How do I focus an 18-55mm lens at infinity for stars and the moon?
Asked 8/20/2015
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My 18-55mm zoom lens does not have an infinity mark. For night photography, I want to shoot stars and sometimes the moon. I’m confused about the difference between autofocus, manual focus, aperture, and focus points.
If I set the lens to autofocus and open the aperture to the lowest f-number, does that effectively mean infinity focus because more light is entering the lens?
If I switch to manual focus and turn the focus ring all the way to one end, is that the same as focusing at infinity?
Also, what do multiple autofocus points do in night photography, and can they help when photographing the moon?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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What if I set my lens to Auto focus...
Assuming you are focussing on stars bright enough for your autofocus to pick up. If not try to focus on the moon or some other bright object in the distance with autofocus. Since stars and star trails are very faint, to capture them properly throughout the frame you are fighting against two competing factors affecting image quality. When you have your aperture wide open, sure you'll get lots of light but depending on the lens, the frame edges will not be as sharp as the center of the image. However if you make your aperture as small as is available on the lens, then you are introducing chromatic aberrations. The sweet spot is somewhere in between the two extremes. Say your lens has f/4 to f/22, then I'd try f/11 or f/13 to start and then test some sample images to see where your particular lens exhibits the least softness and least chromatic aberration.
Infinity focus: ... Isn't it going to capture the same image like I am having in Auto focus?
Only if the autofocus picked the same object as what you are using to focus in manual. However if your question is about copying the mechanical position of the rings from autofocus to manual operation, be aware that some lenses have clutches that turn past infinity though the lens mechanism inside stays put. The best option is to pick a bright object like the moon or a distant light bulb, autofocus and then turn the lens to manual focus to retain that last autofocus point. This way you won't have to repeat focussing in manual mode. Again I'd use f/11 or f/13 or whatever is the sharpest for that lens. Avoid extreme apertures.
What is the use of 8-focusing point in Night photography?
None if you are shooting stars and star trails. Choose single point focus instead. The only role it will play in some cameras is to drain the battery faster which in-turn may affect your time lapse calculations for stars and star trails.
Originally by user39114. Source · Licensed CC BY-SA 4.0
user39114
10y ago
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Generated from our catalog & community — verify before relying on it.
No. Aperture and focus are separate controls. Using the lowest f-number lets in more light, but it does not set the lens to infinity focus.
For stars, autofocus often struggles unless there is a very bright target. A practical method is to autofocus on the moon or another distant bright object, then switch to manual focus so the lens stays there. If autofocus cannot lock, use manual focus and test carefully.
Also, turning the focus ring all the way to the end is not always exact infinity on modern lenses. Many lenses can focus slightly past infinity, so the end stop may not be the sharpest setting for stars. Take test shots and check magnified playback if possible.
Autofocus points only matter if autofocus can detect enough contrast/light. At night they are often not useful for stars, but they can help place focus on the moon if the camera can lock onto it.
One more note: the sharpest aperture for night scenes is often not the lens’s widest or smallest setting. Many lenses perform best somewhere in between, so testing different apertures can improve edge sharpness.
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