How to scale an image or increase its resolution

A photo that you want to download or print for a specific 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 fix the problem.

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

• The dimensions of the image in pixels, like 3024 by 4032 for a modern iPhone photo. Each pixel represents captured information, a sample of the hue and intensity of light that 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 preferred display size. (You can also map to centimeters, of course.) Most images are captured at 72 ppi, which results in an iPhone image with the above dimensions of 42 by 56 inches (107 by 142 cm) if printed or displayed at 72 ppi.

In many cases, the right amount of information is in the image and you don’t need to edit it. Instead, you must change the scale so that it is mapped so that a website, printer driver, or service accepts it. For example, printer software may recommend 300 dpi from an input image to produce a 6 or 8 color inkjet print at 1200 dpi. (Since screen pixels represent millions, if not billions of colors, and printer dots can only reproduce a few colors (as few as black plus cyan, magenta and yellow on cheap inkjet), a higher print dot density is needed to simulate the color of the original.)

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

1. Start preview.
2. Open your image.
3. Picking out File > Duplicate or hold down the Option key and choose File > Save As work on a copy of the image.
4. Picking out Tools > Adjust Size.
5. Uncheck “Resample Image”, which changes the file data.
6. Enter the new resolution number, for example 300 ppi.
7. With inches (or cm or mm) selected for the context menu in Width and Height, you can preview the revised unit dimensions after entering the new resolution figure. Click OK.
8. Save the picture.

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

To “upscale” an image, follow steps 1-4 above, then:

1. Make sure the Resample Image option is checked.
2. Enter a new dimension in units (inches, cm, or mm) or a new width or height in pixels. (With “Proportional scale” checked, Preview automatically scales the other dimension proportionally to the ratio of the image, saving you from having to calculate this).
3. Click on OKAY.
4. Save the picture.

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

High-end photo editing software includes more sophisticated scaling routines. Adobe Photoshop offers several via a context menu that lets you choose and preview based on the type of image you’re resizing. You can choose one algorithm for high-contrast screenshot or image and another for well-lit outdoor shooting.

You can also look to software that incorporates machine-learning AI scaling that can intelligently pick up patterns in images to smooth and fill in enlargements for a more realistic look. You can’t add information from a void, but the algorithms can remove hard edges and pixelation while smoothing out tones, especially on faces.

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

This Mac 911 article is in response to a question submitted by Macworld reader Linda.

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