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pixel/mm ratio for 3D previz
So my question is basically this:
I can set the focal length of a camera (in mm), so how many pixels equals a millimeter (or inch) in a 3D composition?
I want to use AE to do some pre-vizualization for a short film. I’ve done a lot of 3D work in AE, but never needed to represent true physical space. So here’s another way of asking the same question: If I take a real picture of a 12″ square with a 35mm lens, how large (in pixels) should my 3D AE square-layer be to match the photo (with my AE camera set to 35mm of course)?
If you’re out there Dave LaRonde, I get the impression from some of your posts that you know the answer.
Thanks,
Adam -
5 Replies
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Thanks, Dave. Hmm… I’ve been doing some tests. In the camera settings window, I can select between inches and pixels as units, and when I divide those numbers (regardless of comp size) I get 72 (px/in).
Unfortunately, I don’t own a camera with prime lenses to test with.
I’ll try a few more things and report. Maybe there’s other folks out there who could benefit from this knowledge.
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I think I have some answers. Basically, the idea of doing 3D pre-viz in AE works! The pixel-inch ratio is arbitrary, you just need to be consistent. Check out this test:
I used my HVX at it’s widest setting (=32.5mm lens) to take a shot of some walls in my apartment. I picked a point in the room to be my ZERO point (x=0, y=0, z=0) and then I measured EVERYTHING (mostly importantly, camera placement/angle). I drew up some sketches of the walls (to scale) and brought it all into After Effects. For this test I made 1 ft = 72 px.
I set-up everything in the compostition by the measurements I made in the real world, and this is the result.
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I thought the same thing about the “72” coincidence, so I rescaled the images in PhotoShop and did it again with 1 ft = 100px, and it looked exactly the same. So it seems to be arbitrary, as long as I’m consistent.
If anyone else tries this, I recommend using the ratio 1ft = 100px. The file sizes are manageable, and the conversion is very simple. For example, if my AE camera is 600px away from the subject, I know to put my real camera 6 ft away.
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In case anyone is interested, I made a graphic to show the pixel/mm relationship
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For 3D pre-viz in After Effects, the pixel-to-mm (or inch) ratio is arbitrary as long as you stay consistent. I usually pick a scale that’s easy to work with, like 1 ft = 100 px. Then I match my AE camera distance proportionally to the real-world measurements. This keeps everything to scale without worrying about exact mm-to-pixel conversion.
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