The Secret Keyboard Shortcut for Comparing Before/After Edits in Photos on the Mac
Photos on the Mac provides so many editing tools that it’s easy to lose track of how an edited image compares to the original. You can always use the Revert to Original command and then undo it but that’s fussy.
Instead Photos provides a Show Original button in the upper-left corner between the window controls and the Revert to Original button. Click and hold it to see your original image; let up to see the edited version again.
Even easier press the M key on your keyboard. The only thing either of those techniques won’t do is show the effect of cropping; to see the un-cropped original, press Control-M. And if you just want to see how a particular set of adjustment controls affected the image click its blue checkmark to turn it off and back on.