Optical Engineering
Part 4: Filed and aperture, stop, pupil, vignetting Herbert Gross
Summer term 2020 www.iap.uni-jena.de 2 Contents
. Definition of field and aperture . Numerical aperture and F-number . Stop and pupil . Vignetting Optical system stop
. Pupil stop defines: 1. chief ray angle w 2. aperture cone angle u . The chief ray gives the center line of the oblique ray cone of an off-axis object point . The coma rays limit the off-axis ray cone . The marginal rays limit the axial ray cone
stop pupil
coma ray marginal ray y field angle image w u object aperture angle y'
chief ray 4 Definition of Aperture and Field
. Imaging on axis: circular / rotational symmetry Only spherical aberration and chromatical aberrations
. Finite field size, object point off-axis:
- chief ray as reference
- skew ray bundels: y y' coma and distortion y p y'p
- Vignetting, cone of ray bundle not circular O' field of marginal/rim symmetric view ray RAP' chief ray - to distinguish: u w' u' w tangential and sagittal chief ray plane O entrance object exit image pupil plane pupil plane 5 Numerical Aperture and F-number
image . Classical measure for the opening: plane numerical aperture (half cone angle) object in infinity
NA' nsinu' DEnP/2 u'
. In particular for camera lenses with object at infinity: F-number f f F# DEnP
. Numerical aperture and F-number are to system properties, they are related to a conjugate object/image location 1 . Paraxial relation F # 2n'tanu'
. Special case for small angles or sine-condition corrected systems 1 F # 2NA' 6 Generalized F-Number
. More general definition infinity
of the F-number for systems finite with finite object location object DExP u' u'o
. Effective or working F-number with f' s' f '1 m s' and D D m D sinu' ExP ExP p EnP 2s' 2 f '(1 m) 2n' f (1 m)
we get as a relation with the object-in-infinity-case
eff 1 1 m F# F# 2n'sinu' mp
m is the system magnification, mp is the pupil magnification Properties of the pupil
Relevance of the system pupil :
. Brightness of the image Transfer of energy
. Resolution of details Information transfer
. Image quality Aberrations due to aperture
. Image perspective Perception of depth
. Compound systems: matching of pupils is necessary, location and size Entrance and exit pupil
field point of image upper object marginal ray point U on axis U' W on axis lower marginal chief point of ray ray image upper coma ray
lower coma ray stop outer field point of object exit entrance pupil pupil 9 Pupil Mismatch
. Telescopic observation with different f-numbers . Bad match of pupil location: key hole effect
F# = 2.8 F# = 8 F# = 22 a) pupil adapted
b) pupil location mismatch
Ref: H. Schlemmer Vignetting
. 3D-effects due to vignetting
. Truncation of the at different surfaces for the upper and the lower part of the cone
object lens 1 aperture lens 2 image stop upper truncation chief ray
lower sagittal coma truncation trauncation rays 11 Vignetting
field . Artificial vignetting: stop truncation Truncation of the free area of the aperture light cone
D
axis 0.8 D
truncation
AExp . Natural Vignetting: Decrease of brightness according to cos w 4 due to oblique projection of areas and changed photometric distances field angle w imaging with imaging without vignetting imaging with vignetting vignetting
complete field of view 12 Vignetting
. Illumination fall off in the image due to vignetting at the field boundary