Physics Questions about Scanning

Hi have a physics/photography question here.

If I want to get a big 2d pattern off a thin flat sheet (let’s just say a 2mx2m pattern off a flat paper)
If I use my phone camera (not wide lens) take a top view picture of it, is the pattern distorted compared to the original (like slightly fish eye effect)?

I know obviously the hight of camera (facing down towards a pattern on the ground) will determine the distortion and perspective caused by field of view. (And in an ideal world, a humongous telecentric lens will serve my need with an isometric image). How high in the air should my phone camera be to get a negligible result? My acceptable error margin is about 2mm.

I was also looking at handheld scanner bar as an option, but they expensive, and mostly for scanning documents. Thanks

PS. I’m going to fix camera angle with perspective skew in Photoshop, so camera angle error is not a problem

No idea how high you’d have to be, but here are some suggestions for alternative ways of getting the orthographic image you want which won’t require being on top of a ladder or a skyscraper.

I’d use Reality Capture photogrammetry. Once you have the 3D model of the subject, you can then get an orthographic view. Accuracy should certainly be better than 2mm. This video explains how to get an orthographic view from Reality Capture.

There’s a small charge for processed scans - probably a few pennies. Alternatively you could use Reality Scan photogrammetry for free and then import to Blender and get an ortho image that way. Photogrammetry involves taking lots of photos of the subject to generate a point cloud, but you don’t have to be far from it or worry about distortion. It also saves needing to do deskewing.

If you can borrow a 3D scanner with colour capture that can be handheld, that would give the best accuracy and would be a better option than photogrammetry.

Lightroom can correct perspective distortion and might possibly be able to modify the image to an orthographic view. Deskew first. Also lay a grid over the subject so that you have a reference photo to check settings against. 3D scanning with scanner or photogrammetry will probably be easier and give more accurate results.

If using 3D scanning, then depending on the subject, if it has large areas with no features, it will help to put down some marker dots (or cut up blue masking tape into smallish pieces).

Others may have better suggestions.

ETA - if the pattern is on 2mx2m paper, then you could also scan it in sections using a book scanner (the lid folds fully back). But if the sheet is stiff, that’s not going to work because of the raised control panel. Happy to lend mine in return for a donation to the space if you can collect and return it to Holloway.

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Could you take a picture of a big grid from the same place and work out the distortion from there?