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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?
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).
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:
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.
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.
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. :)
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.
