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Resolution math question


Jason Chan

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If a project is shot in 4K 4:3 in anamorphic and then the frame is cropped to the aspect ratio of 2.1:1, what will be the resolution after the crop? 

 

Any experts out there can solve this? And perhaps break it down how you get your answer? Thank you! 

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It's pretty easy math if you know the exact resolution in pixels of your capture format. You say 4K 4:3 but that doesn't really mean anything....the two standards of "4K" are UHD (3840x2160px) and DCI (4096x2160px), neither of which are a 4:3 aspect ratio. Also, your final ratio is interesting....2.1:1 doesn't sound like a standard I have heard of. 2.39:1 or 2.40:1 are the common cinemascope final aspect ratios. Perhaps you meant a 2:1 pixel ratio for the anamorphic desqueeze? That is part of the process of shooting 2x anamorphic lenses on digital. 

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Following off Chris's response,

On film the aspect is not 1.33:1 (4x3) but about 1.2:1 to start. They anamorphic aperture on the film camera is actually taller than the Academy aperture. There’s very little cropping. But on 4K HD the math works out like this:

4K equals how many pixels. At what aspect? (Depends on the camera) Then figure out how many pixels there be at half of 2.1:1  which would be 1.05. Anamorphic of course it doesn’t add resolution it’s an analog compression scheme so whatever your sensor would be if you crop it to ratio of 1.05 would be the resolution of anamorphic.

When you expand it out and effectively double the horizontal pixels it’s still just doubling pixel so you’re not actually gaining resolution there are more pixels but there’s no more information in those pixel.

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Ah nice, that is helpful, but I still have to make a bunch of assumptions:

So, assuming you're going to shoot full 6k raw 3:2 "open gate", that is a capture resolution of 6048 x 4032. Assuming you're shooting 2x anamorphic lenses, your desqueeze will be easy math (divide the vertical resolution in half) resulting in 6048x2016. This is a 3:1 aspect ratio, which is way too wide. Multiply your new vertical resolution by the desired aspect ratio, and that will tell you how much horizontal pixels to cut. Assuming 2.40:1 is desired result:  2016*2.4=4838. 4838x2016 is your answer in this scenario. 6048 - 4838 = 1210 pixels that need to be trimmed from the sides (605px off each side) to get that desired result.

Keep in mind, this is assuming you're shooting 2x anamorphic on a full frame camera, which is highly unlikely. 2x anas don't cover full frame, and even if they did, as you can see, you're throwing away a lot of unused pixels. Therefore it would be more common to shoot 1.8x chameleons/Vayzen or 1.5x cooke full frame anas. Whatever you go with, divide that squeeze factor by your vertical resolution and continue on from there.

Because this is a forum full of knowledgeable film people, I'm sure I will be murdered by words in no time if any part of my process is incorrect. 

Edit: I guess you can easily shoot 2x anas on this camera if you don't mind throwing away pixels on both axes until you achieve coverage. In that case, you'll just have to test lenses to determine your crop factor and final resolution.

Edited by Chris Stiles
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