Use the preview app to reduce multiple images at once on a Mac

Convert photos in the preview app
Convert photos in the preview app

Convert photos in the preview app

Today I had another interesting question from a reader: She had problems with software that only allows photos in a certain maximum size. There are many reasons why you want to calculate smaller photos or images on your Mac - saving memory, adapting to a size suitable for the web or the like can be the reason. There are also tons of tools for this that cost more or less money and provide a batch resize function. But I wanted to see whether it couldn't be done with the on-board tools of macOS and found what I was looking for with the built-in preview app of the Mac operating system. This software really has more talents than one might think. And as a little treat, the preview app not only calculates JPEGs but also PNG graphics smaller.

The procedure step by step

Here are the individual steps in detail, how to reduce the size of the images in the "Preview" program:

  1. since the process will directly overwrite the original images, I recommend that you make a backup of the graphics
  2. Marks the photos in the Finder that you want to convert (you can select multiple files by holding down the cmd key)
  3. either double-click on the selected files or right-click (or control-click with the mouse) on the files and then select "Open with" and then click on "Preview.app" in the submenu
  4. Now all photos are open in "Preview" and you select all of them in the left bar (keyboard shortcut cmd + A does the same)
  5. At the top of the menu there is the item "Tools" and in it the function "Size correction" ... you select this
  6. Now a window opens in which you can make various settings. I recommend keeping the proportional change and changing the dimensions to "Pixel" ... now you can enter your desired pixel dimensions and you will get a preview below of how the file size will change
  7. Click on OK if you are satisfied (but be careful! The preview will IMMEDIATELY overwrite the files and DO NOT save them as new files!)
  8. if you want to start all over again, you can cancel the action with the keyboard shortcut cmd + Z and try again ;-)

Zooming out with the preview app explained in screenshots

Here I have documented the process for you using screenshots. Maybe this will help you to understand the individual steps. You can also click on the screenshots to see them larger.

Via the context menu, click "Open with" to call up all selected images in the preview app.

Via the context menu, click "Open with" to call up all selected images in the preview app.

The images in the preview must all be marked: The keyboard shortcut CMD + A does this job.

The images in the preview must all be marked: The keyboard shortcut CMD + A does this job.

Under "Tools" -> "Size Correction" you call up the dialog window via which the new image sizes can be set.

Under "Tools" -> "Size Correction" you call up the dialog window via which the new image sizes can be set.

Here you can see the window in which you can make the appropriate settings for the size conversion.

Here you can see the window in which you can make the appropriate settings for the size conversion.

The info window in the preview app shows whether the resizing worked.

The info window in the preview app shows whether the resizing worked.

I hope the contribution helps you with the conversion and saves a few euros. You don't always have to shoot pictures with Photoshop. Sometimes a small, free tool from Apple will do the graphics jobs too. ;-)

If the contribution helped you, I would be happy if you would share it on the social networks. Thanks!

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11 comments

  1. Kati says:

    Wow, thanks for this tip! It makes life easier for you. Am thrilled! Do you also have a tip on how to rename several pictures at the same time? So give them all the same name, maybe numbered?

    • sir appleot says:

      Hello Kati! Yes, you can even use the Finder without any additional tools. You mark all files that you want to rename and then click with the right mouse button (or with CTRL + mouse button) on the files and then select the item "Rename x objects". Then there is a small menu in which you can set how the files are to be renamed. :)

  2. Kati says:

    Great thank you! I'll try that out at the next opportunity :-)

  3. Dirk says:

    Hello Sir Apfelot,
    Thank you for the tip.
    However, I am looking for a program that is equivalent to the Paint program on the IMac. For example, to briefly mark something, to incorporate a circle or an arrow. Which program were used to process the photos shown above?
    Thank you for a tip.
    Regards

    • sir appleot says:

      Hello Dirk! Oh, you don't need a special program for that. I always do that with Apple Mail! :) You drag a screenshot into a new mail, then you click on the arrow in the picture in the top right of the mail and select "Marks". The screenshot will now open in a mini editing tool that you can use to add arrows, circles, ellipses and text. When you're done, you close it and have the revised version in the mail. I then simply drag this onto the Photoshop symbol and export it as JPG or PNG. But you can also copy and paste it into any other program. I am going to write a little manual about it every day, because it is really 10 times better than all the programs that you can buy especially for this purpose. Apple Mail is my favorite secret weapon ... and it's free!

      • Olli says:

        Moin,

        First of all, thanks for the great tip Sir Apfelot.
        A little hint for quick marking: Open the image in preview, go to Tools and then select "Annotate". There you can easily and quickly insert text, arrows, etc.

        Greetings from China ^^

  4. Hello Sir Apfelot

    Thank you for your tips, described in a very uncomplicated way and I was able to implement it right away :)

    Greetings from Schindellegi

    Monica

  5. Agnes says:

    Hello!
    I've had a storage problem since shrinking my photos. I have now gained 20 Gb in the folder with my images. All in all, however, I was suddenly shown 20 Gb less on the computer. I restarted and am back to the space I had before shrinking. So from 37 down to 19, back to 37. I can see the 20 Gb less in the folder information, but not generally on the computer. I thought there were cache files or duplicates somewhere, but CleanMyMac didn't find anything. What can I do now?
    Thank you very much
    Agnès

    • sir appleot says:

      Hello Agnès! So actually CleanMyMac should clean everything that concerns caches. How did you "downsize" your photos? Maybe this is the problem and the originals are still somewhere?

  6. Klaus says:

    Hi Jens,

    that's basically how I do it. Only in the last step I export the scaled down files to a subfolder instead of overwriting the large ones.

    This way I keep the originals and have the other size copies in a subfolder.

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