and contrast that the black ink adds to the colors.) Some inkjet printers use additional pale colors or extra orange and green ink, but it’s still all about light reflecting off the page. The old advice was to convert images to CMYK before printing, but that’s no longer best practice. Most of today’s print engines (from desktop inkjets to high-end commercial raster image processors, or RIPs) will happily convert RGB to CMYK for you as the print is sent, and they’ll know exactly how to do this to best suit their own idiosyncrasies. Convert the image yourself with the wrong settings, and your prints could look awful. If you’re sending files off to be printed it’s worth asking, but the answer will almost always be to leave things in RGB. That’s good in other ways too, as a CMYK version of an image is less flexible and less suitable for different uses than an RGB original.
When you look at the final printed result, remember that print simply can’t reproduce the deep lushness of fully saturated RGB colors. Any print will always be a little more dull, although you probably won’t notice this if you don’t hold the result next to your screen.
Your images are almost certainly in 8-bit RGB, not least because that’s all that the JPEG format supports. Each of the three color channels uses eight bits of data for each pixel; 256 levels from full-strength color down to nothing, enabling roughly 16.8 million different colors to be represented. ON1 Resize can convert an image to 16-bit color, but that’s unlikely to be any benefit for a final-stage printing tool. This is great for photo-editing adjustments, but only really if the original was in that mode.
Let’s look at how to apply the techniques…