I recently got the Sony Cyberhot-RX100II mainly as a camera I can carry around on a daily basis. Looking at a quite impressive low-light performance, I was wondering whether usable night sky images can be taken by this camera.
The focal length is variable in the 10.4 - 37.1 mm range, the maximum aperture changes with the focal length - from f/1.8 at the wide end to f/4.9 at 37.1 mm.
So, formally the maximum absolute aperture is achieved at 37.1, since the aperture is 37.1/4.9 = 7.6 mm, while at 10.4 it is 10.4/1.8 = 5.8 mm. Does this mean, that for this camera the maximum zoom level is optimal for star field images in terms of light collection capabilities?
Hi Glider2014,
I have been looking at your question trying to decide how to answer in analytical way. My gut feeling told me that despite the smaller clear aperture of the short focal length, that it would be what I would use for astrophotography. But the larger clear aperture is supposed to be "better". So after a
little research online, I found an interesting article (
Comparing Lenses for Nightscape Photography) which I used answer your question.
First I'm assuming that when you are asking which focal length would be "optimal for star field images in terms of light collection capabilities" you are referring to
untracked, tripod mounted camera imaging. If that is not the case then what follows will not answer your question.
The article in the above link, entitled "Picking a Great Lens for Milky Way Photography", makes the point that to rate a lens for its astrophoto capabilities you must consider three factors:
- Area of maximum lens opening (in square millimeters)
- Angular area covered by the lens (in square radians)
- Maximum exposure* for essentially untrailed star images (in seconds)
* Maximum exposure (in seconds, using the 500-rule) = 500 ÷ lens focal length (in 35mm equivalent)
The author uses these three factors to come up with a measure to compare different lenses which he calls "Untracked Astrophoto Rating", which is just the
product of the above three factors. Following the methodology he used, I computed the rating for your camera at each end of its focal range and came up with the following
Untracked Astrophoto Rating:
- For 10.4 mm @ f/1.8, the rating is 507
- For 37.1 mm @ f/4.9, the rating is 20
So
the short focal length is by far the better lens to use for star field images. This matches my gut response to your initial question. The disparity in rating between the two focal lengths (short versus long) is a bit surprising, until you look at the numbers. While short has just over half (0.58) the area in square millimeters, it has 12 times the angular coverage and approaching 4 times (3.6) the maximum seconds exposure. When you multiply these together, you get the 25 times factor favoring the 10.4 mm f/1.8 lens.
It would be interesting to see how your real-world images at these focal lengths compare with the above analytical comparison.
--
Best Regards,
Russ
Strange colours are a combination of light pollution and post-processing. Just about all the shots taken that night had some cloud obscuring the stars.
