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I have a 18-55mm zoom lens with no Infinity symbol on it. Let's assume I want to capture stars in night. There are three ways to do it (If I am not wrong)

  • Use Auto focus (If your camera is able to set it)
  • Use manual focus
  • Use focus point to set focus

    Auto Focus: Now, What if I set my lens to Auto focus and put it to lowest F-stop (No zoom) number Is it not equivalent to setting infinity focus because I am allowing lens to capture the maximum light and allowing light to hit lens in parallel way?

    Infinity Focus: As I mentioned that I don't have infinity sign on my lens. So, I set my lens on MF mode and rotate focus ring in clockwise (Assuming Infinity is on that side) and the zoom ring is exactly at the same position (same f-stop number) as I had in Auto focus mode. Isn't it going to capture the same image like I am having in Auto focus?

    Last question is What is the use of 8-focusing point in Night photography? Will it play role when I have to capture only Moon (Assuming I am able to set the focus point on moon in night)?

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    You might want to re-word your actuay question, as its like asking what is the difference between Darjeeling tea and a cat. Stick to one question per, erm, question please. – Digital Lightcraft Aug 20 '15 at 8:00
  • @DigitalLightcraft I just did. Is it appropriate? – Amit Pal Aug 20 '15 at 8:01
  • Much better :-) – Digital Lightcraft Aug 20 '15 at 8:17
  • First things first, never rely on the positioning of the infinity (or other) symbol being acurate, particularly on cheaper lenses. – Digital Lightcraft Aug 20 '15 at 8:20
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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.

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    I've found that for absolute sharpness in astrophotography the focus position needed for the moon and the focus position needed for stars are not exactly the same. Even when photographing the moon and Jupiter in the same frame during a conjunction several years ago, I had to choose between centering the moon or centering Jupiter in terms of focus. This is particularly true of longer focal length lenses, where even the center of the moon and the edge of the moon (which is half the moon's diameter further from the surface of the earth) need slightly different focus positions. – Michael C Aug 20 '15 at 19:58
  • @MichaelClark, very good point indeed. Sharpness is not just about luminosity and resolution, but also color rendition. On bright scenes this is not as noticeable as it is in low-light astrophotography. Prime lenses and lenses with low-element counts show this better than zooms and high-element count lenses. – Emacs User Aug 20 '15 at 20:11

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