Re: Eyepiece and virtual image

Posted by Julian Smith III on
URL: http://confocal-microscopy-list.275.s1.nabble.com/PSF-with-DIC-tp3781952p3788414.html

Yes--what you describe is quite true--but the point is that you do have
to refocus the microscope to make the projected image sharp. In fact,
there are old methods for using that to take photos with plate-film
cameras, where the image was refocused on the camera's ground-glass screen.

But I think the answer to your original question is that the
intermediate image is not quite at the front focal point of the
eyepiece, so the eyepiece isn't really projecting an image to infinity.
In fact, when you look in the eyepiece, the apparent (virtual) image
distance is roughly 25cm away. My understanding is that this is
(historically speaking) so that you could look in your upright monocular
microscope with your left eye and at your drawing paper on the table
next to the microscope with your right eye.

That said, I think the physical distance between the actual front focal
plane (image at infinity) and the intermediate image (image at 25 cm) is
probably pretty small. I'm not able to walk through the math to get
there....

Julian


Joachim Hehl wrote:

> I would say it is in focus. But yes it gets a bit sharper when
> adjusting the focus. And the wider I go back with the paper the bigger
> the image is, like a projector. Maybe you try it out with one of your
> scopes?
>
> Joachim
>
> On [DATE], "Guy Cox" <[ADDRESS]> wrote:
>
>     But is this image truly in focus? If you adjust the microscope
>     focus, does it get sharper?
>
>     Guy
>
>
>     ------------------------------------------------------------------------
>     *From:* Confocal Microscopy List
>     [mailto:[hidden email]] *On Behalf Of *Joachim Hehl
>     *Sent:* Thursday, 8 October 2009 8:03 PM
>     *To:* [hidden email]
>     *Subject:* Re: Eyepiece and virtual image
>
>     Dear Guy,
>     The point is that I neither adjust the focus nor lift the
>     eyepiece. I look through the eyepiece (without my glasses, I am
>     short-sighted), focusing and then I only hold a piece of paper in
>     front of the eyepiece and can see the picture on it. And this also
>     happens to my normal-sighted colleagues.
>     Joachim
>
>     On [DATE], "Guy Cox" <[ADDRESS]> wrote:
>
>         You can always get a real image from an eyepiece by
>         refocussing so that the first image is in front of the focal
>         plane of the eyepiece - either adjust the microscope focus a
>         little or lift the eyepiece slightly in its tube. If you see a
>         sharp image without refocussing from your normal viewing
>         position, it probably means that you - like me - are long-sighted!
>
>         Guy
>
>
>
>         Optical Imaging Techniques in Cell Biology
>         by Guy Cox CRC Press / Taylor & Francis
>         http://www.guycox.com/optical.htm
>         ______________________________________________
>         Associate Professor Guy Cox, MA, DPhil(Oxon)
>         Electron Microscope Unit, Madsen Building F09,
>         University of Sydney, NSW 2006
>         ______________________________________________
>         Phone [phone] Fax [phone]
>         Mobile [phone]
>         ______________________________________________
>         http://www.guycox.net <http://www.guycox.net/>
>
>
>
>
>         ------------------------------------------------------------------------
>         *From:* Confocal Microscopy List
>         [mailto:[hidden email]] *On Behalf Of
>         *Joachim Hehl
>         *Sent:* Thursday, 8 October 2009 7:36 PM
>         *To:* [hidden email]
>         *Subject:* Eyepiece and virtual image
>
>
>
>
>         Dear all,
>
>         I have a –maybe stupid- and not confocal but “basic optic”
>         question:
>
>         As you can read in all textbooks concerning microscopy and
>         geometric optics the objective produces a real, inverted and
>         magnified image since the distance from the object to the
>         object front lens is bigger than one but less than two focal
>         lengths of that given objective.
>         This intermediate image is then magnified by the eyepiece.
>         Since the intermediate image lies exactly in the front focal
>         plane of the eyepiece the result is a virtual, true sided and
>         magnified image which occur in the infinite space. Our eye
>         with its optical components is then producing a real image on
>         the retina.
>         So far, so good.
>         By definition, a virtual image can not be captured on a
>         screen. BUT: When I hold a piece of paper in front of the
>         eyepiece in a distance bigger or smaller than the back focal
>         plane of the eyepiece (the distance I use when I look through
>         it with my eyes) I am able to capture a pretty sharp image of
>         my object on the paper. Why is this? I should not since it is
>         a virtual image?
>         Thanks for your input!
>
>         Joachim
>
>
>         Joachim Hehl
>         LMC-Light Microscopy Centre, ETH Zurich Hönggerberg
>         Schafmattstrasse 18, HPM F16.1
>         CH-8093, Zurich, Switzerland
>
>         Web: www.lmc.ethz.ch
>         Phone: [phone]
>         Natel: [phone]
>         Fax: [phone]
>         e-mail: [hidden email]
>
>
>

--
Julian P.S. Smith III
Director, Winthrop Microscopy Facility
Dept. of Biology
Winthrop University
520 Cherry Rd.
Rock Hill, SC  29733

803-323-2111 x6427 (vox)
803-323-3448 (fax)
803-524-2347 (cell)