function PSF = PSFsimp(phi, age)
%PSFsimp A simplified calculation based on the point spread function 
%(PSF) calculations of Ijspeert, 1992.
% phi is in degrees
% pupil = 4 mm
% m = 0.142
% Default age is 0 years
switch nargin
	case{1}
		age = 0;
	case{2}
	otherwise
		error 'too many arguments'
end


AF = agefact(age);

term1 = 1/(1 + (AF/7));
term2a = (11.6 * 10^6) / (1 + (phi/0.0044)^2)^(3/2);
term2b = 84 * 10^3 /(1 + (phi/0.06)^2)^(3/2);
term3 = 1 / (1 + (7/AF));
term4 = (1.8 / ((1/AF^2) + (phi/6)^2)^(3/2)) + 0.13;

PSF = term1 * (term2a + term2b) + term3 * term4;