Somehow I missed this thread.
When we shoot photos of the night sky we actually do refine the focus on a star. There are two reasons.
1. You can't really use a high f-stop. It's dark. There's almost no light. The Earth is spinning. If you close down the aperture, then you have to take a longer exposure. This is going to elongate the stars and you'll have star trails instead of pin-point stars. If you actually want star-trail photos that's great, but if you want Milky Way shots or deep-sky shots then it's going to ruin the shot.
Consequently these shots are usually taken at or very near the wide-open setting for that lens.
2. This means that the f-stop wont save you if you miss focus. So you'll want to be careful to dial in the focus as fine as possible.
The common technique (this is just to help focus - not for the real exposure) is use live-view mode, switch the lens to manual-focus mode, crank the exposure to max (max out the ISO, set the aperture to wide-open, and set the shutter speed to 30 seconds). This is because Canon uses "exposure simulation in live view" and that will make it brighten up the points of light for each star.
Find a nice bright star (this time of year, that would be Sirius - follow the three stars of Orion's Belt to the "left" (east) and the brightest star you find in that direction is Sirius. It's the brightest star in the night-time sky.
Center, focus the lens to near the infinity mark (so that's "close" but just not perfect) and then zoom the live-view to 10x magnification. Now *carefully* refine focus to bring that star down to a pin-point.
Don't forget to return the exposure settings to what you actually plan to use. Point the camera to the section of sky you want to shoot (without bumping the focus ring).
Now you can take your shots.
You can also get focus-masks such as the "Sharp Star" focus mask by Lonely Speck or you can buy a "bahtinov focusing mask" (that's not a brand, it's a type of focusing mask). Both of these cause points of light to produce diffraction spikes which will converge at a common center point when you've nailed perfect focus.
I have taken shots at f/8... but only on a tracking head (no landscape ... deep sky objects). If you can spare the time and the alignment of the tracking head is bang-on accurate then reducing the aperture can help correct for lots of lens issues such as field-flatness (even focus across the field) and other optical issues. But if on a fixed tripod you generally have to shoot at a very low focal ratio or you wont get enough light before the Earth spins too much.
Tim Campbell
5D III, 5D IV, 60Da
