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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)

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For focusing on stars, I suggest using a Bahtinov mask, which uses purposely-created diffraction spikes to determine correct focus.

Bahtinov mask focus example
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.

  • Does the mask still work if the bright "star" you pick is Jupiter or Venus? – Mark Aug 18 '16 at 21:58
  • @mark Depends on your focal length. The pattern is based on being pointed at a bright point light source. If your focal length is short enough that the planets appear as points you should be ok. If your focal length is long enough that they begin to appear disc-like, then no. – tittaenälg Aug 18 '16 at 22:23
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    @mark for your 18-55mm lens, you will have several stars in view to pick from. They will all have cross hatches. They're all at the same effective focus distance: infinity, for all practical purposes. – scottbb Aug 18 '16 at 22:29
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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.

  • I've shot the Moon, Jupiter, and stars all in the same frame. They each required slightly different focus positions to bring each of them to absolute greatest sharpness. – Michael C Aug 18 '16 at 22:32
  • What do you mean "add an infinity marker"? How do you do that? – bearmohawk Aug 19 '16 at 9:06
  • @bearmohawk you can just use a marker pen, or score a little mark in the lens body, crossing over the edge of the focus ring and the rest of the lens, so that you can turn the focus ring and see when these line up – laurencemadill Aug 19 '16 at 10:00
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    or cut a 1/8 inch wide strip of masking tape and tape it to the focus ring and adjacent point on the body if you have an aversion to scratching your lens. – user57200 Oct 4 '16 at 14:52
  • Infinity marks don't work on any focus-by-wire lenses, nor on any Ultrasonic (USM - Canon), Silent Wave (SWV - Nikon), Hyper-Sonic (HSM - Sigma), Super-Sonic (SSM - Sony), Supersonic Drive (SDM - Pentax), Supersonic Wave (SWD - Olympus), or Ultrasonic Silent (USD - Tamron) drive lenses that allow the focus ring to "slip" when rotated past the limits of the focusing elements range of motion. – Michael C Jul 23 at 22:35

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