DAWN AND DUSK

Jim offers a considered study of optics for use in low light

- Jim

Dawn and dusk can be the two most productive times of day for the airgun hunter because they are the times that the day and night shifts of the animal kingdom are swapping over. I’m not an airgun hunter these days, but dawn and dusk are still special for another reason, in that they are the two times of day when the wind is most likely to be calm enough for outdoor airgun accuracy testing, and there are few better ways of spending a summer evening than outdoors with an airgun.

As light levels fall, our eyes start to lose the ability to perceive colours, starting with red, and ending with green and blue, and we can see less and less fine detail, because the colour photosensors in the eye (called ‘cones’) are concentrated toward the centre of the retina, and are densely packed, so when the light level falls too low for cone vision, we lose both colour and detail.

The other type of photosensors in the eye are called ‘rods’; they cannot distinguish colour, but they are more sensitive than cones, so they continue to operate when the cones cease to work. Rods are distributed mainly toward the outside of the retina, and are less densely packed than cones, so they don’t provide much in the way of detail although, on the plus side, they do make our peripheral vision very sensitive to movement.

In normal daylight, we see mainly with the cones (called photopic vision), as light levels fall we use both cones and rods (mesopic vision), and at very low levels, only the rods function (scotopic vision).

BRIGHTNESS