6D - go for longer exposure, or higher ISO?

General question here - and there may not be a definitive answer, but I'd be interested in seeing opinions, esp. backed by example.

With all other things being equalized, is it usually better to get as LONG an exposure as you can (without unwanted trails,etc) or to keep an exposure shorter and bump up ISO?

I was out last night trying to dial in my new Rokinon 14mm and it seems that I get a better frame at 10sec and ISO 1600 than at 20sec and ISO 800. Do I lose anything, light-gathering-wise, at the shorter exposure time? Noise didn't seem to be a huge factor.

Here's two examples in the neighborhood of Sirius, greatly magnified, no processing just trimmed for POTN. Any thoughts, other than move to Montana? TIA

When you increase the ISO it makes the sensor more sensitive to light, thus introduces more noise.

*First set your exposure as long as possible without trails. Your max exposure about half-way between the pole and meridian is 800/effective focal length (on a fixed tripod).
To get a longer exposure do these things
-Reduce focal length (zoom out)
-Shoot at subjects near the poles (polaris doesn't move much, and can be shot with long exposures)
-Get a better tracking setup or better alignment

*Second, set your ISO. Set it such that the histogram spike (representing black) is at about 20%.

If you are in a spot with a lot of light pollution, and you have a very good tracking setup, you may get a histogram spike above 20% with you minimum iso. In that case, you might as well stop-down your aperture to get more sharpness until that spike goes down. An example of this would be someone with a performing a drift alignment on a good equatorial mount, shooting with a 6D and a 300mm F/2.8 near Chicago. The mount will be able to shoot 10-minute subs, but with that lens even on a dark night in Chicago, you're just going to see pink.

If you have very little light pollution, you may be using a lot of iso, AND a lot of exposure time. In this case, it would be a good investment in time and $$ to get better tracking to take advantage of your lucky skies.

Now take as many pictures as possible, and stack them with software like deep sky stacker. The more total imaging time you have, the better. To get the good images you see posted around here, you're going to need a fast lens (or scope), a good tracking mount, a good polar alignment, and a lot of frames to stack.

I shoot in a orange/yellow zone on the light pollution map with a 60D, 300mm F/4, and a ZEQ25 mount. I get the polar alignment as good as possible, then start testing how long of subs I can get with tonight's alignment. Sometimes I can get 1 minute shooting near the meridian, sometimes I can get 3 minutes (unguided). Then I just adjust ISO as necessary and fire away. An hour of subs will give a pretty good image of many deep sky objects.

single 1-minute sub ISO200 F/4

Thanks, MPBOWYER - with the 14mm Rokinon/Samyang/Bower lens, I can go 30s without any noticeable star trails. Then the key just seems to be setting the ISO to strike a balance between sky sensitivity and horizon light pollution. I've noticed that around here, when I shoot off toward the west, I get so much LP from the Wilmington/Northern Delaware area it's ridiculous. North is just as bad, as I'm only about 30 miles from Philly. Guess I've gotta wander out in to the middle of the pineys..

800/fl is really pushing it IMO. I use more like 600/fl on FF (400/fl on crop). It does vary with where you're shooting in the sky, though. If you're aiming toward the poles, you an go longer than if you're shooting toward the equator.