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Re: [OM] diffraction, aperatures, f-stops, extensions

Subject: Re: [OM] diffraction, aperatures, f-stops, extensions
From: "Brian Swale" <bj@xxxxxxxxxxxxxxx>
Date: Sun, 10 Mar 2002 13:44:50 +1300
Hello Dean

Something else occurs to me with respect to your difficulties; and it may not 
be a welcome suggestion; but I think you may need to address it at some 
stage. Anybody please correct me if I have this wrong.

Since exactness of focus is a critical matter in this work, you need to be 
satisfied that when your focussing system tells you that it is focussing 
accurately, it really is.

If the mirror of you camera is a slight bit off the 45 degree angle, the 
accuracy of what you interpret will be off. You might find it rewarding to have 
the collimation of your camera checked.

As I understand it - open to correction.

Brian

Date sent:              Sat, 09 Mar 2002 18:13:50 -0600
From:                   "Dean C. Hansen" <hanse112@xxxxxxxxxx>
To:                     bj@xxxxxxxxxxxxxxx
Subject:                Re: [OM] diffraction, aperatures, f-stops, extensions

> Brian Swale wrote:
> 
> > Hi Dean
> > [slight snip]
> >
> > How are you checking the exactness of your focus?
> >
> > Brian
> 
> Hello Brian,
>     Well, this was one of the two problems.  I needed to work with the diopter
> adjustment to get the double cross-hairs into sharp focus.  Good call!
>     The other culprit was stopping down too much.  As I write on my latest
> posting to the OM list, the f-stop with the best resolution/depth of focus
> combination is f2.8.  Image quality declines at f4 and gets really bad at f8.
> We learn something every day.
> Best wishes,
> Dean

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