What rules? – after awhile I think strict photographic rules start slipping away – just use them as guidelines.
Milky way at the Pinnacles August 2012 single frame shot, Canon 550d Tokina 11-16mm @ 11mm F2.8 ISO 1600 30s. Light painted foreground. One of the first milky way images I had with clouds in it. Light pollution from Perth lighting up the clouds.
The idea is to balance getting as much light into the sensor without exposing for too long otherwise star trails will be evident.
Shutter: – Exposure settings for a milky way image often uses the “rule” of 500 or 600. The rule is based on what sort of size image you expect to have the image at – so the more that you zoom in, the more evident the star trailing will be. So based on this, the shorter the better (for an unguided image). I’ve seen a much more in depth calculation – which I think used many other factors like the latitude and other factors. But using such a formula I feel over complicates it.
Using 600 this is 600/(adjusted focal length) should be the shutter speed that you use in seconds to avoid material star trails. So using a full frame camera with a 17mm lens – would be 600/17 = 35 seconds. Using a crop sensor canon camera that has a multiplier of 1.6 and an 11mm lens would be 600/(11×1.6) = 35 seconds. But if you blew up the image to a reasonable size, you’d still notice some trailing. Using an 18mm on a canon crop sensor camera it would be around (600/(18×1.6)) 21 seconds (in which case may want to increase the ISO).
Having said all of that though – I tend to use 30s more than any other shutter exposure. Depending on the purpose, I may go a little longer or shorter. For example if I’m doing a time lapse – doing 60s shutters maybe entirely reasonable to create more of a sense of motion with the stars moving. When I shot a milky way panorama (26 images) at Sugarloaf rock in Dunsborough – I used 37s exposures to get a bit more light for the moon lit foreground. Doing single images I’ve used 20s and 25s – but I think I prefer getting a little more light in.
Aperture – I’ll have to confess to almost always just going F2.8 or F4 – being the widest aperture I can on the lens to gather as much light as possible. This may cause the image to be a bit more soft due to the wide aperture, but as part of the balancing act – I want more light.
ISO – I’ve generally used 1600-3200. I’ve used more and less, but am more comfortable at the moment with 3200 using my Canon 6D. When using my Canon 550d (crop sensor) I was happier with 1600. Even when I started with the full frame I kept using 1600 – however the noise performance is quite manageable going higher. I tend to like 3200 as the more light captured brings out the milky way more. Updated 1/8/14 – very much loving ISO 6400 in a recent shoot trip at the Pinnacles – the MW has been brought out more. If you consider taps (light sources) all at different speeds the fainter lights more is collected, however the brighter lights a lot more is being collected – so the milky way is more contrasted against the darker background. With the Canon 6d – the noise for an additional stop of ISO doesn’t seem to be too material.
I haven’t really been too fussed with mirror lockup (I have done so in the past though) when using wide lenses – but feel that the time it takes to burn the image into the sensor, it hasn’t really been necessary. When shooting at longer focal lengths (eg 200mm) I found it very important – but had much shorter shutter speeds.
I think the important thing though is to test the various settings when imaging and don’t rely on any sort of “rules” – if you’re “comfortable” at ISO 1600 – take a few images at ISO 3200 and 6400 and see what happens, reduce the shutter or push it out a bit further and so on. Don’t expect to get much milky way in if you expose for ISO 100 F11 for 2s – or even ISO 100 F11 for 30 minutes (unless you have a guided mount) as the starlight concentrated on the camera sensor will move due to the earth rotation.
On a cold moonless night on a dark location pointing away from potential light pollution you may be able to expose more. Light polluted areas would have to expose less and moon luminosity will also affect how much you can expose for.