One of the most awesome tools in Photos isn’t new, but for some reason, it’s quite a bit less obvious than it was in iPhoto. So let’s say you’ve taken a picture, and something in the image isn’t angled correctly. Here’s an example from my own library:
See how the horizon line there is off-kilter? Believe it or not, this is pretty simple to fix; in iPhoto, there’s a big ol’ “Straighten” button within Edit mode that’ll let you do just that. In Photos, that option is still there, but the tool is less “here’s a straightforward button” and more “look, a pretty design element!” Oh, Apple.
In any case, to straighten an image in Photos, first double-click on the image you want to correct, and then choose the “Edit” button in the upper-right. When you do so, you’ll see your tools appear along the right side. “Crop” is the one you’re looking for.
Within “Crop,” there are a lot of options. You could drag the handles along the edges of your image to, well, crop it. You could also use the buttons at the bottom to change the aspect ratio, flip the image horizontally, or let Photos attempt to straighten and crop your picture (by clicking the “Auto” button).
Alternatively, straightening the image yourself is pretty easy, even if it’s not as obvious as I’d like. When you’re in Crop mode, a small tilt wheel will appear along the right side of your chosen photo.
Clicking and dragging on that wheel will let you adjust the angle of your image, and in the case of this picture of mine, I’m gonna use the helpful grid lines that appear to make my horizon perfectly straight.
Boom! And that’s all there is to it. Of course, you can click on the “Done” button in the upper-right when you’re happy with your work, or the “Revert to Original” button is there in case your editing goes horribly awry. As mine often does.