The right aperture for capturing stars?

Question

I recently had the chance to photograph an astronomical event - and this was my first foray into night-sky photography. I had two lenses that I used:

• 85mm f/1.2 shot at f/1.2 through f/2.8
• 16-35mm f/4 shot at f/4

Exposures were all over the place, but I noticed that more stars were captured using the 85mm f/1.2 than were captured at f/4 using the 16-35 - even when shutter speed was lengthened so as to create an equally exposed shot.

As I see it, the potential causes could be:

• Since the stars are moving, they would need to have enough brightness to show up before moving otherwise they would not be captured at all (think super long exposure in a public place). My exposures did range into the 1 - 2 minute range, so this might be the cause to some degree.
• The aperture could also be a cause by limiting the amount of light to a degree that certain faint stars will just never have the lux to be captured?

What was actually at work here to limit the capture of the stars, from existing in the image to not existing at all?

Answer 1

For extended objects like the northern lights, the f-ratio determines the brightness, as for normal photography. For point sources, like stars, the light collecting area (effectively the focal length divided by f-ratio, squared) determines how bright the stars are, since you always end up with a point image regardless of the focal length. (Note that the effective light collecting area is NOT the same as the size of the front element - at 16mm f4, the effective light collecting area is only a 4mm diameter circle, while your 85mm at f4 has an effective light gathering area of a 21.25mm diameter circle - so it catches about 25x as much light[ (21.25 / 4) squared ].

To avoid star trailing due to the earths rotation, aim for an exposure of around 400 / focal length or less. So at 25mm, you should be able to go to around 400/25 = 16 seconds. (Different folk use different numbers to 400 - it depends how picky you are, and whether you're using a crop or full frame sensor, but 400 should be in the right ballpark (it also depends on where in the sky you're aiming)).

Some lenses work well wide open for astrophotography (Like the Nikon 180/2.8 ED), others may need to be stopped down several stops for acceptable quality. (Most of the faster Nikon 50mm lenses need to be stopped down for AP - so while the faster versions can be handy for terrestrial photography, there's little point in paying the substantial extra cost if you only want one for AP).

Also, note that best infinity focus may not be exactly at the end stop, and that when stopped down, diffraction spikes from the aperture blades may give you a bit of a starburst filter effect around bright stars (there will be twice as many rays as the number of aperture blades (for odd numbers of blades) - each blade causes a diffraction spike either side of the star. Thanks to Michael Clark for reminding me that with an even number of blades, the number of rays will be the same as the number of blades, instead of twice that, since the diffraction spikes from opposite blades end up on top of each other).