I am not certain what you want.
Try this:
f=@(x,y) sin(x).*sin(y)-0.5;
fp1=fimplicit(f,[-10,10,-10,10]);
fp2=fcontour(f,[0,10,0,10]);
.
EDIT — (17 Apr 2021 at 19:43)
Another option:
f=@(x,y) sin(x).*sin(y)-0.5;
fp1=fimplicit(f,[-10,10,-10,10]);
fp2=fcontour(f,[0,10,0,10]);
fp2.LevelList = linspace(0, 0.5, 25);
.