A question on aspect ratios

[Frank Olean]

If I shot with HD cameras at a native 1.78:1 ratio (1920x1080), how should I crop to…

1.85:1?

2.39:1?

On blu-rays I find different values and I don't know why. For example, with 1.85:1 I see black bars on the top and bottom with 22 pixels each (1920x1036) and with 20 pixels each (1920x1040). But on UHD blu-rays I always see the same thing, bars on the top and bottom with 42 pixels each (3840x2076).

In the case of 2.39:1 something curious happens. Most blu-rays are 2.40:1 instead of 2.39:1, that is, black bars on the top and bottom with 140 pixels each (1920x800). I find 2.39:1 (1920x804), and black bars on the top and bottom with 138 pixels each, on rare occasions. 

What is the correct pixel value? 22, 20? 140, 138?

[Nicolas POISSON]

Based on calculus, 1.85 format cropped into FullHD should be displayed as 1920x1038, with 21 pixels black bars on top and bottom. I read somewhere that the resolution shall be a multiple of 4, although I do not know why (maybe linked to the macro-blocks of the compression algorithms ?). Starting from 1038, the closest multiples of 4 are 1036 or 1040, which leads to black bars of 20 or 22 pixels height. Maybe the variation you see is just different people choosing different closest multiple ?

Doubling the resolution, the problem does not exist any more: 2076 is a multiple of 4 so the exact height of 42 is acceptable.

[Frank Olean]

Thank you very much for this information, Nicolas.

Edited May 15, 2022 by Frank Olean