Resizing vs resampling images in Preview

PreviewPreview is a useful tool on the Mac and is perhaps one of the most used. It is a viewer for a wide range of files and it even has some editing capabilities too. For example, if you load a photo or other image, there is an option to resize it on the Tools menu. It looks straightforward, but should you tick the Resample Image box? What does it do to the image? Is it important?

Whether you resample or not when resizing a photo is very important indeed and it makes a big difference to the result. Sometimes resampling is good and should be selected, but other times it is not and it should be avoided. So how do you know when to use it?

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Load an image into Preview - you can do this simply by double clicking a .jpg file in a Finder window. Go to the Tools menu and select the option to resize it. The width and height can be displayed in several different ways, but I will use percentage because it makes it easier to see what is going on.

Resize images in Preview

Resample an image

Notice that the image size is displayed at the bottom and it is 3264 x 2448 pixels. Make sure that Resample Image is ticked and then change the size to 50%. The resulting image size is shown below and on clicking OK it will be changed to 1632 x 1224 pixels, which is exactly 50% of the original. That makes sense. Notice that there are fewer pixels and this means that information is being discarded. In fact, at 50% it is throwing away every other pixel.

Resize images in Preview

If you shrink the picture by 50% and throw away pixels, you cannot then restore the original by making it twice as big. Once the pixels are gone they are gone.

You can make images larger, so if the image size is increased to 200%, making it twice as big, it would then be 6528 x 4896 pixels. It adds pixels and it does this by guessing what those pixels might be. For example if there is a red pixel next to a yellow pixel it might add an orange one in between.

Making images larger by resampling them makes them fuzzy. The amount of fuzziness depends on the amount they are enlarged by. Making them smaller throws away pixels, but the image remains sharp and it is just smaller. If you throw away too many pixels then thin lines can disappear if they are among the discarded pixels.

Pixels on a TV or computer screen, tablet or phone are a fixed size and number. HD resolution, which is common on TVs, computers and phones, is 1920 x 1080 pixels. The only way to show the image is to throw away pixels and reduce the number from 3264 x 2448 to 1920 x 1080. This web page is about 1000 pixels wide and images are 500 pixels wide or smaller. Pixels are thrown away by resampling to make images fit on the web page.

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Don’t resample

Turn off resampling by clearing the tick box. In the previous image the resolution was 72 pixels per inch, but with resampling turned off, it is 144 pixels per inch. Notice that the resulting size is still 3264 x 2448 pixels and nothing has changed except the pixels per inch.

Printers print by drawing pixels on paper, but unlike computer screens, TVs, phones and tablets, a printer can vary the size of the pixels. It can make them smaller or bigger are desired. By making them smaller, it can squeeze more into every inch of paper. This has two effects and one is that making the pixels smaller makes the image sharper. Think of ‘retina’ displays used on mobile phones and tablets. Printers can make the pixels so small you can’t see them and the image looks super sharp.

The other effect of increasing the resolution is to make the image smaller when printed. Change the height and width setting to inches like this:

Resize images in Preview

You can see that the image will be 22.67 x 17 inches when printed. This is an important point and the size and resolution affect only the printed image. No pixels in the image are changed and it has exactly the same as it always had. You can change the resolution willy nilly and it won’t change the image one bit, just the size on paper should you decide to print it.

Resize images in Preview

Setting the resolution to 300 pixels per inch, which results in pixels smaller than anyone can see and therefore a super sharp ‘retina’ image. The size when printed would be 10.88 x 8.16 inches. That’s just small enough to fit on an A4 sheet of paper.

Resize images in Preview

If you wanted to print on A3 paper which is twice as big, you would have to lower the resolution. At 150 pixels per inch the image would be twice as big, but people with sharp eyesight might be able to see those pixels.

In a nutshell

Resample images when you want to show them on the computer, TV, phone or tablet. This changes the image and pixels are lost, so keep the original image untouched in case it is ever needed again and save the image to a new file.

Don’t resample image that are to be printed. You can just increase the resolution to make it smaller or bigger. It’s not worth printing above 300 pixels per inch because that’s beyond what the eye can see, so resample if you are printing an image very small. Set the resolution to 300 pixels per inch first and then resample it to make it even smaller if necessary.

Related: Edit multiple photos in iPhoto




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