Find specic quantities within a vector or within a matrix in Matlab

I need to identify the time at which the following values occur, namely: eta = 0 and eta = 0:51;

I should work with 'find' code, but could not get desired value. I know how it works in 'maximum':

t = -10:1:10;
t = t';
eta = -t .^2;
ind = find (eta == max(eta));
t_etamax = t(ind ,1);
t_etamax = t(ind ,1);
eta_max = eta (ind ,1);

Now I need not Maximum but when eta=0 and eta=0.51