2.35, 2.39 or 2.40?
One common misconception about the anamorphic format concerns the actual width number of the aspect ratio, as 2.35, 2.39 or 2.4. Since the anamorphic lenses in virtually all 35 mm anamorphic systems provide a 2:1 squeeze, one would logically conclude that a 1.375:1 full academy gate would lead to a 2.75:1 aspect ratio when used with anamorphic lenses. Due to differences in the camera gate aperture and projection aperture mask sizes for anamorphic films, however, the image dimensions used for anamorphic film vary from flat (spherical) counterparts. To complicate matters, the SMPTE standards for the format have varied over time; to further complicate things, pre-1957 prints took up the optical soundtrack space of the print (instead having magnetic sound on the sides), which made for a 2.55:1 ratio.
The initial SMPTE definition for anamorphic projection with an optical sound track down the side (PH22.106-1957), issued in December 1957, standardized the projector aperture at 0.839 × 0.715 inch (21.3 × 18.2 mm) (aspect ratio 1.17:1). The aspect ratio for this aperture, after a 2x unsqueeze, is 2.3468…:1 which rounded to the commonly used value 2.35:1. A new definition was issued in October 1970 (PH22.106-1971) which specified a slightly smaller vertical dimension of 0.700 in. for the projector aperture to help make splices less noticeable to film viewers. Four-perf anamorphic prints use more of the negative's available frame area than any other modern format which leaves little room for splices; as a consequence, a bright line would flash onscreen when a splice was projected and theater projectionists had been narrowing the vertical aperture to hide these flashes even before issuance of PH22.106-1971. This new projector aperture size, 0.838 × 0.700 inch (21.3 × 17.8 mm), aspect ratio 1.1971…:1, made for an un-squeezed ratio of 2.39:1 (and commonly referred to by the rounded value 2.40:1 or 2.4:1). The most recent revision, from August 1993 (SMPTE 195-1993), slightly altered the dimensions so as to standardize a common projection aperture width (0.825-inch, or 21.0 mm) for all formats, anamorphic (2.39:1) and flat (1.85:1). The projection aperture height was also reduced by 0.01" in this modern specification to 0.825 × 0.690 inch (21.0 × 17.5 mm), aspect ratio 1.1956…:1 (and commonly rounded to 1.20:1), to retain the un-squeezed ratio of 2.39:1. The camera's aperture remained the same (2.35:1 or 2.55:1 if before 1958), only the height of the "negative assembly" splices changed and, consequently, the height of the frame changed.
Anamorphic prints are still often called 'Scope or 2.35 by projectionists, cinematographers, and others working in the field, if only by force of habit. 2.39 is in fact what they generally are referring to (unless discussing films using the process between 1958 and 1970), which is itself usually incorrectly rounded up to 2.40 (instead of the correct 2.4). With the exception of certain specialist and archivist areas, generally 2.35, 2.39, and 2.40 mean the same to professionals, whether they themselves are even aware of the changes or not.
Read more about this topic: Anamorphic Format