Watch Dunkirk in IMAX 70mm (15/70) not in 70mm
Some have seen Dunkirk in 70mm and most of you have no idea that it was the "wrong format" to watch it in.
If its only written 70mm on your ticket, you're actually watching the "theatrical 70mm version"as the image is showing above. You have to look for IMAX 70MM.
As you can see from the image above, the IMAX 70mm is projected horizontally compared to the others that are vertically, yes that's how huge of an aspect ratio it has compared to the 70mm that most of you thought was the real deal. Not saying you don't know your stuff but before I reserved my tickets I did my research and even after doing so I still ended up watching the "wrong 70mm version" because I wasn't looking for the right one which was confusing since I'm not to familiar about these formats. Lucky me, I live in L.A and there's a couple of IMAX 70MM that I can choose from. So here's a site that will lead you to IMAX 70MM (make sure to look for IMAX).
Also watching a movie in film is risky nowadays because most theaters projects it digitally and I find this wrong and untrustworthy. So this site spots Fake 70mm. Link
Make sure when you show up there, if you can have a quick peek at the projector room. You should find something similar to this in there if its real IMAX 70MM, here's one of the projectors used for Nolan's Interstellar (2014):
Just thought sharing this would help, also here's a format guide more in dept about the differences.
MUST SEE DUNKIRK IN IMAX 70MM ;)