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[OM] Re: Simple question about 'clear' screens

Subject: [OM] Re: Simple question about 'clear' screens
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Wed, 04 May 2005 07:34:15 -0400
I've never understood this clear screen/aerial image business.  Why is 
focusing the aerial image on the camera different from focusing the 
aerial image on a microsope or telescope?

Chuck Norcutt

Mark Dapoz wrote:

> On Tue, 3 May 2005, Jay Drew wrote:
> 
>>Doesn't the varimagnifier help in that instance, especially if you've
>>focused the crosshairs on the 1-11 or 1-12 while at the 2.5 setting?
> 
> 
> Yes.  To quote from the OM bible (Mannheim, p299) about this whole issue:
> 
> "Focus the eyepiece of the Varimagni finder unit until the hairline graticule
> of the screen appears clear and sharp.  These crosshairs are actually pairs
> of fine lines, 0.05mm apart, intruding into the centre circle from the two
> sides, top and bottom.  When the Varimagni is correctly focused, you should
> clearly see the two separate lines of each pair.  With the combined camera
> and Varimagni finder magnification (5.5 x at the normal Varimagni setting)
> the visual depth of field is small enough so that you see the image sharply
> only if it is correctly focused in the plane of the graticule marks.  So
> adjust the camera focus until this image appears at maximum sharpness.  The
> lens must of course be at full aperture during focusing.
> 
> As a check move the eye slightly to and fro sideways behind the eyepiece:
> at the sharp focus setting the image detail must not shift relative to the
> graticule lines.  If it does, this slight image parallax means that the image
> projected by the camera lens is not exactly in the correct graticule plane.
> Adjust the camera focus (not the Varimagni eyepiece focus) accordingly.  By
> switching the Varimagni to 2.5X magnification (11 x overall) you get a still
> better check of the aerial image sharpness, though at the cost of not seeing
> the whole picture area."
> 
> So there you have it...... mark
> 
> 
> 
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