Unlike humans, the arrangement is quite different – a single ‘facet’ and a single lens covers a retina of many sensory cells, where each sensory cell contributes one point or ‘pixel’ to the final image, so the retinal sensory cells are the optical units as far as resolution of the final image is concerned. In insects, however, each facet encloses one ommatidium containing just 7 to 11 sensory cells.
In the human retina, in its most sensitive region (known as the fovea) some 175 000 sensory cells per square millimetre are packed into an hexagonal array. In the insect, the compound eye contains anything from about half a dozen ommatidia to 30 000 or more. In the insect, the compound eye contains anything from about half a dozen ommatidia to 30 000 or more. For example, the wingless silverfish have only a few ommatidia, or none at all, whilst the dragonfly has about 30 000 ommatidia in each compound eye.
Visual Acuity- insects only contain 7 to 11 sensory cells and for dragonflies which consist of 30000 ommatidia (which is considered the highest) has only While humans already contains 175000 sensory cells per square meters alone. This adds up to about 20 times higher than the ommatidial density of the dragonflies eye. This allows human eyes to detect greater spatial resolution.
Contrast- In dim light, an optical system needs to collect more light in order to see clearer. In general insects especially those diurnal ones are limited by the small apertures of each ommatidium in the compound eye, hence detect weak contrast especially in bright daylight or dim light. And in order to survive the insect eyes collect lights for about 0.1 second to form a given image. However for dragonflies, they may have apposition eyes with wider facets and they may collect light over a longer period (up to 0.5 seconds) before integrating the signal to produce the final image.
Humans are diurnal, and although they have a degree of night vision, they are not particularly good in twilight than dragonflies.
Colors- color vision in humans depends upon the three types of opsins- red, green, blue. Dragonflies however, have a greater scope than us. Dragonflies can see ultraviolet and see the plane of polarization of light (ability may help them to navigate and to identify bodies of water)
Temporal resolution (motion sensitivity)- refers to the measurement with respect to time. Often there will a tradeoff between temporal resolution of a measurement and its spatial resolution.