As some of you have pointed out, the shortcuts used to navigate through an open document in Photoshop (to make sure that you don’t miss any spots from sensor dust for example), are slightly different than when navigating through an open document in Adobe Camera Raw and Lightroom.
Tapping the Home and End keys in ACR and Lightroom take you to the top-left and bottom-right corners of the picture, respectively. These shortcuts are the same as Photoshop. Likewise, tapping the Page-down key takes you down 1 full screen: the same as Photoshop.
Unlike Photoshop, however, if you’re already at the bottom of the image in ACR or LIghtroom, tapping the Page-down key again takes you back to the top, and to the right by 1 full screen. So, if you start at the top-left of the picture, pressing page-down repeatedly will take you through your image, 1 screen at a time, till you’re at the bottom-right corner of the picture. Page-up does the same thing, but in the opposite direction.
In a nutshell, think of your picture like a book, with the top-left corner as the beginning, and the bottom-right corner as the end. Press Home to visit the beginning, then press Page Down till you get to the end. By doing so, you will see every single pixel of the image at least once.
Folks on the ACR and Lightroom team (myself included) think this variation is an improvement over Photoshop, because for those of us who need to do final inspection of their pictures (e.g., to make sure there aren’t any dust spots, etc.), it’s important to have an easy way to make sure we’ve seen every part of our pictures up close. With Photoshop, I have to remember where I am in the picture, because if I’m in the bottom-left corner of the picture, then tapping the Page Down key does nothing. In ACR and Lightroom, I have a guaranteed way to see all the pixels in the image, and Page Down/Page Up shortcuts allow me to continue navigating regardless of where I am in the picture.
You might not agree – which is absolutely fine, but now you know why the behavior is different between the programs. : )
Thank you Eric for helping me to explain this and for offering the book example above!