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tangential plane

A tangential (also called meridional) plane in 3D is in general a plane containing the optical axis.

The tangential plane through a point is the plane in 3D that contains the point and the optical axis.

The tangential ray trough a point in 3D is the ray in the tangential plane through the point, headed to the centre of the entrance pupil.

In the following interactive graphics you can move the green point (click to change directions from x-y to z). The rays leave the point direction entrance pupille and ‘fill’ it completely.

Perpendicular to it is the sagittal plane.

telecentric

With telecentric lenses the image size of an object doesn’t change when the object changes it’s distance from the lens (a little).

a telecentric lens has the same DOF of an entocentric lens (with the same F-number).
Say, the image doesn’t change size when coming closer, but the image starts to blur.

see “comparison entocentric – telecentric – pericentric