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On the kit 18-55 lens that came with my Nikon D5100 I have noticed that when trying to focus on infinity for shots in dark conditions (star-trails, etc) the lense tends to move past infinity out of focus and need to be brought back a little to get a clear infinity focus point.

Is there a good trick for getting a clear infinity focus when in pitch dark conditions?

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Focus infinity.. AND BEYOND !! –  Berzemus Jul 18 '13 at 10:10

1 Answer 1

up vote 7 down vote accepted

It is not just cheaper lenses. Many modern lenses, especially Auto Focus zoom lenses have this characteristic. There are several reasons for it:

  • Unless a lens is parfocal the exact point of infinity focus shifts as the lens is zoomed in or out, and so obviously there will be a point where infinity for one focal length is past infinity for another.
  • As temperature and other environmental conditions change, the various materials that make up a complex lens expand and contract at slightly different rates. This affects focus position for infinity.
  • Auto Focus lenses use fairly strong motors to move focusing elements quickly. By leaving a little extra room past infinity, the lens designers allow the motors to power the focus assembly all the way to infinity without bumping against a hard stop that could reduce the life expectancy of the motor and other focus components.

The best way to get clear focus for astrophotography is to use Live View at high magnification to manually focus the lens, then leave the focus set at infinity and turn off Live View.

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We actually have a very nice existing question and answer on astrophotography and getting the proper focus - photo.stackexchange.com/questions/23972/… –  dpollitt Jul 18 '13 at 0:42
    
Thanks. I understand why now. Although, a lot of the higher end lenses have an infinity marker on the lens. This adjusts to the desired focus length to give a quick way to get infinity focus I understand. Is this correct? –  Tim Jul 18 '13 at 0:43
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@Tim The accuracy of the infinity mark on a lens varies pretty much on a lens by lens basis. For astro-photography, where the focus needs to be so precise, you are much better off using a magnified Live View to manually focus the lens. –  Michael Clark Jul 18 '13 at 22:12

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