Why did an 85mm f/1.2 capture more stars than a 16–35mm f/4?

Question

I compared two lenses for night-sky photography: an 85mm f/1.2 (used between f/1.2 and f/2.8) and a 16–35mm f/4 (used at f/4). Even when I adjusted shutter speed to get a similarly bright overall exposure, the 85mm images showed more stars than the wide zoom.

Why would some stars appear in one image but not the other? Is it mainly because faint stars need enough light before they trail due to Earth’s rotation, or because the smaller aperture at f/4 simply collects too little light for those stars to register at all?

user61668 · Accepted Answer

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).

Why did an 85mm f/1.2 capture more stars than a 16–35mm f/4?

2 Answers

Your Answer

Related Questions