When shooting 2D objects (e.g., artwork), what is the easiest way to make sure that the optical axis of the camera is aligned perpendicular to the object? That is, how do I avoid any perspective distortion?
Of course I could just try to stare at the tiny viewfinder or the LCD screen and try to guess when it is aligned properly, but all too often you notice the tiny distortions only afterwards when you look at the photos on a large screen (for example, if the object is a perfect square and you crop it tightly, tiny distortions result in uneven margins around the object). Tethered shooting is one option, but what other tools or tricks one could use?
The best solution so far is the following: Replace the object with a mirror. Then (using liveview & zoom), align the camera so that I can see the reflection of the very centre of my lens precisely in the middle of the picture. Then put back the object and take the picture.
It works but it is not perfect; for one, it is not that easy to both align the camera perpendicular to the object and simultaneously keep the horizon level. The horizon is of course easy to fix in post, but it would not hurt to get it right directly.