This is such a simple trick, but it’s incredibly handy if you do a lot of editing in iPhoto. If you’ve gone in and made modifications to an image—you’ve enhanced it, bumped up the contrast, or what have you—holding down the Shift key in edit mode will show you the original file so that you can compare it with the changed version. For example, here’s a photo I took of a Fungus Beetle:
Let’s say that I’ve decided that I need to crop this image in pretty closely. If I select the photo, click on “Edit” in iPhoto’s lower-right corner, and use the crop tool to make that change…
…I can then compare my edits to the original with the Shift key. If I hold down Shift, I’ll see my unedited photo; when I release that key, I’ll see the cropped version. Get it? Shift: Original uncropped bug. Without Shift: Big ol’ closeup beetle. This is a very fast way to make sure you like the changes you’ve made to an image before you move on to something else.
Keep in mind, too, that you can use Shift in this way to see your changes forever (even after you’ve quit the program and reopened it) as long as you’re in edit mode. iPhoto remembers what you’ve modified, so you can always see what you’ve done in the past, or you could go back to where you started by picking Photos> Revert to Original.
A safety net! I like it. Heck, I need it for how clumsily I edit photos. No one’s ever going to confuse me with Ansel Adams.