I recently sent an image file to a client who had plans to enlarge it to more than five feet wide for a trade show display. Straight out of the camera, my file was more than 5,000 pixels on the long side, and the printer’s specifications said it should be 7,000 pixels, so I fired up Photoshop, used the Image Size controls to increase the file size, saved it and sent it over. But what I didn’t realize was that I had sabotaged my chances for success by making at least one crucial mistake.
First, you’ll want to look at your image file at 100% enlargement so that you know what it looks like prior to enlargement. I actually made a screenshot (on Mac computers, it’s Command+Shift+3) so that I could go back and compare the quality and details after the up-res is complete. The main thing to look out for is angled lines that appear straight and smooth in the original file can become stair-stepped, jagged edges after increasing the resolution.
The second step is actually waiting. Specifically, you want to wait to do any sharpening (or at least all but the mildest RAW sharpening) until after you’ve increased the file size. This was the mistake I made, having created a very sharp finished image file that I then tried to up-res. That file full of contrasty edges really played havoc with Photoshop’s ability to keep smooth diagonals smooth, as you can see in the accompanying screenshots. If you use Lightroom or another RAW processing program that allows for adjustments to “clarity,” you’ll want to keep that slider fairly low, as well. The more contrast and sharpness you create, the harder time Photoshop is going to have seamlessly interpolating the missing pixels.
With your image open in Photoshop, choose Image Size from the Image dropdown menu. You’ll see a window with the pixel dimensions of your file, as well as the document size. With all three check boxes checked (Scale Styles, Constrain Proportions and Resample Image) you’ll input new pixel Dimensions at the top of the window. Choose the longest dimension you need and modify it, and watch to see that the other dimension follows suit. (If it doesn’t, you didn’t have “constrain proportions” checked.) You’ll also see that the document size changes, too.
The key to quality upsizing in this window, though, is down at the bottom: Resample Image. You’ll want to use Bicubic Smoother, which Photoshop helpfully explains is “best for enlargements.” If you’re dealing with an image that has a subtle gradient (say a prominent blue sky) you may have better luck with straight Bicubic resampling. What you definitely want to avoid when up-sizing an image is Bilinear and Nearest Neighbor resampling. The Nearest Neighbor example here, with its jagged lines and loss of quality, shows just how problematic it is for upsizing an image file.
Once you’ve upsized the image using Bicubic Smoother resampling, you’ll have a new, larger image file. Depending on how much it grew, any degradation of the image quality will fall somewhere between imperceptible and totally unacceptable. The less Photoshop has to interpolate (fill in the spaces between the original pixels with guesses about new pixels) the better your results will look. While there’s no hard and fast rule, if you can keep the new size to within 120% of the original file’s dimensions, you’re going to be much happier than if you push it farther. Things really start to fall apart fast at greater than 150% enlargement.
Once you’re ready to save the file, that’s the time to apply the appropriate sharpening (again while viewing the image at 100% to ensure you’re not messing up the pixels) using your preferred Photoshop sharpening technique. For me, that’s Unsharp Mask or, more often, Smart Sharpen in the Sharpen section of the Filter menu. Then, save the file as either a TIFF (which is naturally uncompressed) or a JPEG with minimal compression. Too much compression is going to degrade the image file more noticeably. In my experience, a minimally compressed JPEG is all but indistinguishable from an uncompressed TIFF.
If upsizing image files is a standard part of your workflow rather than a once-a-year occasion, you might consider investing in software such as Perfect Resize from onOne software. Though Photoshop has made vast improvements to its capabilities in recent generations, the technology in Perfect Resize (once known as Genuine Fractals) was the industry standard file resizing software. Hopefully, though, your camera can provide a native file that’s large enough for the vast majority of your work. If not, it may be time to consider investing in a camera with a larger sensor. It’s the sensor, after all, that determines the pixel dimensions of your pictures.