Help with the design of an eye in TikZ

Question

In my attempt to create an eye icon to illustrate something in my notes, I came up with the following:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=0.25]
\filldraw[color=black] (0,0) circle (1.5cm);
\fill[color=white] (0,0) circle (1cm);
\draw[very thick](-3,0) .. controls (-2,2) and (2,2) .. (3,0) .. controls (2,-2) and (-2,-2) .. (-3,0)--cycle;
\end{tikzpicture}
\end{document}

This produces:

The problem I would have is to resize the diagram in that I do not like how the inner circles are independent of the eye design. May be using nodes would help a lot. Essentially I would like the inner circles shrink and expand with respect to the outer shape. I know that scale and transform shape can help but I am looking for something flexible like \tikzeye{<height>}{<width>} or \tikzeye{<scale>}. Any other designs or variants are welcome. I know the dingbat package has an \eye command defined but it is not too flexible.

EDIT:

I like the clipping idea. If a point P1 and P2 are on the circle and point Q1 and Q2 on the curve, how can make these points, P1 and Q1 and P2 and Q2 be the equal respectively given that P1 and Q1 are at the top and P2 and Q2 are at the bottom? See image below.

\begin{tikzpicture}[scale=0.25]
\filldraw[color=black] (0,0) circle (1.5cm);                                 % Circle with points P1 and P2
\fill[color=white] (0,0) circle (1cm);
\filldraw[color=blue] (0,1.5) circle (3pt) node[above] {{\footnotesize p1}}; % Point P1 on the circle.
\filldraw[color=blue] (0,-1.5) circle (3pt) node[below] {{\footnotesize p2}};% Point P2 on the circle.

\draw[very thick,yshift=6cm](-3,0) .. controls (-2,2) and (2,2) .. (3,0) .. controls (2,-2) and (-2,-2) .. (-3,0)--cycle;                                                                % Curve in question
\filldraw[color=blue] (0,7.5) circle (3pt) node[above] {{\footnotesize q1}};% Point Q1 on the curve.
\filldraw[color=blue] (0,4.5) circle (3pt) node[below] {{\footnotesize q2}};% Point Q2 on the curve.
\end{tikzpicture}

Answer 1

This is my version of your eye, trying to give an aspect a bit more organic:

\def\eyepath{(-3,0) .. controls (-2,1.8) and (2,2.2) .. (2.7,0) .. controls (2,-1.2) and (-2,-1.4) .. (-3,0)--cycle;}


\begin{tikzpicture}[scale=0.25]
\clip\eyepath;
\filldraw[color=orange!50!black] (-.2,.2) circle (1.5);
\fill[color=black] (-.2,.2) circle (0.7);
\fill[color=white] (.3,.5) circle (0.2);
\draw[very thick]\eyepath;
\end{tikzpicture}

In relation with your question, you can put this tikzpicture inside a \scalebox (from graphicx package) to get any desired size. Note also that my eye is slightly assymmetrical. You can use \reflectbox to get the other eye :-)

Update

I could not resist it, and I tried a more realistic eye (probably not in the spirit of the question, sorry!)

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}
\usetikzlibrary{calc,decorations.pathmorphing}
\begin{document}

\def\eye#1{\scalebox{#1}{
\def\topedge{(-3,0) .. controls (-2,1.8) and (2,2) .. (2.3,.3)}
\def\bottomedge{(2.3,.3) .. controls (2,-2.2) and (-2,-1.2) .. (-3,0)}
\def\eyepath{\topedge -- \bottomedge --cycle;}

\begin{tikzpicture}[scale=0.25]
  \clip\eyepath;
  % Iris
    \filldraw[color=orange!30!black] (-.2,.2) circle (1.4);
  % Shadow on iris
    \filldraw[color=orange!40!black] (-.3,-.1) circle (1.1);
  % Iris lines
    \foreach \a in {0,5,...,360}{
      \pgfmathparse{25+28*rnd}
      \fill[orange!\pgfmathresult!black, decoration={random steps, segment length=1pt, amplitude=0.3pt}, decorate, line width=0.3pt  ] (-.2,.2) -- ++($(\a+2*rnd:.8+0.3*rnd)$) -- ++(\a+90:3pt) -- cycle;
    }
  % Pupil
    \fill[color=black] (-.2,.2) circle (0.5);
  % Sun reflection
    \fill[color=white] (90:.8) {[rotate=-30] circle (0.2 and 0.12)};
  % Shadow of the eyelid
    \draw[line width=2.5mm, draw opacity=0.1, line cap=round]\topedge;
  % Eyelids
    \draw[line width=1mm, red!40!white!80!black, line cap=round]\bottomedge;
    \draw[line width=1.2mm, red!40!white!60!black, line cap=round]\topedge;
  % Lacrimal
    \fill[red!40!white!80!black] (-2.8,0) circle (.25);
    \fill[white] (-2.7,.1) circle (.03);
\end{tikzpicture}
}}

\eye{1}
\end{document}