Autcorrelation Via FFT and Zero Padding

2016 5 23
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Hi,
I would like to understand conceptually why when calculating the Autocorrelation Function of a vector x via fft, the best way is to zero padding in the way
nFFT = 2^(nextpow2(length(x))+1); % nFFT = 2*length(x) will do it as well
F = fft(x-mean(x),nFFT);
and no, for instance
nFFT = 2^(nextpow2(length(y)));
Why do the way we pad the vector influence so much the approximation of the Autocorrelation?
Thank you.

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