How to change frequency bins in specgram to improve recall rate?

[thesby]

I find that bins of frequency of specgram is exactly NFFT/2 + 1. But how can change the bins of frequency, and not change the NFFT ? NFFT affects how many sample to use in FFT. But why does it also affect bins of frequency?

[worldveil]

https://electronics.stackexchange.com/questions/12407/what-is-the-relation-between-fft-length-and-frequency-resolution