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Re: [OM] Question on Focus

Subject: Re: [OM] Question on Focus
From: Paul Wallich <pw@xxxxxxxxx>
Date: Thu, 18 Apr 2002 12:59:18 -0400
At 9:40 AM -0700 4/18/02, Dave Shupe wrote:


I am nearsighted (-2.0) with a slight astigmatism.  I've noticed lately that
I can focus my binoculars at any distance, and see sharply without my
glasses.  My question is, why can't I focus the camera the same way.

I understand why it would not be good to do so, kind of like handing the
binoculars to someone with good vision, they have to refocus.  Therefore If
I could focus without glasses my pictures would be blurry.  Just wondering
how the camera focus system actually works.

It's more complicated than that. The lens and mirror in an SLR project an
image on a little ground-glass (plastic) screen that's the same distance
away from the lens as the film. So when an image is in focus on the screen
you see in the viewfinder, it's in focus on the film.

But that screen in the viewfinder is only a couple inches away from your eye
and lying down flat, so there's another (set of) lens(es) in the viewfinder
that make the focusing screen appear to be at a comfortable viewing distance,
say somewhere between 1 and 5 meters. And that apparent distance is where your
eye has to be able to focus to be able to tell whether the image is right.
Some OM's have a built-in adjustment to change the apparent viewing distance,
others have lenses you can slip onto the finder. If you used either of these
solutions you would be able to focus your camera without your glasses (assuming
the astigmatism wasn't too bad) but no one else would be able to use it
without monkeying with the finder.

paul

--
Paul Wallich                                            pw@xxxxxxxxx

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