《Lens Design Considerations》.ppt

Lens Design Considerations Hazel McInnes – 4th March 2008 Lens Design Titles Geometric Considerations FoV EFL DoF Relative Illumination Resolution Best Focus Aberrations Many Choosing a Lens Number of Elements Stray Light Coatings General Rules Materials ‘Branding’ Advances in Lens Design Geometric Considerations What Makes A Lens? Refractive Index Difference Curvature of surfaces Optical Power “Focusing” Aperture F/# = EFL/D Magnification - m Object to Image DS’ = DS.m2 EFL is paraxial approximation of the distance from the lens to the focused spot for parallel light (light from a point source at infinity) Image Formation and Field Of View Magnification m is ratio of H’ to H. Field of View (2?): full angle subtended by the sensor horizontal field in object space. Simplest lens: the pin-hole camera F-Number and Depth of Field Lower f/# = Bigger aperture More light on sensor Shorter depth of field Lower resolution? Depth of Field Dependancies The Effective Focal Length of the lens is dependant on the required image height (sensor diagonal) and the Field of View. Dictates Hyperfocus along with F# and pixel size. Near and far field points can be calculated given the hyperfocus and the focussed object distance (s) Depth of Field – 1/4” (851) Illumination and F/# Brightness B (Luminance in Cd/m2) is conserved Illuminance E (lux) = 2p.B - Lambertian Typical Lens RI Note the Knee Point in the curve – this is the image circle. Chief Ray Angle 556 measured RI and RI Ratio This is an example of the 556 module. RI is 47% and the RI Ratio is 0.92 Microlens Array RI is controlled by the primary lens and by placement of microlenses above each pixel. Accuracy of mask placement is ±110nm. Trading-Off F/#, MTF, Depth of Field against Low Light Performance Increasing F/# implies : - Increased depth of field Increased tolerance of design to construction INCREASE in low light limit (i.e. lowered performance). Resolution and Theoretical Limits Resolution and MTF Modulation Trans