Question about Planes

[EyeFitWell]

Hi all! Maybe you can help a student out...
I'm just beginning courses for the semester and we're talking about planes (sag, tangental, etc.)
My question is a litte hard to explain but I'll do my best...
When we say, for example, something is in the Sagital Plane I know that the Sagital plane is the plane which would "cut the object into left and right halves."
The question is, if something is in the sagital plane, does that mean it falls directly in the center between left and right halves? Or does that mean that all things parallel to this "cut" are considered in that plane? Or does it mean something else entirely?
And what is the "object" that would be described as being in a plane? Are we talking about rays? Or distortion? Or cylinder powers? Just an example of when these "planes" are used to describe placement...
I'm just having a hard time wrapping my head around the concept of being "in a plane." I'm sure someone can clear this one up for me!
Thanks in advance!

[lensgrinder]

For an off-axis object point the Tangential planes is the plane that contains the chief ray and the optical axis. For an off-axis object point Saggital plane is the plane that contains the chief ray and is perpendicular to the tangential plane. I have included a very generic picture to describe this, but the only program I have in my office is MS Paint, so excuse the picture.

When you are discussing saggital and tangential errors you are discussing off-axis performance or some sort of abberation.

[Darryl Meister]

It's important to distinguish between an anatomical "sagittal" plane, which cuts the body in half, and the "sagittal" plane of a spectacle lens, which is the plane perpendicular to any "tangential" plane of the lens, as Lensgrinder noted. The tangential planes of the lens are the "radial" meridians, like spokes on a way or cylinder axis values. Consequently, since there are 360 degrees of possible tangential planes and the sagittal plane is perpendicular to any one of them, the sagittal planes describe a circle around the lens at any given distance from the center. This link should further clarify these planes.