Solve equation with 1 unknown numerically

I have the following script that defines a function and sets up and equation:

    H = @(f) sum(log(f));
    f = rand(1, 1);

    syms a
    H(f)-H(f-a)

I want to solve H(f)-H(f-a)=0 for a. I tried using fzero in the following manner, fzero('H(f)-H(f-a)', 0), but this doesn't yield me anything useful.

Answer 1

[Star Strider]

This works:

      H = @(f) sum(log(f));
      f = rand(1, 1);

      a = fzero(@(a) H(f)-H(f-a), 0)

The value of a is, expectedly, zero.