how to zoom in on a corner

You can also change the zoom preferences so that zooming by clicking will center where the cursor was clicked. So click on a corner, it will zoom a bit and center, then you can use scrubby zoom to get the magnification that you want.

(EDIT: I removed my post because it was for Camera Raw; I now realize this thread is in the Photoshop forum so I posted the wrong info — sorry. In Photoshop, scrubby zoom won’t solve this problem because it works differently. Try other suggestions, like Chuck’s.)