Re: Eyepiece and virtual image

Posted by James Pawley on Oct 08, 2009; 9:51pm
URL: http://confocal-microscopy-list.275.s1.nabble.com/PSF-with-DIC-tp3781952p3791012.html

Hi all,

This topic causes lots of confusion at the UBC Live Cell Course.

We try to emphasize three things:

1. That any lens will create a real image of any object on its "front" side at some distance on its "back" side as long as the object is more than one focal length away from the optical center of the lens.

2. The magnification of this image will be directly proportional to the ratio of the distance between the central plane of the lens and the image divided by the distance between the object and the central plane. (i.e,  With any converging lens, you can get any magnification as long as you are flexible about where your object and your images planes are.)

3. Therefore, the three reasons for using different lenses for different magnifications are

        a. by having different focal lengths, these objectives can produce real images of specific sizes (magnifications) without having to change the object (specimen) and the image (CCD) planes.

        b. Low mag (i.e., long focal length) objectives can be located with their optical center farther from the object and be optimized to cover larger areas of the object while high magnification (i.e., shorter focal length) objectives can be mounted with their optical centers closer to the object and optimized to be free from aberrations at higher numerical aperture.

        c. Most important of all: the aberrations inherent in all simple, refractive lenses can only be compensated for at one particular set of object and image planes.

So yes, an infinity objective (plus its tube lens but either in the presence or the absence of an occular) will form real images of the structures found at any plane in the object that is more than one focal length beyond its optical center. The location of this image along the optical axis will determine what plane in the object will be in focus there.

BUT such an image will only be free from aberrations as long as it is focused on one plane in the object. For infinity-conjugate objectives, the ideal object plane is that found at the front focal plane of the objective and it will focus light from this plane into an (almost) perfect "intermediate" image on a plane 1 cm below the lip of the tube holding the ocular.

The ocular itself is located so that its optical center is one focal length above this intermediate image plane meaning that this image emerges from the ocular as though it were coming from an object an infinite distance away. This is important because, on a binocular viewing unit, the two ocular tubes are parallel to each other and so you would like the optical axes of your eyes to also be parallel, a condition that will only occur when your eyes think that they are focused on distant object. (The idea that this set up was designed to allow one superimpose the image from the microscope with that of an image being drawn by hand about one meter away is a useful approximation, but isn't quite true) .

Yes,  you can find real images of any plane in the object projected onto some plane above the "eye point" or "exit pupil" of the ocular, (or if you remove the ocular, at some plane down the tube) but any such image will only be free from aberrations if the image plane is at infinity (Which is why one uses a special projection lens rather than an ocular to project the image onto, for instance, a CCD camera).

I hope that this helps,

Cheers,

Jim Pawley

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)

--

James and Christine Pawley, 21 N. Prospect Ave. Madison, WI, 53726   Phone: [phone]