The Relative Effectiveness of Camera Lenses

Exposure is only part of the requirement for pleasing auroral pictures. It is always best to have an interesting foreground and a nice star background, so as to give a sense of scale. In particular, a brilliant star background greatly enhances auroral photographs, particularly if the stars are not trailed. So it is well to understand that star exposures do not depend on the f/number of the lens used but rather on the effective aperture of the lens, which is given by the focal length divided by the f/number.

The table below shows the relative effectiveness at capturing auroras and stars of some lenses that might be used for auroral photography. The 50-mm, f/1.0 lens has been used as a point of reference. The table shows a little-appreciated fact: although wide-angle lenses may give a good sense of auroral scale, they are very inefficient at recording stars. The longer-focal-length lenses often provide much better star fields. Of course the longer the focal length, the smaller the area of sky captured on film. Balancing field of view, exposure time, and the richness of the star field is the challenge of auroral photography.

Relative Effectiveness of Camera Lenses for Aurora Photography
Focal Length   (mm)	f/number	Effectiveness  for auroras	Effectiveness  for stars	Stars/aurora  ratio
15	2.8	0.125	0.01	0.08
24	1.4	0.50	0.12	0.24
35	1.4	0.50	0.25	0.50
50	1.0	1.00	1.00	1.00
50	1.2	0.70	0.70	1.00
50	1.4	0.50	0.50	1.00
85	1.2	0.70	2.00	2.85
100	2.0	0.25	1.00	4.00
100	2.8	0.125	0.50	4.00

A final factor to consider is contrast of the aurora in the sky. Moonlight will diminish contrast; a Moon with up to 25 percent illumination gives pleasing results, as the light illuminates the landscape without brightening the sky enough to degrade auroral contrast. However, once the Moon is half lit, auroral contrast will begin to suffer. Auroral photographs taken near full Moon will be very washed out except in the case of extremely bright auroras.