DARK KNIGHT/PS3 Aspect Ratio Query...

[DNYCE]

Howdy all... New to this site, so my apologies if this topic has already been addressed... I'm also just now beginning to play with all my HD stuff....

Playing "The Dark Knight" BRay on my PS3 I noticed some aspect ratio changes in certain scenes:

The beginning bank robbery scene of the movie filled the whole screen of my 52" LCD, but in the very next scene, it displayed it in letterbox... I saw this occur with several other scenes and became puzzled as to why the shift in aspect ratios, and began to second-guess my hardware.

I began to surf this site and read its forums on aspect ratios and quickly learned a lot from what aspect ratio is listed in the back of the dvd boxes and which ratios fill the whole screen and which don't. The details on the box for the video display for the Dark Knight BR read:

"1080P High Definition 16x9 Variable 2,4:1 and 1.78:1 (IMAX Sequences)"

(I understand the first & last , but the "Variable 2,4:1" part confuses me)

So, please correct me if I'm wrong, but my understanding of this is that the scenes in which I see the whole screen filled is because ONLY those scenes are IMAX sequences, and as soon as those scenes are over, the ratio drops (is that an appropo term?) to a 16x9 letterbox ratio? If what I surmise is correct, then gee, why not display the whole disc in that IMAX aspect? Any insight on this would be greatly appreciated.

Has anyone else noticed this? Is my hardware OK? And what is the "Variable 2,4:1" mean?

Thank you in advance.