Understanding image resolution
Hans Weichselbaum discusses the various image-resolution options and dimensions, and clarifies which options to use for a variety of image-use situations
The topic may sound trivial, but you’d be surprised at how many professionals, photographers, desktop publishers, and printers are confused by the concept of image size and resolution. No matter what you’ve whipped up in Adobe Photoshop, sooner or later you’ll need to either upsize the file for a super-large print or scale it down for the web — and that’s when the pixels and inches, the ppi, dpi, and lpi, will come back to haunt you.
Size verses resolution; monitor verses printer
The image size is determined by nothing else other than its pixel dimensions. A 12-megapixel camera will give you 4000x3000 pixels to work with. If the image is going to be displayed electronically (monitor, website, projected on a screen), you only need to look at the pixel dimensions; there’s no need to worry about the image resolution. The resolution of a monitor or projector is determined by the hardware. For example, I am looking at a screen 52x32.5cm that displays 1920x1200 pixels. This works out to be 36.9 pixels per centimetre or 93.8 pixels per inch (ppi). If an image is displayed on my monitor at full screen, and it has fewer pixels than my monitor’s native 1920x1200 pixels, it’s going to be up-sampled by the display driver. If the file has more pixels, it will be down-sampled. That’s all done by the display driver, and there is no need to worry about image resolution — though you might notice the poor quality of an up-sampled image.
Alternatively, if the image is heading to a printer, the image resolution becomes important, and you need to look at the ppi settings. Your photo is made up of image pixels — tiny blocks of colour that have no predetermined size. ‘Resolution’ is the number that determines how many pixels get packed into a given space, which, in turn, controls how big or small those pixels are. Commonly, resolution is referred to as ‘dpi’, which stands for dots per inch. This term isn’t strictly correct — it should be pixels per inch, but it means the same thing.
The Image Size dialogue box
If the monitor doesn’t show you the true resolution of an image, what can you trust? Well, you can rely on the Image Size dialogue box. In Photoshop, you’ll find it under Image > Image Size (see Image 1). It doesn’t just show you the size and resolution — from this screen, you can also control both of them.
A lot has changed since the early versions of Photoshop. The new image preview shows you the results of the options you enter before you commit to them. Nothing happens to the file size if you leave the Resample box unchecked. You can change the dimensions (in centimetres or inches) or the resolution (in ppi or pixels per centimetre), and the overall file size will not change — you can’t do any damage to your image.
However, if you tick that Resample box, the program gets ready to change the resolution and the overall pixel dimension of your image. In other words, you can up-sample your image if you need a bigger file for a large print or down-sample to get a small web image. The challenge is to manage this without sending the image quality down the tube.
Remember that ‘up-sampling’ means that your editing program adds new pixels that weren’t originally there. This is called ‘interpolation’ and is done by guessing the colours of the new pixels by looking at existing ones. Needless to say, this won’t give you any image detail that wasn’t there to start with.
Photoshop gives you a whole pile of options in the drop-down menu next to the Resample check box. In the early Photoshop days, there were just three choices: Bicubic (the one to use for photographs), Nearest Neighbour (for line art with hard edges), and Bilinear (which is really useless). Today, Adobe makes it a lot easier by helping you to select the right algorithm for enlargement or for reduction (see Image 1).
Note that there are two options for enlargement — in my next article we’ll compare the two, plus some third-party specialized upsizing software. In short, the Preserve Details (enlargement) option sharpens areas of fine detail, which tend to get softer by up-sampling. The problem here is that image noise also gets enhanced, and, therefore, you’ll get a Noise slider when you choose this option. The other alternative, Bicubic Smoother (enlargement) works similarly to the old Bicubic setting but blurs the interpolated pixels slightly to get a smoother and more natural transition. Bicubic Sharper (reduction) will give you better results when you need to down-sample your images to make them smaller. Of course, you can forget about all those subtleties and simply choose Automatic to let Photoshop pick the best method, whether you’re making your image larger or smaller. Interestingly, the Crop tool and the Free Transform command use this algorithm as well.
Needless to say, the Photoshop guru will prefer to use the standard Bicubic setting, and apply his or her own sharpening/softening/noisereduction magic setting for the perfect outcome.
It’s important to remember that you protect the image’s quality when the Resample option is turned off. If you plan to print the image, you can fiddle with the resolution without doing any harm to the image. You’re just changing the pixel size not the number of pixels. If the Resample check box is ticked, this means that you are changing the number of pixels, either forcing the program to interpolate new pixels or to throw pixels away.
But there’s more: the latest Photoshop version also sports a Fit To dropdown menu (as seen in Image 2). Here, you get a handy list of commonly used sizes, and you can add your own presets too. Note that the given print settings all come in 300dpi — I’ll explain more about that later. Clicking the gear icon in the dialogue box’s top-right corner displays just one menu item: Scale Styles. When selected, this option tells Photoshop to resize any layer styles as well. This is important, because otherwise that pretty drop shadow that you added might end up bigger or smaller than your resized image.
Resizing images for print
Commercial printers routinely ask for 300dpi files (remember: this should actually read ‘ppi’). This figure goes back to the resolution of the first LaserWriters in the mid 1980s, and 300dpi has become something like a magic number for printing (like the 72dpi for monitor display). Strictly speaking, laser and offset printers work with lines per inch (lpi), and that’s the unit we should use instead of ppi.
In offset printing, the print resolution is determined by the halftone screen frequency. The file resolution should be between 1.2x and 2x the screen frequency. Newspapers are printed with a coarse 80-line screen. A 2x factor will give us a file resolution of 160lpi. Glossy magazines typically use a 133-to-150–line screen, and high-quality art is printed with a 200-line screen. At this high level of screen frequencies, a factor of 1.5 is considered sufficient. This gives us a maximum file resolution of 225 for glossy magazines and 300lpi for the best fine-art print.
The Fit To drop-down menu (seen in Image 2) gives you an Auto Resolution option (see Image 3). If you know the lpi of the printer, you can enter it in the Screen field, and Photoshop calculates the resolution (in ppi) for you. You’ve got the choice of three quality settings: Draft gives you
a resolution of 72ppi, Good multiplies the lpi by 1.5, and Best uses a factor of 2
hen you send your files to a professional printer, it’s always a good idea to ask what resolution they want and what colour profile they prefer. If they don’t know what you’re talking about, find another printer — fast!
Today’s high-resolution inkjet printers work by putting 1440 to 5760 drops of ink per inch onto paper. Now, here we are talking dots or drops per inch, so dpi is the correct unit. Every pixel of your file needs to be dithered with many ink drops to simulate thousands of colours. You certainly don’t need to (and you shouldn’t) feed your printer with 1440 to 5760ppi files.
First, a print resolution of 1440dpi (dots or drops of ink per inch) is plenty for matte and fine-art paper, and you will be hard-pressed to tell the difference between 1440 and 2880dpi, even on the best photo-gloss paper. Second, the resolution of the file should be an integral divisor of the printer resolution. Most experts agree that a file resolution of 240ppi is sufficient for photographic images, which happens to be 1/ of 1440. If you are a
6 perfectionist, I suggest you make a series of prints of one of your supersharp photos. Resample the file (preferably down-sample) to resolutions of 180, 240, 300, 360, and perhaps 480ppi, and print to identical physical size. Use the highest-quality setting on your printer. You will probably need a loupe to distinguish the 240ppi print from anything higher.
Due to the viewing distances, poster prints can be done at fairly low file resolutions, typically 150ppi down to 75ppi. If the file resolution is too low, we will start to see individual pixels, most evident in the ‘jaggies’ on diagonal sharp lines. If you print a generic-size image (for example, 5x7 or 8x10 inches) straight from Photoshop, you can turn on Scale to Fit Media in the Print dialogue box (see Image 4). The program will calculate the resolution for you, but you won’t have any control over how the image is being cropped to fit the page.
Another clever way of resizing images is to use Russel Brown’s Image Processor, which can be found via File > Scripts > Image Processor (see Image 5).
This script comes in particularly handy when you want to resize a whole folder full of images. It also allows you to resize landscape and portrait shots to a maximum dimension by entering the same number for both height and width (don’t worry, you won’t get square images).
A list of ways to resize images would not be complete without mentioning the Save for Web command. It gives you superb control when downsizing images for the web. We’ll discuss it in another instalment when we focus on web images.
In the next issue, we’ll look at various ways of up-sampling an image using Photoshop’s options and compare the image quality we obtain with that obtained using some third-party programs.
Image 2 — the Fit To options of the Image Size dialogue box
Image 1 — Image Size dialogue box in Photoshop CC 2015
Image 4 — scaling print size in the Print Setup interface
Image 5 — the Image Processor script
Image 3 — the Auto Resolution option for offset printing