Upsizing your images
Hans Weichselbaum explores the notion of ensuring the best-quality image is maintained when up-sampling and down-sampling your photographs
In the last edition, we looked at the concept of image resolution. A figure of 300 pixels per inch (ppi) doesn’t tell you anything until your image gets onto paper (the same goes for ‘dpi’, which stands for ‘dots per inch’, or ‘drops per inch’). The size of a digital image is determined by the number of pixels, and nothing else. For example a 12-megapixel sensor will give you a 4000x3000-pixel image.
Only before printing do we look at how many pixels we get per centimetre (or inch) of paper. Our example of a 4000-pixel-wide landscape image will give you 571ppi for a 17.78cm (seven-inch)–wide print. This figure drops down to 235ppi if you want a 43.18cm (17-inch) print, slightly larger than A3.
Optimum file resolution for printing
How many pixels per inch do we need for best quality? You’ll often hear the number 300ppi for printing and 72ppi for monitor display. The 72ppi figure is nonsensical, because it will depend on the physical size of your device and the number of pixels it can display. The resolution can be more than 500ppi for a typical smartphone display.
Commercial printers will routinely ask for 300ppi files, and, in the previous article, we looked at the historical reasons for this figure. It is still a good ballpark figure to aim for when your images are heading for a printer. Epson inkjet printers work with a native resolution of 360ppi (or 720ppi if you tick the ‘Finest Detail’ checkbox). This means that your image is going to be up- or down-sampled to 360 or 720ppi depending on your print settings. The printer driver does the resizing automatically for you.
You do not need to panic if your file resolution works out to less than 360ppi. The 43.18cm print from our 12-megapixel camera (235ppi) will still look great. The trouble starts when you want to print your 12-megapixel camera file at A2 size (594mm/23.4 inches), which works out to 171ppi, or if you are in the habit of cropping your images. As a rule of thumb, you will get good results from 240ppi prints viewed close up and from 180ppi photos hanging on the wall and viewed from a distance. If the ppi gets too low, you’ll get aliasing (the ‘jaggies’), and the image will need to be upsized.
Printer drivers in general do an excellent job of upsampling your images to 360ppi, or whatever the native resolution of your printer might be. You can also rest assured that the commercial printer you’re using has a professional raster image processor (RIP) doing the best possible job of upsizing.
The dark art of up-sampling
Despite all these assurances, many people get nervous when they think of feeding a small file into a black box (the printer driver), yet out comes the perfect 360ppi file. As we saw in the previous article, Adobe Photoshop gives you a number of choices when you need to up-sample your image files. Obviously, there is more than just one option.
‘Up-sampling’ means asking the computer to ‘invent’ new pixels between existing ones. Needless to say, there is no way that you can add detail that wasn’t in the file when it came out of the camera. Perhaps you remember details in the original scene, but they were not recorded because the resolution wasn’t high enough. The software will interpolate between existing pixels, and add more based on the average value of those pixels around them. This just gives the illusion of increased resolution, but it’s only more pixels showing the same thing.
The problem is that the image gets softer, losing sharpness and contrast when it gets resized (the same applies to downsizing). Some interpolating algorithms are better in keeping the image sharp, but are more likely to produce resizing artefacts. The challenge is to find the best program, which can vary from image to image.
If your image comes from a scan, there is a simple rule: never up-sample. Especially film scans: the film grain and the scanner’s sampling grid form interference patterns, which tend to make things worse when upsizing. For the best quality, scan at the maximum optical resolution, and then down-sample to the required size. Digital cameras, on the other hand, are fairly tolerant of resizing. If the original capture was in focus, using the right software and up-sampling by 200–300 per cent can give you very good results.
A few years back, before digital cameras took over the photographic scene, there were many secret recipes that promised the user a superior resampling quality. One trick was to resize the image not in one go but in a number of smaller steps. Users went to great lengths writing Photoshop actions, but today the available algorithms will be difficult to beat.
Third-party software for upsizing
There are several programs that claim to do a better job than Photoshop. One of the best-known software programs for resizing images is Perfect Resize, which was originally called ‘Genuine Fractals’ when it launched in 1996. Distributed by ON1 software, it costs US$80, and you can download a 60-day trial.
It works as a stand-alone or as plug-in for Photoshop, Adobe Elements, and even Adobe Lightroom. The interface is clean and intuitive (see Image 1), and you simply enter the new image dimensions (per cent, pixels, centimetres, or inches) and the program does the rest. The large preview shows you exactly what you will get, and there are also options for sharpening, film grain, tiling, and gallery wrap. On the left side, you’ll find presets for common printer/paper combinations.
Another program promising you the best possible results is called ‘PhotoZoom’ and comes from the BenVista group (see Image 2). There are two versions, the Pro edition, which weighs in at €169, and the more affordable Classic version for €69.
You can test-drive this program, but you’ll get watermarks all over your enlarged copy. The Pro version is a pleasure to work with — it has a large preview, which can be set up as a split preview. The program offers 12 resizing algorithms, and you get a live view of what the various options are doing to your image. There is the usual Unsharp Mask filter, plus controls for artefact reduction, crispness, and vividness. On top of that, you’ll get handy presets, tailored for regular/soft/detailed photos, reducing