Olympus-OM
[Top] [All Lists]

[OM] determine precise infinity focus for a lens

Subject: [OM] determine precise infinity focus for a lens
From: "Richard Locke" <dtlocke@xxxxxxxx>
Date: Tue, 31 Oct 2000 07:53:40 -0600
Here's how to determine the precise infinity focus for a lens (I should say at 
a given temperature, as temp. apparently will cause focus shifts in some 
lenses...)
1) construct a mask to place over the lens; the mask should have two holes of 
about equal size.  Cut two holes in the mask of about 1/3 the total diameter of 
the mask.  The holes don't have to be round or even.
2) stick a small slice of an address label or similar onto the rotating part of 
the lens where you can line it up against fixed markings on the lens.  Make 
marks every milimeter on the lens.  The idea here is to be able reproduce the 
exact focus setting.
3) mount the camera/masked lens to a fixed tripod and go outside on a clear 
night away from direct light sources.  Take a log book.  Aim the camera at a 
group of bright stars with the lens all the way open.  I'd recommend Kodak 
Elite Chrome 200 slide film.  In northern hemisphere, you want to aim just 
about due south, about 20 degrees south of directly overhead (this is where 
stars appear to move the fastest.)
4) set the focus to infinity and open the shutter using the bulb setting for 45 
seconds
5) cover the lens for 30 seconds (with your hand, a hat, whatever)
6) rotate the lens one milimeter using the marks as a guide, expose for 30 
seconds, cover the lens for 30 seconds, repeat.  (Going 1/2 mm at a time is 
even better if you have the patience.)  Keep a log of each setting.
7) close the shutter, get the film developed, and examine under high 
magnification.  You will have star trails on the film.  The longest star trail 
is the 1st.  When the lens is not precisely focused you will see "fat" or 
double trails.  Where the star trail is the thinnest is your precise focus 
point.

This technique is described in Robert Reeves' "Wide Field Astrophotography" 
book.  Let me know if anything is confusing.

-Dick


< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
  • [OM] determine precise infinity focus for a lens, Richard Locke <=
Sponsored by Tako
Impressum | Datenschutz