Craig,
That's a fantastically detailed response so thanks very much. I was wondering if something like that could be used in a different situation. I'm thinking of going on a Northern Lights Cruise, which is basically a service vessel running up and down the Norwegian Coast right up into the arctic and back. It doesn't stop anywhere for very long so most of the time would be at sea. To do any aurora photography I'd be battling against two main problems
1) ship movement, and
2) engine vibration.
My guess is that a tripod would not help due to (2) in particular, so instead of doing hand-held long exposures with both (1) and (2), would it be possible to take a series of 20 exposures at, say, 1/10th sec hand-held and combine them in software to give the equivalent of a 2 sec exposure? What do you think?
I don't have PS but if it's possible in PS, it must also be possible in Affinity Photo, and I'd be happy to get into that.
A number of problems at work here!
1. Earth's rotation giving the appearance of stars moving.
2. Camera position moving relative to foreground.
3. Camera not completely stable due to engine vibration.
I like the idea of shorter exposures, then stacked, the problem is you need an anchor point/s. (No pun intended!) If you shoot with foreground, some land on the horizon included, then just align this in each shot. That will allow for camera position movement, (but not camera shake or motion blur) it won't prevent star trailing but shooting wide for a total accumulated exposure of less than say 20-30 seconds you should be fine anyway. I'd still shoot off a tripod though, with a remote release in continuous, (or intervalometer).
To decide how long those exposures should be I think you will actually need to do some test shots on the ship. I would actually do that then make my decision on how to frame up. Take a test shot pointing at something on the ship, try a sequence of test shots, even up to longer exposures. You would be amazed, due to effectively in camera averaging sometimes a small vibration part way through a long exposure is not visible, because the parts of the exposure during the time the camera is not shaking burn onto the sensor for more of the time and you don't see the small shake. Once I'd established what the limit for ship vibration would allow I'd hope to take some auroras when the ship was not sailing, but if it was going to be, hopefully on calm water, I'd during the day (do some test shots framed up on some distant land) to see how quickly you get motion blur on the land. This then gives you a maximum exposure length from the camera's relative movement to the land point of view.
Pretend you get to 1 second maximum, that would be ideal as it could be usable. Remember what I said earlier about the iso and aperture being the only parts of the exposure triangle that are relevant to the exposure of bright moving things like stars. Well that applies to stars after the 500 rule, basically beyond the time that they have moved pixel position on the sensor and are being recorded on a different pixel. Well, with auroras I guess that would happen sooner, as they appear to move quicker, and in post, we are just going to add the green blur together! Pick an ISO and aperture that brings the aurora out as bright as possible, say ISO1600 and f4 minimum. Don't worry if it appears bright, or dark even, the real magic will happen in post with the additive of all the exposures, but with more control over dynamic range headroom.
I still think you will struggle, and what ISO you need to go to will be affected by the length of the exposure you can achieve. If you have a faster lens, then use it. My only other thoughts are to frame up without the land, but include some bright stars, then use these as anchor points to re-align the images in PS. You will need to ensure that the images are sharp though, and not ruined by ship movement still, but my head is hurting from thinking if the stars are corrected for earths rotation what happens to the aurora movement?!
Tail Fade Star Trails;
Insignificant by
Craig Hollis, on Flickr
Blue Hour Blend with Image Averaged Milky Way;
Crowfield Milky Way by
Craig Hollis, on Flickr
Aurora, f/2.0 20 seconds ISO 800
Jokulsarlon Aurora Borealis by
Craig Hollis, on Flickr
