rjlittlefield wrote: ↑Fri Dec 04, 2020 9:44 am
.....The image formed by the lens in each ommatidium in a fly's eyes is real and inverted, just as it would be for any short simple lens. If we were seeing those images, then each color blob would show an inverted image of the hand.
So, I think that what we're seeing in your image is really a blurred view of the outside world, the hand or the flower, seen through a hexagonal mask formed by the borders of the ommatidia. I expect that your preparation of the fly's eye has destroyed each ommatidium's ability to form an image of its own.
...
Land's book has a fine account of how all this works. I strongly recommend it.
--Rik
Thanks again for the comments! I've read one article by Land, but skimped the book... I'll go back to it!
Yes, my preparation of the fly's eye was quite crude and not really delicate... I had to brush away all internal structures, cone and baffles, since they were soft tissues. However the bulk of the refractive power should still be in the hard shell, the front curvature of each ommatidia; under normal microscope, they still produce an image, pretty sharp too; I've measured the focal length at about 120 um; ommatidia diameter is about 30 um. Below I'll brag some excuses on why the single ommatidia images are not very clear in the photos:
Mostly is narrow aperture and poor focus: it was f/20 nominal on the relay, and (estimate) about f/10 object side; not much resolution available for an image inside a 30 um hexagon. At high aperture (low f#) each ommatidia would project an image with higher resolution but much larger than the ommatidia itself thus all small images would overlap, with global result much worse than the images posted. Stopping down reduces this overlap. In the eye of the live bug, each image is contained by dark baffles (that I had to brush away).
The focusing point: in the center I've put it not exactly in the image, but a bit shifted toward the ommatidia lens, so the black contour of the ommatidia become a bit focused too (otherwise it will be more blurred); away from the center, the **** eye is approximately spherical (~3mm diameter; I cut a cap of 2mm) so the focus wanders across the field: I tried out different microscope objectives to find the one with more or less the same field curvature, with the best focus surface a sphere of radius matching the eye, but think I'm still far off. So the only quick way to get all in one shot was to close down the aperture.
I wrote to vendors of the big 4 microscope manufacturers, "what is exactly the field curvature of your objectives? I need it for a project...", you may imagine the answers.
It is possible to focus at infinity (hand, flower) but then the hexagonal mask get totally blurred: even at the closest aperture (f/32 nominal) the effective entrance pupil is about 0.15mm diameter (objective fl 2mm), much larger than the ommatidia.
The other excuse is the relatively narrow FOV of each ommatidia, about 12°; while the flower or the hand are much larger, about 120° (they were right in front!); so each ommatidia pictures just a piece of finger or petal; it is (barely) visible that such pieces (the color blobs) are inverted, on the "wrong" side of the finger or of the petal.
The photo has been taken by a fisheye objective, with the entrance pupil (point of view) in the center of the eye dome; so each ommatidia looks at a different direction.
So, I believe that the images can be improved with better skills, optics and patience. I'll try to post all the setup details and thinkering, but in a while, I've exaggerated the last weeks and now I have to look after the wife and the real jobs

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Giorgio