how to copy a photo and edit it and keep the original

There are several different ways to do it. If that picture is in your Mac's Photos library, you can duplicate it in the Photos app (select the image, then Image > Duplicate or ⌘ D), and edit the duplicated picture within Photos.

If that picture is not in your Photos Library, you can duplicate it in the Finder using File > Duplicate (same key combination) and use your favorite image editing tools.

Either method leaves the original picture untouched.

When you edit a photo in Photos, the Photos.app is always preserving the unmodified original photo automatically. You cannot prevent it. The original and the edited version will be stacked as one photo, but you can always revert it to the original version.

So there is no need to duplicate the photo to save the original, unless you want to use the original and the edited version at the same time in a project, or want to have two differently edited versions.

In Photos for Mac you can export the unmodified original as a file with the command "File > Export > Export unmodified original".

John, this lossless workflow is one of the things I really love about Apple's photo managers. It has been the same in iPhoto and Aperture, and is the same in Photos across all devices, as you probably know.

And I like the way it is implemented. Duplicating a photo does not create a second original master file in the library, all duplicates are referencing the same original image file (that is one of the reasons why some cleaning apps are making a mess of the Photos Library, when they replace the originals by the edited versions to save storage).

I am duplicating my photos frequently, for several reasons:

• When I have imported a RAW & JPEG pair and want to see both the JPEG and the RAW original at the same time.
• When I want to change the adjustments, but not lose the previous adjustment
• When I want to have a black&white version in addition to the colour version
• Or for artistic reasons, when I want to compare differently cropped versions

There is a difference between importing a photo twice and duplicating it. When I import a photo twice into a library that is not syncing with iCloud, Photos will store two different original files in the library package and the size of the library will increase.

léonie brought up a good point about the Photos app. The original photos are always saved in their original state, and can be reverted at any time.