Photographing corona & aureole

Corona rings are difficult to photograph. This is because they occur very near to the light source; usually this is the sun or moon (altough I sometimes see corona around the major planets also, as well as around streetlights and such). Whenever the unobstructed light source is in view, this will cause glare and over-exposure unless the optical part of the camera is extremely clean (and even then, you will get reflections from the inner lens surfaces, causing glare).

Corona around the sun: the inner aureole is blue, bordered by a red brown ring.

When to look for coronas & aureoles

Coronal rings are best seen in the small cirrocumulus and lacunosus structures mostly seen when a jetstream is situated over your observation area. These cloud types are very transient, so you have to be relatively fast. The corona aureole, however, is seen always whenever the sun (or moon) is shining through a thin layer of cloud.

If the droplets are uniform in size, constructive interference will occur at correlated angles for every light wavelength, and you will observe many rings. If, on the other hand, the cloud contains a wide variety of droplet sizes, you will in general only see the aureole, as a blueish disk around the light source. This disk is quite small; of the order of 2 to 3 degrees diameter.

Surface fog can also cause spectacular corona rings around the sun or moon, but ofcourse only if the fog droplets are uniform in size.

Film & photography equipment

For corona caused by the sun, you won't need a tripod and cable release, because the light around the sun is very bright. Typically you will take pictures at 1/1000 second, f/16, on 100 ISO film, with a telephoto lens.

Blue corona aureole around both Jupiter (left) and Saturn (lower right).

If, however, you will photograph the corona around the moon, or a planet like Venus or Jupiter, you will need longer exposure times - depending on the brightness of the corona (NOT depending on the brightness of the light source!), and you will need a tripod to keep the camera steady.

Exposure

Light metering for the corona is tricky, because the corona is so close to the light source, which should not be in view when you do the light metering. So, I recommend you to do spot-metering with your camera at the place where you want the corona exposed optimally (typically 3-4 degrees away from the light source; for the aureole, this would be 1 degree - this assuming the most prevalent sizes of cloud droplets).

If your camera does not have spot-metering, you will either have to zoom in on the corona until the light source is out of view, block the light source, or move the view so that the light source is outside view. Blocking the light source is generally best, and a rock or a tree trunk is generally best for this (buildings do well also, but it is less natural).

At daytime, for coronas around the sun, you will generally get properly exposed photos. For nighttime coronas this can be difficult, however, since light metering is a bit unpractical or impossible with the technique as described. What I do is to expose lunar coronas for full moon with between 10 and 20 seconds at f/2.8 on 200 ISO film, and bracket exposures by +/- 2 stops. For corona around planets like Venus and Jupiter, you will need much longer exposures - typically longer than a minute for 400 speed film at f/2.8.

Polarization

Coronas are polarized, but only very weakly so, and not enough to have any significant effect if you use a polarizer on your camera lens.