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I let my film get developed and scanned at a Fuji Lab in Japan and always receive a DVD with images of 4336x3036 pixel resolution, which is an aspect ratio of about 1.428194:1.
I always thought 35mm film has an aspect ratio of 3:2, or 1.5:1.
1.428194:1 is noticeable more "square" than 1.5:1.
What causes this difference, i.e. why are the scans which I receive not closer to 1.5:1?
Does the lab not scan the entire width of the 35mm frame? Or what is the actual ratio of a 35mm frame?
Here's a dirty little secret: 35mm film has no aspect ratio at all until it is exposed. It is just one blank piece of film a specific width (35mm) and any practical length with perforations occupying the outer edges that leave a 24mm wide strip in between the perforations.
What determines the dimensions of the photo is the size of the film plane each specific camera allows to be exposed each time the shutter is opened. Movie cameras that used 135 format film, for instance, classically used a frame 24mm wide and 16mm tall (plus a 3mm gap between frames) as the film was going through the camera vertically oriented (the perforations were on the right and left of each frame). 135 format still image cameras typically run the film through in a horizontal direction and expose about 36mm of width along with the 24mm of height per frame (with the perforations above and below each frame).
Back in the heyday of 35mm film cameras, most U.S. printing labs cropped each frame by around 5% to avoid printing rough edges. Most viewfinders on 35mm cameras were only about 95% coverage (so you didn't see the full field of view being exposed on the film, but rather the 95% that was actually going to be printed by most labs) or had a 100% viewfinder with indexing marks inscribed around the edges of the view screen that showed you where the 95% lines were. There were also technical issues with film that made the outer edges a little less precise than the middle of the frame in terms of optical performance. Japanese labs cropped the long edges only and printed the center 34.2mm x 24mm. Even today the standard 3R print size in Japan is 127mm x 89mm (5" x 3.5") which yields a ratio of ≈1.427:1. U.S. labs once did the same when producing 3 1/2" x 5" prints. When the U.S. moved to the larger 4" x 6" print, labs typically printed the center 34.2mm x 22.8mm of the 36mm x 24mm that was exposed.
It seems your lab is truncating 5% of the width of your negatives only and including almost the full height when digitizing them. If you divide 34.2mm by 24mm you get a ratio of 1.425:1.
As I have been scanning negatives for a while recently, I can offer my own insight here. In many cases, the 135 and 110 format negatives I've been scanning to not have even tone across the entire surface of the frame. With the larger format, there is often a pronounced "vignette" at the edges. For both formats sometimes the film just doesn't lay flat, but in a regular manner, leading to one side or the other often having a lighter or darker grade than the center or other side of the frame.
To combat this issue, I've resorted to cropping all the scanned frames in an identical way. Now, I chose to keep things in the 3:2 ratio, so I am dropping the top and bottom more than I actually need to in order to crop out the unsightly left and right sides. It is possible that the lab you used to develop and digitize your film follows a similar procedure, however in their case they chose not to maintain the aspect ratio.
This is all speculation, honestly, but given my own recent experience, I thought I'd offer the possibility.
