res = 100;
coords = ToPackedArray@
   Table[{i - 0.5, j - 0.5, k - 0.5}, {i, Range@res}, {j,
     Range@res}, {k, Range@res}];
dists = Map[N@Norm[# - res/2.] &, coords, {3}];
opa = (1/(dists + 0.1))^1;
opa /= Max@opa;
ImageAdjust@Image3D[opa, ColorFunction -> "GrayOpacity"]