Scale an image or increase the resolution

A photo that you want to upload or print for a particular purpose may be considered too “low resolution” to meet the needs of a site, print, or online photo service. What options do you have? You can change the resolution of an image to solve the problem.

There are two aspects of a digital image that are often described as “resolution”:

• The dimensions of the image in pixels, such as 3,024 by 4,032 for a modern iPhone photo. Each pixel represents captured information, an example of the hue and light intensity passed through a lens on a camera sensor element.
• The pixel density, or dots or pixels per inch (dpi, ppi), which maps pixels to a desired display size. (You can also create cards by the centimeter, of course.) Most images are captured at 72 ppi, which makes an iPhone image measuring 42 by 56 inches (107 by 142 cm) above when printed or displayed at 72 ppi.

In many cases, the right amount of information is in the image and you don’t need to adjust it. Instead, you need to adjust the scale so that it maps to a website, printer driver, or service that accepts it. For example, printer software may recommend 300 ppi of an input image to produce a 6-color or 8-color 1,200 dpi inkjet print. (Because screen pixels represent millions or even billions of colors, and because printer dots can only reproduce a few colors—as little as black plus cyan, magenta, and yellow on a cheap inkjet—a higher printer dot density is required to match the color of the original.)

You can change the density in the built-in Preview app in macOS:

1. Start example.
2. Open your image.
3. To elect File > Duplicate or hold Option and choose File > Save As to work on a copy of the image.
4. To elect Tools > Adjust Size†
5. Uncheck the “Resample Image” box, which will change the file’s data.
6. Enter the new resolution figure, such as 300 ppi.
7. With inches (or cm or mm) selected for the pop-up menu in Width and Height, you can preview the revised unit dimensions after entering the new resolution figure. Click OK.
8. Save the image.

If you need more image data than is contained in a file, usually because you cropped an image, you can resize it in Preview. Results often look blurry when zoomed in 100% because Preview can’t add new information where it doesn’t exist. It can only fill in new pixels that are adjacent on average.

To “scale up” an image, follow steps 1 through 4 above, then:

1. Make sure Resample Image is checked.
2. Enter a new dimension in units (inch, cm, or mm) or a new pixel width or height. (If “Scaling Proportionally” is checked, Preview automatically resizes the other dimension relative to the aspect ratio, so you don’t have to calculate it).
3. Click Okay†
4. Save the image.

In step 2 above, you can change both the resolution and the unit dimensions if you know the output size and the required resolution. For example, change Resolution to 300 and Width to 12 inches for a 12-inch wide print, and Preview calculates that it should scale the image by 119 percent.

Higher-end photo editing software includes more advanced upscaling routines. Adobe Photoshop offers several through a pop-up menu that lets you choose and preview based on the type of image you’re resizing. You can choose one algorithm for a high-contrast screen capture or image and another for a well-lit outdoor shot.

You can also use software that includes machine learning artificial intelligence scaling that can intelligently incorporate patterns in images to smooth them and fill in enlargements to look more realistic. You can’t add information from a void, but algorithms can remove hard edges and pixelation and smooth out tones, especially on faces.

AI options include features found in newer versions of Adobe Lightroom (Enhance) and Photoshop (Neural Filters); both apps are part of an Adobe Creative Cloud subscription. You can also look into standalone apps or Photoshop plugins, such as ON1 Resize AI ($79.99) and Topaz Gigapixel AI ($99.99).

This Mac 911 article answers a question from Macworld reader Linda.

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