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Hey, John--

John Runions wrote:

> Ok everyone, here is a question I've been stewing over for a while.  I 
> know some of you will howl with indignation that I can't figure this one 
> out and probably have me drummed out of the society for good but here 
> goes...  If a small aperture (large f-stop number) gives you longer 
> depth of focus in photography, then how come a smaller pinhole in the 
> emitted light pathway results in reduced focal plane thickness in 
> confocal microscopy?
> 
> Signed, Confused.

I think the answer all comes down to the paths of in-focus and 
out-of-focus rays.  In the case of a small f-stop on a camera, the 
smaller the aperture, the closer the camera comes to being a pin-hole 
camera.  In the case of an ideal pin-hole camera, everything is in 
focus.  With an infinitely small pin-hole, you have infinite depth of 
field because rays originating from any point in space can project 
directly onto a plane.

With a confocal, the further from the point of focus that the rays 
originate, the further they fall from the pinhole.  Thus the smaller the 
pinhole, the more restricted the depth of field.

Now...suppose you tried to cross a confocal with a pin-hole camera and 
substituted a small pinhole for the objective.  You'd lose your confocal 
effect.  This is comparable to using an objective with a small NA or not 
filling the back-aperture of the objective.

Hope that helps.  Just this morning I was thinking about how other 
people on this list know so much more than I.  It's gratifying to be 
able to answer a question like this!

Martin Wessendorf

-- 
Martin Wessendorf, Ph.D.                   office: (612) 626-0145
Assoc Prof, Dept Neuroscience                 lab: (612) 624-2991
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