I think what makes these images so impressive is the depth of field — without the characteristic blurring of far and near objects that typically comes with macro photographs, the violins appear to be vast interior spaces.
I would argue these are not really macros though. They have a wide field of view and are also not that "close". I suspect they likely use something like a gopro to get into the tight spaces.
You can get a large depth of field by stopping down, no need for focus stacking. Also the lens used here was quite wide which increases the depth of field (simplistically speaking). The main challenge in these images is the lighting, which arguably is what makes them look so awesome.
DoF is a geometric thing, it's proportional to (distance to object / focal length)^2. A narrow DoF is caused by the object being close to the camera (in terms of 'number of focal lengths'). (It's also related to the aperture)
Therefore a physically small camera (think a phone camera) will have a wider depth of field, for a given scene and f-number, than a larger camera.
In other words, the inside of a guitar is a relatively large space if you're a tiny camera with a 5mm focal length.
> The images show the insides of a violin, cello, flute, and pipe organ.
The one at the top of the page (2nd in the sequence of smaller images farther down) is not a violin, cello, flute, or pipe organ.
It looks like a classical guitar, although only 5 strings are visible. The missing string would be to the left of those 5, and so may be hidden due to the angle of the shot. (We can tell it is the left that is missing because the 3 strings on the right are more opaque than the 2 we see on the left. On a classical guitar the three high strings are plain nylon, and the three low strings are a nylon core (or sometimes a synthetic fiber blend) with a metal wire wrapped around them).
The 6 strings of a classical guitar are centered over the sound hole and these look to be displaced to the left more than I would expect just from perspective but maybe I'm misjudging the perspective by subconsciously judging it as if this were a room sized space or something like that?
