What is the Optical Center of a Lens?

[er.mayankkapoor]

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

[Colin Southern]

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

[dan88]

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.

[Colin Southern]

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

[teedee]

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

[darekk]

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

[herbert]

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

[darekk]

Canon's focal plane mark

[Hero]

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

[teedee]

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

[krispix]

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.