Prefocus for Astrophotography

Yesterday I found a blog post on by Luis Argerich titled How to Photograph the Milky Way. I posted a link to the article on the Night & Low Light Facebook page and also shared it with the Create Photographics Facebook page (for full disclosure, I’m an admin on both Facebook pages). One of the comments was about how the camera had trouble finding something to focus on. Luis responded with nice tip about using Live View to zoom in and check on the focus. What do you do if you don’t have Live View or if you are still having trouble getting the stars in focus?

Here is something I do and you might want to try: prefocus your lens before you go out to photograph the night sky. It’s not very hard to do if you have lenses that have a little window that shows the focusing distance. Most of my Nikon lenses have them but I do know that some lenses don’t (more about that in a little bit). All you need to do is set the center of the infinity symbol (∞) on the line at the middle of the window. Once you have done that tape down the focusing ring so it will not move. That lens is ready to make images all night of any celestial body you want to point your camera at. Do this to all the lenses you might want to work with and you can carefully change lenses as often as you want to. One thing to keep in mind when using zoom lenses: don’t tape down the zoom ring. When you tape down the focus ring make sure you only tape down that one ring because you will want to use the zoom feature to frame your images.

The next step is to turn off the auto focus on your camera. The camera will still try and focus even though you have the focusing ring taped off. Some lenses can still be focused by the camera even with the focusing ring taped down. Other lenses will try to autofocus and will push against the tape. That can also hurt the bearings in the lenses if your camera fights hard enough to force the lens to focus. If you lens has a M/A M button on the lens turn that off as well, just to be sure.

So what do you do if you have a lens that does not have the little window that I spoke about earlier? You will need to do a bit of planning ahead. If you know you are going out at night to make some images prefocus your lenses on a distant subject earlier in the day while you have enough light to focus the lens. When the camera has focused the lens take your finger off the shutter release or the focus button (if you use back button focusing) and tape down the focus ring then turn off the autofocus on your camera. Repeat this process for all the lenses you might want to use.
Have fun out there making images of the night sky!

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