# Thread: Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

1. ## Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

Hi there, I've posted this here because my question on photo.net applies to a project I am creating for the Micro Four Thirds format. I use a Panasonic GH1 with a variety of manual focus lenses, as well as the Panasonic 20mm f/1.7 lens.

Perhaps someone here has the answer to my question linked above, and might have missed it if I hadn't posted here as well.

My question on photo.net follows:

Who has some advice on formulas or techniques for finding the exact "optical center" of a Canon FD 50mm f/1.4 lens. When I say "optical center" I don't necessarily mean the physical center of the lens length. I've wondered if the following formula is a valid way of finding it:

IDEA#1:
A Canon FD lens has a "flange to sensor distance" of 42.0mm. I don't know if this is also the "Back Focal Length" (BFL)? Therefore is this true:
[Effective Focal Length] - ["flange to sensor distance"] = "the optical center"
e.g. 50mm - 42mm = 8mm, where the "the optical center" is 8mm from the rear optical surface of the lens.

This formula wouldn't work, or at least it has confusing results when the "Effective Focal Length" is shorter than the "flange to sensor distance".
e.g. 24mm - 42mm = -18mm

IDEA#2:
Another thought is to just measure for the physical distance between front and rear optical surfaces, i.e. "the exit pupil distance".
["The exit pupil distance"] / 2 = "the physical center"
e.g. A Canon FD 50mm f/1.4 lens has about ~44mm long "exit pupil distance" from front to rear optical surfaces.
So... 44mm / 2 = 22mm, where 22mm is "the physical center" but perhaps not "the optical center".

Can anybody offer an exact way of measuring "THE OPTICAL CENTER"?

Cheers,
Dan

2. ## Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

Hi sorry to double post a thread question, but I wonder if some Canon users might have an answer to this question here.

The question basically is (as written on photo.net):

Who has some advice on formulas or techniques for finding the exact "optical center" of a Canon FD 50mm f/1.4 lens. When I say "optical center" I don't necessarily mean the physical center of the lens length. I've wondered if the following formula is a valid way of finding it:

IDEA#1:
A Canon FD lens has a "flange to sensor distance" of 42.0mm. I don't know if this is also the "Back Focal Length" (BFL)? Therefore is this true:
[Effective Focal Length] - ["flange to sensor distance"] = "the optical center"
e.g. 50mm - 42mm = 8mm, where the "the optical center" is 8mm from the rear optical surface of the lens.

This formula wouldn't work, or at least it has confusing results when the "Effective Focal Length" is shorter than the "flange to sensor distance".
e.g. 24mm - 42mm = -18mm

IDEA#2:
Another thought is to just measure for the physical distance between front and rear optical surfaces, i.e. "the exit pupil distance".
["The exit pupil distance"] / 2 = "the physical center"
e.g. A Canon FD 50mm f/1.4 lens has about ~44mm long "exit pupil distance" from front to rear optical surfaces.
So... 44mm / 2 = 22mm, where 22mm is "the physical center" but perhaps not "the optical center".

Can anybody offer an exact way of measuring "THE OPTICAL CENTER"?

Cheers,
Dan

3. ## Re: Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

Admin note: You double-posted so I merged them and left a redirect in the Canon forum. This makes it easier for folks to follow.
~~~

It would help to know why you want to know this or what it is you're trying to accomplish with it.

The optical center of a lens is often called the nodal point, but in fact there is a front and rear nodal point and they aren't at the optical or physical center of a lens. Other relevant optical points are the principal points, as well as the entrance and exit pupil.

Personally, I think one could argue that half-way between the principal points is the optical center, or one could argue that half-way between the front and rear nodal points is the optical center, but these may or may not define the same point depending on lens design. One could also argue that half-way between the front of the front element and rear of the rear element is the physical center of a lens, but again it would help to understand what exactly you are trying to determine and why.

4. ## Re: Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

I believe the 42mm number is the flange focal distance, not just the flange to sensor distance. So if you place the flange 42mm from your sensor, you'll be in focus with the lens. No need to calculate anything else. Others can chime in.

5. ## Re: Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

Canon defines "flange back" and "flange-focal distance" as the distance from the camera's lens mount reference surface to the focal plane (sensor plane).

Canon defines "Back focus" as the distance along the optical axis from the apex of the rearmost element to the focal plane.

By "optical center", I assume you mean the principal point (specifically the second or rear principal point, in a complex lens design).

Your Idea #1 is correct for identifying the rear principal point. It always lies a distance away from the sensor plane equal to the focal length of the lens. In some lens designs the rear principal point may be farther forward than the front principal point--in telephoto lens designs the rear principal point can actually be in front of the lens' frontmost lens element!

With retrofocus designs, the rear principal point is positioned to the rear of the rearmost lens element, just as you describe with "confusing results"--the rear principal point of the 24mm lens is, in fact in the mirror box of the camera.

You'll find all this (and much, much more) in a copy of Canon Lens Work. Now even available for download at http://www.canon-europe.com/Support/...ork_iii_en.asp. The section with answers for you is the Optical Terminology and MTF chapter.

Best of luck with your project!

6. ## Re: Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

This is great help so far thanks so much. I'll write some more tonight about the reason I need this info, which might help you clarify which method I should measure with. Thanks,
DAn

7. ## Re: Finding the OPTICAL CENTER of a Canon FD 50mm f/1.4

My conclusions are in the photo.net link at top.