Re: Using a DSLR (was Re: North Korea satellite observed)

From: Marco Langbroek (
Date: Fri Dec 14 2012 - 15:17:09 UTC

  • Next message: George Roberts: "Re: Using a DSLR (was Re: North Korea satellite observed)"

    Op 14-12-2012 16:07, Gavin Eadie schreef:
    > I've an associated question regarding focusing.  I've never been able to focus really well on star fields, and seem to often catch stars as little blobs of light [a recent posting here, into Orion, also exhibited a little of this].  Autofocus doesn't work well (at all?) in the dark of course, and manual focus is tricky -- visual focus throughout the lens doesn't have enough light or precision; or I can set focus under lighter conditions and carefully not touch the lens, though I suspect temperature changes cause some drift.  Also, I can't just rotate my lens to the physical limit and expect that to focus at infinity because it travels a tiny bit past infinity.  Obviously many people in this group have solved this problem and I'd much appreciate some suggestions.
    You need a camera with a "live view" option - i.e., one that can show a live 
    image on the LCD display. For Canon EOS that is the 450D and later models, for 
    Nikon or other brands I do not know.
    Point to a bright star. Put the camera on "live view" and the lens on manual 
    focus. Then manually focus on the star image on the LCD screen: zoom in on the 
    screen if the camera has that option.
    This is the *only* way to properly focus.
    Autofocus will not work, and the "infinity" mark on modern autofocus lenses is 
    only an approximation at best. Some lenses are moreover very prone to focus 
    shifts with temperature change.
    - Marco
    Dr Marco Langbroek  -  SatTrackCam Leiden, the Netherlands.
    Cospar 4353 (Leiden):   52.15412 N, 4.49081 E (WGS84), +0 m ASL
    Cospar 4354 (De Wilck): 52.11685 N, 4.56016 E (WGS84), -2 m ASL
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    Twitter: @Marco_Langbroek
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