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I am using a canon 10-18mm wide angle lens on a Canon t3 camera.

I want to try shooting some stars at night. So I want to focus the lens to infinity.

But the lens has no hard stop to it, or any sort of visual indication. The focus ring just turns infinitely. So I have no clue how I am supposed to focus this lens to infinity.

Is my only option to auto focus it to a distant building tape the ring?

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As others already said, when shooting at night you almost always need to focus manually. When doing astro photos, if you can find a bright star or planet, you can try to manually focus on them using your camera live view feature.

If you can't find any star or planet bright enough, you can focus on a light that's far enough to be, essentially, at a distance indistinguishable from infinity from any practical point of view. Such a distance is called hyperfocal distance.

Hyperfocal distance depends on the camera sensor, lens focal length and aperture, and you can find several calculators, tables or even mobile apps to know it.

For your Canon T3 camera with a Canon 10-18 mm lens, the hyperfocal distance is about 4 m (at 18 mm and at f/4.5). This means that if you focus on a flashlight slightly farther than 4 m, anything farther than that flashlight will still be in focus, even the stars.

See the following diagram from Hyperfocal Pro app for Android, that shows that anything at a distance bigger than your hfdwill appear in focus (hfd stands for hyperfocal distance and DOFstands for depth of field - the interval of distance where everything appears in focus).

enter image description here

If your flashlight is bright enough, you can even use autofocus to focus on it.

So you can use the following workflow to quickly and effectively focus for the stars:

  1. Get the hyperfocal distance for your camera+lens system.
  2. Put a flashlight ad a distance farther than your calculated hyperfocal distance, and point it at your camera.
  3. Use autofocus to focus on the flashlight.
  4. Once the camera is successfully focused, check the focus with live view (use maximum zoom) and then disable autofocus (I suggest to tape the focus ring on your lens, to avoid any accidental change of focus).
  5. Point your camera at the sky, check again your focus using live view, and then compose your photo and take it.

Once you focus on infinity the first time, you often don't need change focus again as long as you're shooting anything farther than hyperfocal distance. But remember that after changing focal length (zooming) you need to focus again.

  • What is the hyperfocal distance for 10mm at f4.5? – ScottF May 9 '18 at 3:43
  • As said above, hyperfocal distance depends on your camera sensor, you lens focal lenght and aperture. With a Canon T3i, a 10 mm lens at f/4.5, the hyperfocal distance is about 1.2 meters. There are several smartphone apps that do the calculation for you, but if you prefer you can do the calculation from this website, and even print out a table to keep at hand for future reference: dofmaster.com/dofjs.html – gerlos May 9 '18 at 16:17
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    @gerlos Keep in mind tha all hyperfocal calculations are also based on intended display size and viewing distance as well as assumptions about the viewer's visual acuity. If they're not explicit in the DoF calculator, they're probably based on an 8x10 inch display size viewed at a distance of one foot by a person with 20/20 vision (or 20/15 in the case of Zeiss). – Michael C May 9 '18 at 22:17
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    For astrophotos the stars are still a blurry mess if you only focus just past 4 meters at 18mm and f/4.5, even though that is what the hyperfocal charts indicate. This is because we expect stars to be points, not circles and because we tend to pixel peep our images at enlargements that are significantly larger than 8x10". Looking at a 24MP image on a 24" HD monitor at 100% is like looking at a piece of a 60x40" print! – Michael C May 9 '18 at 22:21
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    @MichaelClark is right. Final result depends on the size of the circle of confusion you plug in the hyperfocal calculation. Some calculators let you tune it to suit your needs. – gerlos May 10 '18 at 10:19
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It is almost always necessary to manually focus astronomical subjects. The AF systems in most cameras can't focus on small, dim objects in the sky. Even when they can, their margin of error is usually too great to give the kinds of results most people desire when doing astrophotography. The same is true of focus markings on lenses that have them - they're not accurate enough for critical work with point sources of light such as stars.

Because infinity focus changes with focal length and other environmental factors, most modern lenses allow the focusing elements to go past infinity. Many lenses with high speed AF motors allow an even greater buffer past infinity focus so the focus motor is less likely to bump against the end of travel when trying to focus at infinity.

With digital cameras manual focus using the viewfinder can be difficult even in bright light. It's even harder in dim light. Fortunately there is a better way if your camera has a Live View shooting mode. Set the lens to "manual focusing", use the Live view magnifier to zoom in on a section of the sky (in your case the moon) and manually focus until it is as sharp as you can get it. Since AF is turned off, the lens will stay focused at that distance as long as the focus ring is not moved. You can then exit live view and shoot using the viewfinder to compose. Just be careful not to touch or move the focus or zoom rings on the lens.

  • So to explicitly touch on my question....I can not actually focus to infinity directly. I just have to focus on a distant object. – ScottF Apr 19 '17 at 18:20
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    Why would you say you can't focus to infinity directly? Focusing on a distant enough object is focusing to infinity. Stars are certainly distant enough for that. – Michael C Apr 19 '17 at 19:46
  • I mean I can not set focus to infinity without actually focusing on something. Eg on my lens that has a focusing display, I can focus to infinity by setting it to infinity on the dial. I didn't need to actually look through the view finder or go through any process. I just set my lens to focus to infinity. – ScottF Apr 19 '17 at 20:05
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    @ScottF In which case you are setting it approximately to infinity and probably not close enough to satisfy most users doing astrophotography. – Michael C Apr 19 '17 at 20:15
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    @Scott: Manually setting the dial to infinity is pretty inaccurate: we're talking about a wiggle room of fractions of millimeters on the dial. Checking through liveview's magnification feature whether you've actually nailed the focus is strongly advised. Personally, I'd like to avoid waiting several minutes out in the cold, just to see my patience being rewarded with a blurry image. ;) – ParaDice Apr 20 '17 at 8:40
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For shooting stars, you don't need to focus at infinity. You need to focus at something a bit lesser than infinity to get everything tack sharp. You can take around 5-10 photos to decide at which position you are going to hit that point.

Alternatively you can mark that point both in the ring, as well as its adjacent place (in the lens) to make your life easier. :)

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Whatever method you use, mark the lens after you are satisfied you found true infinity. From Astro to street shooting, this is so much quicker than ‘checkiong’ your focus each time, especially with wide angles.

Also worthwhile to ‘mark’ other distances for zone focusing when stopped down and using a hyper focal distance method of gaming maximum DoF.

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    This does not work with the Canon 10-18mm lens, as the focus ring is "fly by wire" and turns infinitely. – ScottF Mar 7 '18 at 15:40
  • A mark on the lens may work with many lenses, but should be used with caution, since it can also give unexpected results because of thermal expansion (or reduction) of lens internals that can slightly change the focus. – gerlos May 10 '18 at 10:25

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