It's a legitimate question: how does the resolution of HDTV at 720p, 1080i, and 1080p compare with what we see in the cinema?
720p, for example, provides 720 rows of pixels, each row containing 1,280 pixels, in an image whose width-to-height ratio is 16:9. The horizontal resolution in "TV lines" (TVL) is 1,280 times 9/16, or 720 TVL. The number of TVL (720) is the same as the number of pixel rows because the pixels are square.
Meanwhile, experts agree that the best, most detailed film elements contain enough fine-detail information to make "2K," "4K," and even "6K" scans worthwhile, when film is being transferred to video by the latest and greatest in film scanning equipment. A 2K scan has 2,048 pixels per row, which at a 16:9 aspect ratio comes to 1,152 TVL. 4K doubles that to 4,096 pixels per row, or 2,304 TVL. And 6K scans yield fully 6,144 pixels per row, or 3,456 TVL.
So one might think that what we see at the local movieplex has resolution that good. But maybe not. According to a SIGGRAPH course called "The Technology of Digital Cinema," “Because of the statistical nature of frame unsteadiness and its attendant resolution loss, the vast majority of motion-picture viewers have been enjoying their
entertainment experience with a good deal less than 700 TVL ... .”
This is a quote from A. Kaiser, et. al., in ”Resolution Requirements for HDTV Based Upon the Performance of 35mm Motion-Picture Films for Theatrical Viewing,” SMPTE Journal, pp. 654-659, June 1985. It seems to be saying that film as projected has a lot less resolution than film per se does.
Not only does "frame unsteadiness" or "image unsteadiness" contribute to that resolution loss, so too do things like "MTF generational loss." MTF is short for "Modulation Transfer Function," the source of an unavoidable loss of detail that occurs any time an image is focused through a lens (see this excellent explanation for more about that topic).
When film is duplicated from its highly detailed original camera negative, going through interpositive and internegative generations en route to making a release print, there are multiple opportunities for MTF to blur the results. Then there's the MTF of the projection lens to consider, and, as noted, the "frame unsteadiness" or "image unsteadiness" that arises because (among other reasons) sprocket holes can be punched in the film with only so much precision.
So, if A. Kaiser is to be believed, the resolution that greets your eye on the screen at the local Bijou is generally less than that of 720p HDTV.
That in no way means that when the film comes out on HD DVD or Blu-ray high-def discs, it won't have all the resolution it nominally should (1,080 TVL). To the contrary, if it is a recent film, it will probably have been 2K- or 4K-scanned from the original negative, or at worst a first-generation interpositive, which provides all the resolution anyone could ask for. Some of that plethora of detail will in fact disappear in the downconversion to a 1080p/24 video master. Even so, with 1,080 TVL, the video master and the eventual hi-def DVD will out-detail anything you typically see at the movies.