A rule of thumb method to calculate the maximum exposure time, for a particular focal length, before star trails become noticeable. It's simply 600 / focal length = exposure time (in seconds).
I can't help you with the stacking thing unfortunately
but I gotta ask why use f/8?
I use 2.8 on my 16-35 with the focus set to infinity..
That way you can keep your ISO down to a minimum and get less noise at
the same amount of time.. using the 600 rule...
If you have a 30 Sec exposure using ISO 6400 @ f/8 you can go down to
ISO 800 @ f/2.8 and still have a 30 sec exposure..
You'll still get the sharpness you need..
What is the 600 rule?
For a 50mm lens, 600/50 = 12 seconds.
For a 15mm lens, 600/15 = 40 seconds.
By keeping the exposure to those times or less, the stars appear as dots, not trails.
Wow...talk about a VERY informative post for a noob!!!!
Thank you very much!
ps. Do you have any insight into what stacking photos are? Is that similar to HDR photography?
There is many programs who can help you with stacking, and you use stacking for many things, which among them are stacking panoramas, HDR or focus stacking. There are great tutorials on this if you enter these names on youtube. I use Photoshop CS 6 for this, and Photoshop 5 before that and I think it works great.
Example one Panorama picture (and you can see the result earlier in this post with my shot from Shanghai). I used a 5D II with a 16-35 II. I turned the camera to portrait mode (vertically, to get more sky) and shot eight pictures free hand. What I normally do is to make sure that the pictures overlap with about 25%. After getting them on the computer just go to photoshop - file - Automate - Photomerge and then choose the option that work best for your shot.
Example two Focus Stacking. Although a lowres version of this picture, this picture is taken with the 17mm TS and is comprised of 28 pictures in one.
Focus stacking can be especially useful in macropictures as DOF is very shallow.