ISO for star trails

CaveDweller

CaveDweller

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Paul
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This has probably been covered more than I've had hot dinners but what would be the best ISO to use for star trails? I'm using a Canon 550D with Magic Lantern. Am I correct in thinking that a higher ISO (eg 800) will pick up the fainter stars over a lower ISO (eg 100)? Also...I have the Starstax program for stacking pictures, when all the images are stacked will this process inherently decrease the noise?

Cheers.
 
Hi Paul it's mostly all covered in my guide to Startrails as linked in my signature, generally I'll shoot as high of an ISO as I'm comfortable with my camera, to the point of exposing the scene if the light pollution is an issue you'll generally top out at about 3200 as you'll get to the point of actually starting to over expose areas of the sky because of the light pollution in a 30 second exposure

Generally the stacking process will cancel out some noise as it averages out the scene to an extent and noise is not static in frame except for hot pixels so generally will be smoothed out a little
 
Yes, higher ISO will show more stars. Doubling the ISO will mean that you capture stars half as bright.

Yes, the stacking algorithm will reduce noise (strictly the signal:noise ratio), by roughly the square root of the number of stacked images. So stacking 4 images will mean the noise is halved. Stacking 100 images reduces S:N by about 10 fold.
 
Cheers both. I will have to try it the next clear night. Hopefully I can get away with using a high ISO with my 550d since stacking the shots will reduce the noise. It's noticeable at ISO800 so I'll have a play around one night to see how far I can push it.
hollis_f said:
Yes, higher ISO will show more stars. Doubling the ISO will mean that you capture stars half as bright.

Yes, the stacking algorithm will reduce noise (strictly the signal:noise ratio), by roughly the square root of the number of stacked images. So stacking 4 images will mean the noise is halved. Stacking 100 images reduces S:N by about 10 fold.
Click to expand...

You lost me at the word "yes" lol. I get what your saying though. Just wanted to double check before I crank up the ISO and end up with a useless image once it's all stacked, never done it before so I'm new to this. cheers.
 
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