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Thread: What is the Optical Center of a Lens?

  1. #1

    What is the Optical Center of a Lens?

    Hi friends,

    I have recently joined this forum and found the stuff and ur discussion very exciting.

    I cam across a term while reading some document related to lens.
    The term is "optical center". I searched reasonably through the web for this term but couldnt get the exact idea that what is it.

    Please share the knowledge if u have some idea.

    cheers
    Mayank

  2. #2

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    Re: what is optical center

    Quote Originally Posted by er.mayankkapoor View Post
    I cam across a term while reading some document related to lens.
    The term is "optical center". I searched reasonably through the web for this term but couldnt get the exact idea that what is it.
    Hi Mayank,

    It's the theoretical point that all light rays travel through when entering the lens (keeping in mind that the image that falls on the sensor is unside down and around the wrong way from what on the "outside" of the lens.

    If you do a search on "nodal point" you might get a better idea (nodal point is what it's more commonly known as, but it's not quite technically accurate).

    We use this point to rotate the lens about when shooting panoramas that need to be stitched; if you don't rotate the camera around it's optical center then objects in the foreground will appear to move relative to objects in the background as the camera is rotated, and make stitching very difficult.

    For all intents and purposes, it's usually VERY close to the front element of your lens, although if you have a nodal bar you can work it out for yourself very easily by putting a couple of objects into the field of view (one near, one far) and see if they change position relative to each other as you rotate the camera. Once there's no relative movement than you've found the right point.

    Does this help?

    PS: Welcome to the forums - great to have you with us Please help us to help everyone by joinging in the conversations as much as you can

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    Re: what is optical center

    Ahh... I love these forums. I didn't even ask this question but I did learn something today

    Mayank, see this page - I found it helpful in explaining.

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    Re: what is optical center

    Quote Originally Posted by dan88 View Post
    Ahh... I love these forums. I didn't even ask this question but I did learn something today
    Make sure you tell all your friends about us then

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    Re: what is optical center

    Thanks guys, especially Colin.

    I am making a pano head & needed to work out the mounting distances.

    Where in the body is the focal plane, where the image is focused? I am trying to measure the range I will need to allow for camera adjustment. It appears to require 120-160mm range if I measure to the absolute back of the body to the front glass of the lens for full range of lens settings.

    My camera is a Nikon D5000 with an 18-55 lens as well as a telephoto lens.

    Trust this makes sense.
    td
    Last edited by teedee; 8th December 2011 at 12:30 PM. Reason: more info

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    Re: what is optical center

    The thick lens has 2 principal points (= optical centers ?):
    http://software.canon-europe.com/fil...Book_10_EN.pdf
    p. 195 (4 of 25): Principal Point
    p. 207 (16 of 25), Fig. 38 and 39: Principal point interval (see attachment)
    (http://www.canon-europe.com/support/...ork_iii_en.asp)

    This is useless theory, but how long can be the principal point interval ?
    I have found only this:
    Google Books: Introduction to geometrical optics By Milton Katz, p 156
    Attached Images Attached Images
    Last edited by darekk; 8th December 2011 at 01:40 PM.

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    Re: what is optical center

    Quote Originally Posted by teedee View Post
    Where in the body is the focal plane, where the image is focused?
    Not sure about the Nikon cameras but on a canon the focal plane of the sensor is drawn on the camera body as a little white line through a circle, usually next to the eyepiece. On my camera it is about 7mm from the back of the body which makes sense as there has to be something behind the sensor.

    The Nikon should have a focal plane mark on it somewhere.

    Regards,

    Alex

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    Re: what is optical center

    Canon's focal plane mark
    Attached Images Attached Images

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    Re: what is optical center

    Nikon has a similar marking (top righthand side on my D40)

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    Re: what is optical center

    Thanks everybody. Yes! The D5000 also has such a mark on top right. It tried its best to hide, disguised very cunningly as a black raised mark on a black body, but this eagle eyed sleuth found it.
    Thanks again
    td

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    Re: what is optical center

    Quote Originally Posted by teedee View Post
    Thanks everybody. Yes! The D5000 also has such a mark on top right. It tried its best to hide, disguised very cunningly as a black raised mark on a black body, but this eagle eyed sleuth found it.
    Thanks again
    td
    Nikon have always been a bit secretive about the precise location of the focal plane. On the Nikon F (1965) it lined through with the top of the serial number which is etched on the body next to the rewind handle. Having said that it's quite clear, in white on the right hand side on my D700 & D300s.

  12. #12

    Re: What is the Optical Center of a Lens?

    Nodal Point: One of the two points in a compound optical system, located so that a light ray directed through the first point will leave the system through the second point, parallel to its original direction. Also called axial point. This is NOT the optical center.

  13. #13

    Nodal Points not to be confused with Optical Center

    Lenses have two related nodal points with the property such that a light ray entering at one angle leaves the lens at the same angle. In other words the entrance path is parallel to the exit path.

    This is NOT to be confused with the ONE Optical Center which acts geometrically like a pin hole. The focal plane is (usually) on a flat, photosensitive image plane on which all rays are focused. It is the optical center that is used for panoramas. The Optical Center is used as a reference point for perspective drawing and rendering systems.

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    Re: Nodal Points not to be confused with Optical Center

    This is a rather ancient post (started back in 2009)..

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    Re: Nodal Points not to be confused with Optical Center

    Just an FYI for alpha people (as opposed to alpha types) - It's the same mark in white on the black camera body for Sony alphas. It's on the top of the camera to the right of the viewfinder about 5mm forward of the back edge of the top of the camera.

    virginia

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    Re: Nodal Points not to be confused with Optical Center

    Most good cameras mark the film plane or sensor position with the mark illustrated above

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