I carry the bubble level in the shoe but use it by holding it against the viewing screen (with some black masking tape on the side which contacts the view screen to avoid scratches). There are a couple of advantages here. One-the viewing screen has proven to me to be parallel to the sensor (at least on the 5D and 5DII). Two-the bubble level is small, cheap and conveniently carried in the shoe. And three and most important, when shooting at twilight, the image on the screen provides enough illumination to read the bubble.
Karl inquires......can you share any more details on your technique for using a bubble level on the 5DII viewing screen? Which make/model of bubble level you use... how you level the camera left-right... and the accuracy you're getting?For leveling left to right, I eyeball it with the help of a gridded focusing screen in my camera. This won't help Karl. He might want to look at some kind of leveling plate for his tripod? The Manfrotto level I use is from B&H. Hot Shoe Bubble Level
The reason I ask is that I'm shooting old, tilted ranch buildings for a cultural resource survey on a wildlife refuge. There's not a true horizontal or vertical in any of the buildings and accurate camera level is critical, both in the up-down pitch for leveling the 24 TS-E and left-right roll for the buildings' verticals....
Juergen Sattler over at the Largeformat Forum suggested this.
I'm not thrilled with having to carry more spare batteries but....it looks like it has potential. It can be "calibrated" if you have a crooked hot shoe and is available in audio or visual signaling versions: ZigView available from Adorama