Focus and shooting stars

I had a first experience with attempting to shoot stars last night in sedona, az. I used a 1D Mark III set on bulb with a 50 mm 1.2 and ISO of 100-200, varied times but most at about 25 seconds. My focus was set on infinity which I assumed would give me optimal focus. obviously, true focus is hard to tell on the view finder. When I was done, no shots were in focus, I understand the rotation issues that can cause blurring, but this should not happen at exposures this short, any suggestions?

The simple approach:

1) Set lens to AF.
2) Point camera a bright star and press shutter half way with focus point on star.
3) Once the star has snapped to focus, reset lens to MF and be sure camera is set on M.
4) Point camera to area of sky you are imaging. Be very careful not to touch the lens while composing your shot
6) Using the camera delay timer or a remote switch(best) trigger your exposure.

I use this method at short focal lengths(18-55mm) and its very effective. It's a quick way to get focused without repeated test images and focus tweaking.

At higher lens focal lengths there are other devices and techniques for achieving critical focus. Some swear by live view for focus, others use an Anglefinder C, some use laptops to review focus. The same applies to telescopes where even more devices and methods are available.

I hope this is helpful.

I am really confused. Understand the concept except that shouldn't a lens set on infinity automatically be in focus for an image that is light years away. I did that with manual focus and it didn't work so obviously I am wrong. But conceptually, I do not understand.

Most lenses I've used don't have true "infinity" focus at the infinity mark.

Don't you have Live View on the Mk III? If so, see if that is easier to use to get the focus right. On my 50D, live view at 10x makes it easy to get astro shots focused well.

I know that some lenses are not capable of infinity focus and that some have room beyond infinity. I'm not familiar with the 50mm, f/1.2, although I'd like to have one for improving my widefield work. I'm not sure exactly what is happening in your case, I wish I did. I hope someone will come along with a more definite explanation. The majority of what I do at night is with telescopes so I am usually thinking in terms of manual adjustment to achieve critical focus, aside from occasional widefields where I use the method I decribed.

The 50mm f/1.2 should work the same exact way as my 18-55mm IS as far as focus issues . As Steve mentions set the lens to "Auto" , trigger the shutter button half way , when star snaps to focus keep finger steady while using your left hand left index finger to slide the focus switch back to manual focus then afterwards don't touch the lens to refine it . The Auto mode will fine tune it properly and better than you can manually . I've tried the manual focus technique with the 18-55 and the best way is to zoom in at 55mm , do the focus technique mentioned , then adjust the lens back out to 18mm not touching the focus ring . Setting the lens to infinity manually does not guarantee a fine true focus . You still have to visually look into the viewer or use LiveView if your camera has LiveView for focus . If using LiveView besure and use the 10x enlargement for focus .

Yes, as Steve and pingflood said, the problem is that the infinity mark, and the hard stop on the focus ring, don't correspond to the true infinity setting. With the modern lenses I have used the infinity setting on the focus ring is always past the true infinity setting. It may be possible to get autofocus to work using the procedure Steve described, and it should also be possible to focus accurately using live view in manual focus mode as pingflood suggested.

Don