Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It only takes a minute to sign up.
Sign up to join this community
Last night, when I was attempting to do some astrophotography, I found it rather difficult to focus on the stars/tress/buildings in nearly pitch darkness. The Live View feature doesn't show much as it is so dark, and you can't see through the viewfinder either (when it's dark)
The one single lens I have, an 18-55mm IS Canon lens doesn't have an infinity marker on the focus ring. It's just a standard lens. Also my first lens (I'm an amateur)
For focusing on stars, I suggest using a Bahtinov mask, which uses purposely-created diffraction spikes to determine correct focus.
Bahtinov mask by Justin Dolske, from Flickr. CC BY-SA-2.0
This image montage is an example of a Bahtinov mask on a telescope focusing on the star Betelguese. The center image is correctly focused; the other two images are under- and over-focused, respectively.
There are several manufacturers of Bahtinov masks, or you can make your own (there are even online calculators that will generate Bahtinov mask patterns to print out and cut). The size and spacing of their diffraction gratings is dependent upon focal length and absolute aperture.
Personally, I use the SharpStar2 4x4 filter from LonelySpeck (I'm already a 4x4 filter user). I couldn't be happier with it.
Agena Astrophotography makes a mask that snaps into any 77mm UV (or clear) filter. Agena's mask is very inexpensive (about $15), and a cheap UV filter can be had for under $5.
I have found that the narrower/tighter slit pattern on the SharpStar2 filter creates larger, cleaner diffraction patterns than the wider-spaced grid on filters such as Agena's, which makes focusing easier. Regardless, even using patterns like on the Agena, Bahtinov mask focusing is so much easier than without using a mask at all.
An alternative solution to Scott's answer would be to add an infinity marker onto the lens, so you can easily set it to the right point in the future. If you go out on a night when the moon is out, see if the camera can focus on the moon. Set the camera's focusing so it only uses the centre focus point, then once you've focused on the moon at a given focal length (because the focus point may vary if you change the focal length), then you can put a mark on the lens to show were you need to line it up for future use. If you want to shoot at different focal lengths, you can do the same method at each focal length, and test to see if the point changes.
Another option as one of the comments mentions, is to zoom in as far as you can using liveview, and use manual focus.
