Science – Images of auroras and other marvels

This year is an exceptional year for viewing aurora, comets, meteors and other astro marvels in our dark skies away from the light pollution of nearby towns and cities. And it has been a great year for astro-photographers, both professional and amateur. A common question I get asked is why does a camera record a much brighter astro-image than the human eye sees?The answer to the above question is simple geometry. Capturing light is like capturing water in a bucket – the larger the bucket the more water it can capture.

Let us look at our common light-buckets – the diameter of a human pupil in the dark is about 8mm, the diameter of an iPhone lens is 20mm, and the area of a typical astro-lens is about 50mm. And the greater the area of a lens, the greater its light-capture capacity. So, the relative light capture disadvantage of the human eye (value of 1) over an SLR Camera is 40-fold and of an iPhone lens about six-fold.

However, it does not end there, as the refresh time for the human eye is roughly 1/30th of a second, so the eye captures a new image every 0.03 of a second and, of course, the amazing human brain smooths out the images that we see. However, a camera is not limited in its exposure time, so we can increase the camera exposure or light-capturing time up to 30 seconds, if necessary, in order to compensate for the dimness of a distant astro-image. That is another 1000-fold advantage for the camera over the eye. Combining both the area and exposure time advantage, in theory, we could get 40 times 1000 overall optical brightness advantage, or roughly 40,000-fold, which is a very big advantage.

However, these geometric calculations above do not include the inherent evolutionary advantages of the human eye. Also, in capturing an image of a distant faint star, we may not need to use 50mm of the diameter of the wide-angle lens and to avoid star trails we will try to use much less than 30 seconds of exposure. And, of course, the sensitivity of the human retina will be very different to the sensitivity of the camera detector. Finally, the ability of the human brain to amplify a signal is likely to be very different from the amplification (ISO) function of the camera. Nevertheless, while the absolute advantage of the camera may be much less than 40,000, it is not difficult to understand why my camera image of the aurora was much brighter than my eye vision image.