From the American Cinematographer story on TDK:
After shooting was complete, and after the editing process was well under way, DKP 70mm scanned select Imax takes at 8K resolution on a unique Northlight scanner. Then, Pacific Title and other facilities made 2.40:1 extractions from the 1.33:1 Imax negative to conform to the framing and movement decisions made in the Avid by Nolan and editor Lee Smith. That process resulted in a 35mm anamorphic negative, which was combined with effects shots and used to generate 35mm release prints.
To bring scenes shot in 35mm to Imax screens, where images are projected in 1.43:1, DKP 70mm scanned the 35mm interpositive at 4K, and an Imax team in Toronto applied digital DMR (Digital Remastering) processing to degrain and sharpen the images. The process stayed at 4K until the images were filmed out onto 65mm back at Keighley's facility and combined with the Imax material for print. The final Imax print combined the 4K DMR filmout, 5.6K and 8K Imax filmouts, and 18K contact prints from the Imax negative, says Keighley.
People suggested Chris and Wally should have covered themselves by shooting key sequences in both 35mm and Imax, but the 2.40:1 extraction from the Imax frame looks beautiful, he continues. In fact, due to the oversampling, it's probably the best 35mm anamorphic image we've ever seen. If we'd had time to scan the original negative at 6K, we could have produced even higher quality. The information is on the negative 35mm film captures the equivalent of 6K and a color bit depth of 14 bits plus.
As they did with the Imax prints of Batman Begins, Keighley and his team screened each of the 80 Imax prints of Dark Knight in real time to ensure quality. We're a small group of hands-on people who really care about images, he says. We pay attention to all the details all the way to the screen.
Tyler Pruitt - Technical Liaison at SpectraCal