I'm from France and I would ask a question on a point that I don't understand, as it seems that so many people confuse (or may be I'm wrong).

On the camera, there is a symbol (a theta or phi) which is called in English "image plane".

In France some people call it "focal plane", but for me the focal plane is the plane formed of the sum of all secondary image focus points and primary image focus point (F').

This plane is the same as image plane but just in case on image coming from infinity.

For a near object placed before the object focus point (F) , the image is formed behind the focal plane, reversed, and reduced, on the image plane.

Do you agree with this assertion ? Could you tell me if I'm wrong ? (If not I don't know why some people call image plane "focal plane")

Do you have a reference course which explain that ?

  • Yes, you are correct. The focal plane is the plane is the plane perpendicular to the optical axis passing through the focal point.
    – Szabolcs
    Aug 14 '13 at 15:24

Focal Plane and Image Plane are not the same. Image Plane is where the plane where the image is projected (ie, on to the sensor). The focal plane is the plane where objects would appear in focus. The rear focal plane will intersect only where the image is in focus.

  • The OP is right while your explanation is somewhat confusing. Objects at different distance would appear in focus in different planes. The precise way to say it is that the focal plane is where (infinitely) distant objects would appear in focus.
    – Szabolcs
    Aug 14 '13 at 15:25
  • @Szabolcs - from doing some more reading it appears to depend if you are talking about photography terminology or optics terminology. The focal plane of the lens in optics seems to refer to the lens at infinity focus, but it is also used to describe the plane that is in focus for a particular focusing of the lens in the photographic context. I found references to both uses in my research about this question. I included the photographic version in my answer.
    – AJ Henderson
    Aug 14 '13 at 16:19
  • Yes, terminology often differs between fields, or even between subfields. I have a physicist's training so I was familiar with the optics terminology.
    – Szabolcs
    Aug 14 '13 at 16:27

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