Focus stacking and photogrammetry

Having found the excellent focus-stack, I decided to try out a few ways to do macro photogrammetry.

I grabbed one of my smaller fossils, an ammonite about 2 cm across. Not terribly tiny, but challenging to get focus on. I used my Z8 and MC105 VR S lens. I was only really interested in capturing the front of the fossil, so the coverage at the back is poor and shows in the models below.

First up, I used the Nikon Z8 focus-shift to take a series of shots from 15 positions. Each focus-shift set consisted of 25 images with a very shallow depth of field (shooting at f4):

I used focus-stack to merge the sets into 15 photos where the whole specimen is in focus, e.g.:

You can actually see that there’s a bit of distortion on the small plastic box at the very front and back of the image, where the camera was never actually in focus.

Anyway, I plowed on, and threw all 15 stacked images into Reality Capture. It aligned them all, and produced the following model:

As noted above, coverage at the back was deliberately (lazily) poor. But the detail at the front in both mesh and texture is excellent.

Purely out of interest, I tried throwing all 15 x 25 original images into Reality Capture, wondering if it had a sharp filter and could cope with the out of focus areas…

…Nope. Terrible, distorted model, unsurprisingly.

I then noticed that the MC105 VR S can close down to f51, which is pretty absurd. That got the whole specimen in focus in a single image:

This image is straight out of the camera, and the few cm specimen, and it’s box, is fully in focus. So, I chucked those images into Reality Capture too, and the result was great:

For comparison, here’s the models made from f51 shots on the left, and from the focus-stacked images on the right.

Both look great, good textures and good meshes. I’ll keep playing with this. That the lens can go to f51 makes things much, much simpler, but for even smaller objects, focus stacking might be needed, in which case focus-stack clearly produces outputs that work absolutely fine for photogrammetry.