Re: Focussing Lens without Spherical Aberration

redbelly98_at_yahoo.com
Date: 12/29/04

  • Next message: Seidel: "Re: question about MTF" 
    Date: 29 Dec 2004 13:57:28 -0800

    Narasimham wrote:

    >Is z= curv R^2/(1+sqrt(1-(k+1)R^2)+ A R^6 +....+ D R^10 supposed to be

    > an equation to a conic

    > (ellipse,hyperbola) of revolution when A,B,C,D
    > vanish? Cannot a conic be more simply desribed by (z/a)^2 +/- (R/b)^2
    > =1?

    Correct on both counts. However, the equation with "k" and "curv"
    seems to be standard in the optics industry. I'm not entirely sure
    why. However, it does have the following desireable features:

    1. The surface curvature appears explicitly in the equation.
    2. z=0 at R=0, so that for other values of R, calculating z will
    directly give the surface's displacement relative to the center of the
    lens.
    3. In lens design and ray tracing software, simply change the value of
    k to change the type of surface. No need to edit the equation and
    change "+" to "-" or vice-versa.

    Mark

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