Physics Photo of the Week

November 18, 2011  

Dragonfly Eye
Insect eyes are totally different from vertebrate eyes.  Insect eyes are compound eyes.  The compound eye consists of a hexagonal arrangement of individual photoreceptors called ommatidia that cover the hemisherical surface of each eye.  Each ommatidium has a lens at its opening that focuses the light onto the photoreceptive nerve cell at its base that sends a neurological signal to the animal's brain.  The image at right shows a close-up of the hexagonal arrangement of the photo receptors.

Vertebrate eyes, on the other hand, consists of one lens that focuses an image on the array of photo receptors on the retina at the back of the eye.  The main difference between compound eyes and vertebrate eyes is that compound eyes have a lens for each photoreceptor; vertebrate eyes have one lens for all the photoreceptors.  The evolutionary pressure for the vertebrate eyes was a more compact eye for large animals.  If people had compound eyes, the eyes would have to be many times our head sizes in order to see the same field of view as insects.

Compound eyes can see images, but at much less resolution than vertebrate eyes - principally due to the density of photo receptors.  A retina is much more efficient for high resolution.  Compound eyes are most likely more efficient for small animals.  Insect eyes can increase their sensitivity to low light situations by combining the light into adjacent receptors.  This is analogous to some digital cameras increasing light sensitivity by combining the signal from adjacent pixels.  The increase in sensitivity comes with a trade-off of decreased resolution. 

These pictures were taken with a "standard" zoom lens set at 55 mm focal length with no macro attachments.  The beautiful dragonfly had been found dead before being photographed.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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