low light focusing

[hornetzoo]

Can someone tell me how to set my tamron 28-75 to focus at infinity? I have a 20D. The problem is that there it is capable of focusing past infinity, so at night (nearly pitch black), I have no way of knowing if the picture will be exactly in focus.

[tim]

The only way I know is to do it visually. If you want to use AF then use an external speedlight flash, which has a focus assist beam.

I saw an EOS-5 last night, it has the focus assist beam built into the body, which is a good idea.

[hornetzoo]

Thanks, yes I'm trying to do it visually. But in very low light, when I basically can't see anything, this is not really possible. Any other ideas? What if I just focus on a bright light "x" feet away and lock the focus there? How big would the "x" have to be?

[tim]

Personally i'd just try it and see. Set the focus on it's extreme end range, then try tweaking it manually and see which tweak you like best. Focusing on something you can't see's going to be tricky no matter what you do.

If you want to focus on something X feet away then depth of field will come into play. Have a look and play with http://www.dofmaster.com/dofjs.html

[badrotation]

I carry my several million candlepower spotlight with me when I am shooting at night, and shine it on a contrasty object where I want the focus to be... it works about 90% of the time.