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Preview JPG Rotation

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exif rotation
I never noticed that images I rotated in Preview weren’t really rotated. Then I happened to notice a rotated jpg in my Gmail sent items.  It wasn’t rotated.

If you rotate a jpg in Preview and you want it to appear rotated for people you might send it to, clear the checkbox labeled Use Exif Orientation Tag.  The box will appear when you’ve rotated a jpg in Preview.

What’s this all about? Leaving the box checked adds information to the Exif data for the image. That information can be read by some software products that will then display the photo rotated the way you’ve set it.  And your image won’t be altered in the rotation.  If a user opens the image with software that doesn’t read the tag the photo won’t appear rotated correctly.

For example, I saved a rotated jpg with the box checked. Lightroom displays the image rotated. Opening the image directly with Firefox does not rotate it.

The downside of not checking the box is lossy rotation. Pixels are rearranged to create your rotated image, and data is lost in the process. For certain photos you might not be concerned with this, but it’s always good to know how certain software rotates your jpg files. Especially if you don’t have a backup.

More on lossy rotation at Apple Support.

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2 comments to “Preview JPG Rotation”

  1. The (unfortunately only PPC) Contextual Menu Plugin PhotoTool supports lossless transform of JPGs: http://www.pixture.com/software/macosx.php a great way to (even batch) rotate JPGs without re-encoding.

  2. Aha, I’d always wondered why it did that.

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