The tangential (also called meridional) plane through a point is the plane in 3D through the point and the optical axis.
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 Entry pupille and ‘fill’ it completely.
Perpendicular to it is the sagittal plane.
Entocentrical lens with a focal lengthlarger than the sensor diagonal, for example a f=8mm lens on a 1/3“ Sensor (that has a 6mm sensor diagonal).
When we use the word “tele lens”, it must be perfectly clear, which sensor is involved.
(Depending on the sensor) the same lens can be wide angle lens, tele lens or normal lens.
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”