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For fft and ifft, there are corresponding time and frequency arrays on input and output. These have constant spacing delt or delf between points. For an N-point fft or ifft, in either direction always
delt*delf = 1/N.
If you start with certain valid definitions on delt and delf, and subsequently pad an array with zeros, that changes the value of N, and one of delt and delf must change. Usually you are sure of the corresponding input array and have to make the appropriate assumption about the corresponding output array. For example if you have an input frequency array with spacing delf, and double the number of points by doing zero padding, then delt in your new corresponding output time array is half of what it was.
