I seen an never ending search of the "perfect level" both on this site and others. I've seen people bemoan the fact that the level attached to their tripod is perfect; likewise the level on their head. This is being posted in "Medium Format Systems..." as this is the place where I see this quest the most. I've kept quiet until now....
As many may know I am primarily a landscape and nature photographer with a limited amount of wildlife thrown in to shake things up. My primary capture method is medium format using a combination of a tech cam as well as a Phase One DF; using a Phase One IQ160 back. I have a sturdy tripod and head which I use to attach my gear to and all told if I count each and every level system I have at least 2 to 3 depending which one I'm using.
I've tried to have my tripod perfectly level. I then attempted (and failed) to have the head perfectly level after leveling the tripod. I then attempted (and failed again) to have my camera body perfectly leveled once I had the tripod and head near level. I spent nearly an hour one time as an experiment seeing if I could ever get the "perfect level". And failed.
I learned quickly on that I need to trust one leveling system and choose the levels on my tech cam. I figured that the ground I was standing on wasn't level and what I needed was to have the tripod as sturdy as possible and just level the camera. Using this method I've captured multiple images where I stitched them together with near 99% success rate. Then Phase One came out with the IQ back that had yet another leveling device and I tried using that and failed. The failure wasn't so much the fault of the back than it was mine; it took too much time that resulted in the same end result. I've returned to using the levels on my tech cam.
My holly grail isn't so much of getting the perfect level. What I'm attempting to achieve is being able to shoot multiple captures which when stitched together achieve 100% pixel coverage. So far I've gotten as close as 98-99%.
Even getting as close as I have gotten there are times when I look at the image file knowing I was level (where I was standing) and see that the horizon looked "off". It happens and then what you need is good software to correct the problem of perception.
In the end I've learned that in shooting in the outdoors there simply isn't a level place and you need to learn to deal with it. Or, go nuts looking for the perfect level. My recommendation is to pick a level and stick with it.