see wat tweety said is rit... definitely....

but understanding is .....

focal plane is a plane at which any sytem of parallel beam meet.. i mean its locus...

if the rays are parallel to principal axis thn they all r going to be definitely meeting at principal focus... but if it is not parallel to pric. axis but they all are parallel to each other they ud meet somewhre on focal plane....

as said focal plane passes thru principal focus... must.......

Cheers!!!!