Re: Depth of Focus & Formula?`

From: Jon Bell (jtbellj3p_at_presby.edu)
Date: 02/23/05 

Date: Wed, 23 Feb 2005 12:47:30 +0000 (UTC)

In article <sJYSd.3614$MY6.2120@newsread1.news.pas.earthlink.net>,
W. Watson <wolf_tracks@invalid.inv> wrote:
>I'm looking in a microscope, and can see the object in the eyepiece from
>a 1/10"
>out to 3" pretty clearly. Of course, it gets smaller as I go out. Isn't this
>called the depth of focus?

Are you talking about moving your head back and forth behind the eyepiece?
If so, that's simply the lens of your eye changing its focal length to
"accommodate" to the changing distance between your eye and the image that
you're looking at. It's the same reason why you can hold an object within
a range of distances from your eye and still see it clearly. For young
people this range of distances is usually, quite large. For older people
(like me!) the range is much smaller, which is why we need bifocal or
trifocal eyeglasses.

> Perhaps it's how much I can move the microscope's
>knob in and out and still maintain focus, which isn't much.

That's one way to describe "depth of focu" for a microscope.
Alternatively, it's the range of object positions, for fixed lens
position, that produces acceptably sharp images. Note "acceptably" rather
than "perfectly".

> If my head movement example is wrong, then why can I clearly see the
>image over a sizable range? My eye is a lens, so isn't there an area
>(range) of focus?

See above. The lens of your eye can change its shape and therefore its
focal length. As you get older this ability decreases. 

-- 
Jon Bell <jtbellm4h@presby.edu>                     Presbyterian College
Dept. of Physics and Computer Science        Clinton, South Carolina USA

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