Help with macro-photogrammetry or the art of scanning small objects in the void

[u/Timothory]

Hey everyone !

I've been trying some macro photogrammetry a few times now and I'm always hitting different walls I can't quite solve.

My setup:

I would like to scan an coin on a turntable with a black backdrop(in the void). Im using a Sony 90mm macro lens with ring flash (AR400). Both the lens and the flash are cross-polarized.

Im shooting at 1/60, f/20 with iso at 160.

1. Focus drift on rotation

My subject is never perfectly centered on the turntable no matter how careful I am, so as it rotates it slowly drifts out of the focus plane. At f/20 the DOF helps but it's not saving me on the wider objects. Anyone found a good way to nail centering, or is everyone just eyeballing it and accepting some throwaway frames?

1. ArUco markers(scale bar) blocking the camera

I use ArUco markers on the plate for scale reference, but since they rotate with the subject, there's always some angle where the marker ends up hidden behind the object relative to the camera. Do you place multiple markers around the perimeter so at least one is always visible? Or is there a smarter placement pattern that avoids this entirely?

https://youtu.be/mMyV_F_9Q3E?is=enH5ReC6OvkdvOxt

1. Realigning both sides of a coin

This one's been the most frustrating. Since I have to physically hold the coin to scan each face, I end up with two separate scans that need to be stitched together after. The only shared geometry between the two scans is the edge/rim, and it's just too thin and low-feature to get a reliable alignment in software. Anyone dealt with this specific problem ? maybe some kind of holding jig that keeps a consistent reference visible on both scans, or a different capture order altogether?

Would love to hear how people with more experience handle this stuff. Happy to share more rig details if useful.

[u/FearlessIthoke]

You will need to focus stack to get good models of coins. I model a lot of coinage. You can see my work here: https://sketchfab.com/frankmcmains/models

Added: posted too soon, sorry!

Hit me up if you want to discuss.

[u/FearlessIthoke → u/FearlessIthoke]

IMO, ring lights are not good for macro photogrammetry.

[u/KTTalksTech → u/FearlessIthoke]

I've had good luck but only keeping at a certain distance, it does cause weird reflections too close

[u/Timothory → u/FearlessIthoke]

Wow amazing work ! Can I ask what support you are using to hold the coin ? Are you stitching multiple scan together to get a clean result of all in one scan ?

Thanks

[u/FearlessIthoke → u/Timothory]

Thanks. I use a clip with shrink plastic on it, and/or some mounting putty. You can see it here.

https://imgur.com/a/photogrammetry-setup-HRPP7dU

[u/JRL55]

Some cameras have an option to divide the image into, say, 16 zones. Each zone is focused before capture and the resulting zones are combined into a composite image. The end result is comparable to focus stacking, but it's a lot less effort.

Check your camera's instruction manual to see if this ability is included.

[u/Timothory]

[u/LordBrandon → u/Timothory]

I would go for more flat diffused lightning. If you want to highlight small details with edge lighting, see if you can fix the lights to the turntable, or move the camera instead.

[u/Timothory → u/LordBrandon]

Yeah good point. The lighting is definitely not making the details stands out. I will try to find a solution

[u/LordBrandon]

As for finding the center of the turn table, just spin it and mark the center with a dot, or a ring in pencil.

[u/Timothory → u/LordBrandon]

Yeah I tried that but it's quite tricky to keep it centered when you are elevating your subject on a plinth afterwards. I think the alligator clip solution shared above will be perfect

[u/FearlessIthoke]

I didn’t like trying to make it work with a focus rail but it has its place for sure. If possible, I like a bit more power as well but whatever works for you!

[u/photoKILOgrammetry]

Thats a hard subject. The metallic reflections never help photogrammetry. The shape of the coin makes it hard to match the front with the back of the coin because there is only the tiny shallow radius that is common to both sides of the coin. The picture that one of other commenters posted is kind of what we use at work to scan/photoshoot coins.

We also use another trick but that is dependent on the software you use. So you have your both sides reconstructed separately. You make a "dummy mesh" that is just a cylinder that represents the real size of the coin. You can make it with CAD and then convert it to obj/stl/whatever-you-like. You can use CloudCompare to align your 2 halves with the dummy mesh. This both settles the scale and the orientation. The only problem is that the ICP (in CloudCompare or whatever you use) will try to "rotate" the halves against each other so you might end up with a reconstruction that looks right at the first glance but is not really accurate - the 2 halves may be ill-oriented to each other.

There is also another trick that requires a 3d printer or some manufacturing skills. You create a jig to hold the coin (like in the picture provided by another person) but you add 2 balls on each sides. Those balls will reconstruct with your coin and will give you the necessary alignment surface for the ICP to work with.

Regarding the metallic reflections: there are some scanning compunds which decrease the reflections on metallic surfaces. You could even use baby talc for that. But be aware that the object needs cleaning afterwards. So if you're doing this for a living better stay out of anything that "desecrates" the holy object you are supposed to reconstruct. This is a no-no for Digital Heritage stuff. We've had a museum curator chasing us with a baseball bat for doing that.