The maximum frequency you can detect is limited by your sampling rate, but there's not a limit on the precision with which you can break those frequencies up.
It's controlled by a parameter NFFT -- the PSD will compute (NFFT/2+1) values evenly spaced between 0 and the Nyquist frequency.
So say the frame rate is 15Hz and you compute with NFFT=2048, then PSD[970] contains the amplitude at 7.09Hz.
Also, it's not as widely known as the FFT, but if you know roughly the frequency of interest you can use the Goertzel algorithm to calculate a chosen number of bins around that specific freq and then pick the max of them to find the freq of interest, instead of when using the FFT having to calculate a bunch of bins using a large nFFT in order to get enough freq resolution and then discarding 99% of the results. Going further, compared to the original Goertzel, the Generalized Goertzel algorithm does the same thing but allows you to query non-integer multiples of the fundamental frequency: http://asp.eurasipjournals.com/content/2012/1/56
It's controlled by a parameter NFFT -- the PSD will compute (NFFT/2+1) values evenly spaced between 0 and the Nyquist frequency.
So say the frame rate is 15Hz and you compute with NFFT=2048, then PSD[970] contains the amplitude at 7.09Hz.
This was a really cool project by the way!