Short answer: conventional 70mm and IMAX 70mm both use large-format film, but they arrange each frame very differently. In the conventional system, the camera normally shoots on 65mm negative that runs vertically and uses 5 perforations per frame. The release print is 70mm. In film-based IMAX, the negative is also 65mm, but it runs horizontally and each frame spans 15 perforations. In the theater, the print is 70mm 15/70. This changes the physical frame size, film consumption, image shape, and the way the picture fills the screen.
70mm and IMAX 70mm are not the same format
The name can be confusing because both arrive in the projection booth on 70mm film. Kodak distinguishes 65mm as a camera film gauge and 70mm as a release-print gauge. In conventional 65mm, the film travels vertically, with a frame height of 5 perforations. In IMAX, the same family of 65mm film is transported horizontally and the frame extends across 15 perforations.
That is why the term 15/70 appears in film-based IMAX screenings: 15 perforations per frame on a 70mm projection print. Conventional 70mm is normally associated with the 5-perforation arrangement, also called 5-perf.
How much does the frame size change?
The difference is visible in the geometry of the film itself. In a format comparison, Kodak lists a capture area of 52.15 x 23.07 mm for 65mm 5-perf and 70.41 x 56.62 mm for 65mm 15-perf IMAX. These measurements describe the physical area listed by the manufacturer and should not be confused with the final projection aperture, which can involve framing and cropping.
| Feature | Conventional 70mm | IMAX 70mm |
|---|---|---|
| Camera negative | 65mm | 65mm |
| Camera transport | Vertical | Horizontal |
| Perforations per frame | 5 | 15 |
| Capture area listed by Kodak | 52.15 x 23.07 mm | 70.41 x 56.62 mm |
| Typical presentation aspect ratio | 2.20:1 | Up to 1.43:1 in a 15/70 presentation |
| Approximate running time of 1,000 ft at 24 fps | 8 min 53 sec | 2 min 57 sec |
The roll times make the scale of the system easier to picture. Because each IMAX frame consumes much more film length, a 1,000-foot roll lasts about one third as long as a 5-perf roll at 24 frames per second, according to Kodak's film-format comparison.
Why does the camera use 65mm while projection uses 70mm?
Historically, the extra width of a 70mm release print provided room along the edges for magnetic sound tracks. The Kodak technical guide explains that images made on 65mm were printed to 70mm and that the extra width once accommodated six magnetic sound tracks. Later systems used separate synchronized audio controlled by time code.
So a movie being shot on 65mm and shown in 70mm is not a contradiction. One number describes the camera negative gauge, while the other describes the projection print gauge.
What changes in projection?
In conventional 70mm, the 5-perf print preserves a wide image, traditionally associated with a 2.20:1 aspect ratio. The projector advances one five-perforation frame at a time in the film's vertical orientation.
In IMAX 15/70, the frame is laid sideways across the film. The system requires a dedicated projector designed to transport and stabilize a much larger image area. The 15/70 presentation was also developed for very large screens and, when the movie and theater use the full frame, the image can reach 1.43:1. IMAX states on its page for Oppenheimer in IMAX 70mm that sequences photographed in the format can expand vertically to fill the screen at compatible 15/70 locations.
What does the viewer actually notice?
The easiest difference to notice is not an abstract resolution number, but the shape of the frame and how much of the screen it occupies. 5-perf 70mm tends to present a very wide image suited to panoramic compositions. IMAX 15/70 can use a taller frame, revealing more image above and below the center, provided the movie was captured and finished for that framing.
The physically larger IMAX frame also provides more area in which to record detail. That does not mean there is a single exact conversion to a pixel count. Film has no native pixel resolution. Perceived sharpness and detail depend on lenses, focus, film stock, exposure, processing, print generation, projector alignment, the screen, and screening conditions.
It is also inaccurate to assign one automatic color or contrast look to IMAX 70mm. Those qualities depend on the negative, cinematography, lab work, print, and projection. The format increases the physical potential of capture and presentation, but it does not replace cinematographic choices.
Why do some movies change aspect ratio during a screening?
Productions can combine 65mm 5-perf and IMAX 15-perf. Kodak describes this type of workflow in material about Dunkirk, which used both formats and had different release versions. On an IMAX 15/70 print, scenes designed for the IMAX frame can occupy more screen height, while material originating in another format may appear with a different image area.
This means an aspect-ratio change can be intentional. By itself, it does not indicate a projection error. Final framing depends on the capture format, the version prepared for that theater, and the choices made by the director and cinematography team.
How to read a screening listing without mixing up the formats
- 70mm: normally indicates photochemical 70mm 5-perf projection unless the theater specifies otherwise.
- IMAX 70mm or 15/70: identifies the analog IMAX presentation with 15 perforations per frame.
- IMAX: by itself does not guarantee 15/70 film projection because the network also operates digital systems.
- Shot with IMAX cameras: describes how certain scenes were captured and does not guarantee that the screening you choose uses an IMAX 70mm print.
Practical summary
Conventional 70mm and IMAX 70mm belong to the same large-format film family, but they do not use the same frame design. The first uses 65mm 5-perf in the camera with vertical transport and reaches projection as 70mm 5-perf, usually at 2.20:1. The second uses 65mm 15-perf with horizontal transport and reaches the theater as 70mm 15/70, where it can exploit a much larger and taller image, up to 1.43:1 in compatible presentations. For viewers, the difference is most apparent in frame scale, image height, the field of view occupied by the screen, and the detail potential of photochemical projection.
