|1.43 ↔ 2.20
||IMAX 70mm and IMAX GT Dual Laser
|1.90 ↔ 2.20
||IMAX Single Laser and Digital IMAX
||Non-IMAX digital and 70mm
I will explain what each of these are in case any of them are unfamiliar, but the easiest way to remember it is that all IMAX presentations of Oppenheimer have ratio changes, all non-IMAX presentations have a fixed aspect ratio. As stated on the official Oppenheimer website:
IMAX is the only experience to offer the film in 1.90:1 aspect ratio as well as 1.43:1 in select locations.
To understand the reason for the different aspect ratios it will help to first go over some basics about IMAX 70mm and the differences between it and the other formats that Oppenheimer is being released in.
This is the original film IMAX which is projected onto giant screens. Often referred to as 1570 IMAX which stands for 15-perf 70mm, referring to both the size of the film and the number of perforations, i.e. sprocket holes per frame.
IMAX 15-perf film cell replica
The above photo is of part of a promotional replica film cell strip that was given out only at IMAX 70mm screenings of Oppenheimer and only on the first three Fridays during the original planned IMAX run, although the run has now been extended to the end of August. I got my promotional film strip on Friday, August 4th which was the third and as far as I know final week of the promotion. Each of the three weeks the promotional strips had a different set of three photos.
The replica accurately depicts how the images on the film are oriented sideways compared to regular 70mm and 35mm film. For this reason the film moves through the camera and projector horizontally instead of vertically like it does in standard formats. This is why the IMAX image is so large compared to regular 70mm, which is also known as 5-perf 70mm. If you count five perforations in the above photos you can get an idea of how much bigger IMAX is compared to regular 70mm. And 5-perf 70mm film is already much larger than standard 35mm film which is the size that most movies were shot in prior to the digital era.
Approximately 25% of Oppenheimer was filmed in 15-perf 70mm IMAX. These scenes, actually in most cases individual shots within scenes, are shown in the standard 1.43 IMAX aspect ratio. All other shots were filmed in standard 70mm and are shown at a 2.20 aspect ratio.
I created the comparison below by cropping an IMAX shot to the smaller 2.20 aspect ratio. The top view represents the 1.43 aspect ratio of IMAX shots as they appear on the screen. The bottom view represents the 2.20 aspect ratio of 70mm shots.
During the movie it is regularly switching back and forth between these two aspect ratios. Of course it doesn't switch with the exact same image like this, but sometimes a scene might start with a 1.43 IMAX view of a lecture hall that fills the entire screen for the first several seconds, and then it switches to a 2.20 70mm closer shot of the actors that are speaking. You would think that the constant switching back and forth would be distracting but it's not, they made it pretty seamless.
The other answer that someone gave to your question had some reference links for the aspect ratios for the 70mm IMAX presentations. Which by the way yes this is what you saw at BFI. I saw it at Regal Mall of Georgia, one of only 19 theaters in the U.S. showing Oppenheimer in 70mm IMAX. There are 30 theaters worldwide showing Oppenheimer in 70mm IMAX.
For anyone who has never been to an original style IMAX theater, the screen is huge. It is as wide as a standard movie screen in a large theater, however in an original style IMAX theater the screen extends all the way to the floor and to the ceiling. And the ceilings are quite high, at least 50 ft (15 meters) and even higher.
Regal Mall of Georgia IMAX 59 ft x 81 ft screen
For scale notice the two people walking directly underneath the IMAX logo near the bottom of the screen.
The shape of an original IMAX screen is similar to the older television and pre-1950's movie formats which had an aspect ratio of 1.37. Viewing regular films on an IMAX screen looks normal, you don't really notice the blank sections of screen above and below it. But when it switches to IMAX the picture now fills the entire wall floor to ceiling and side to side, with an extremely sharp picture. That's why so many people make such a big deal about it and make an extra effort to get to one of these theaters to see Oppenheimer.
As a side note, the actual camera film used for 70mm films dating back to the 1950's has always been 65mm. The image is then transferred to 70mm print film for distribution to theaters. Originally the extra 5mm was used to hold the magnetic multi-channel sound track. With the advent of digital sound, 70mm prints now only have an optical timestamp which is used to synchronize with a separate sound source. IMAX is also shot on 65mm film.
4K Dual Laser Digital IMAX
Often referred to as GT Laser (Grand Theater, apparently named after the original of the four types of IMAX 70mm film projectors), these dual projector systems are used for IMAX screens more than 80 feet (24 meter) wide (Display Daily).
The IMAX dual laser system is the only digital IMAX that can project IMAX at the same 1.43 ratio as the original 70mm IMAX (Digital Trends).
IMAX dual laser theaters display Oppenheimer in the standard 1.43 IMAX aspect ratio for IMAX scenes, and 2.20 for the scenes shot in standard 70mm.
These are the same aspect ratio changes as the 70mm IMAX presentations, however the method used is a bit different. Besides being digital, the projector's maximum native aspect ratio is 1.90, so an anamorphic lens on the projector is used to expand the projected IMAX image to 1.43. This does not distort the image because during the creation of the GT digital print the IMAX image is digitally squeezed, so the projector in the theater is merely stretching the image back to its original aspect ratio.
There are currently about sixty IMAX dual laser theaters worldwide. The London Science Museum has one, although they are currently showing Oppenheimer on a 70mm IMAX projector.
There are currently 24 theaters worldwide showing Oppenheimer in dual laser IMAX. These can be seen in an Oppenheimer location map that someone posted online. On the left side of the map there are some red check mark boxes which you can use to select or de-select different types of theaters.
4K Single Laser Digital IMAX
Also known as IMAX with Laser, although dual laser GT systems are sometimes advertised with this name also.
This single projector laser system is actually newer than the GT dual laser, and is used for smaller IMAX screens up to 80 feet wide. There are about 300 single laser IMAX theaters worldwide.
The IMAX scenes are cropped to the projector's native 1.90 aspect ratio. The standard 70mm scenes are shown in 2.20 aspect ratio.
Also known as xenon digital IMAX. This is the original digital IMAX which uses xenon projection bulbs similar to standard projectors. This is by far the most common type of IMAX theater, with over 1,000 theaters worldwide (IMAX Corporation).
Similar to single laser IMAX, the IMAX scenes are cropped to the projector's native 1.90 aspect ratio. The standard 70mm scenes are shown in 2.20 aspect ratio.
In non-IMAX theaters, which are all digital except for special film screenings, the entire movie is presented in a fixed aspect ratio of 2.20.
In other words the IMAX scenes are cropped from 1.43 to 2.20. However the movie was produced with these theaters in mind so nothing critical is missed in these presentations, you just don't get the more immersive experience of the larger 1.90 or 1.43 aspect ratios that you get in IMAX theaters.
About 120 theaters worldwide are showing a standard 5-perf 70mm film print of Oppenheimer. The entire movie is presented in a fixed aspect ratio of 2.20.
About 70 theaters are showing a 35mm film print of Oppenheimer. The entire movie is presented in a fixed aspect ratio of 2.35.
Even though the 35mm film frame is smaller, the projected image is enlarged to fill the screen side to side, so the viewing size will be similar to 70mm, although this will result in less image sharpness and brightness than 70mm. Also there is a little more cropping of the top and bottom of the frame due to the 2.35 aspect ratio compared to 2.20.