How to get information on low frequencies of a signal using FFT?

[BrunoIdeas]

Hello. I've made a lab experiment of interference and with a CCD camera photographed fringe pattern which one can understand as a periodic signal.
My question is: When I apply an FFT to the image is there any limit to the amplitude information I may get from lower frequencies?
I read in the internet that for a 1 sec signal, one may be able to obtain information about only freqs above 1 Hz.
Why do I ask this? Because the width of the fringes is of about 1/10 the size of the total image, and I am interested in low frequencies.

Now I am processing the image in ImageJ in order to get some information.

Thanks in advance.

 

[MATLABdude]

Welcome to PhysicsForums!

I'm not quite sure how you're getting temporal measurements (hz) from the FFT of an image (usually in vector space), unless you're asking about contributions from low-frequencies in general.

When you mention fringes being 1/10 the size of the image, you may have things reversed: the contribution from that should actually have a spatial frequency of 10, not 1/10 (think 10 fringes per image):
http://sharp.bu.edu/~slehar/fourier/fourier.html#harmonics

 

[MisterX]

The lowest positive frequency represented in the FFT output will be 1 cycle over the length of data. So, for a 1 second signal, there would be information for ..., 0 Hz, 1Hz, ...

1 * fs/N = 1/(Ts * N) = 1/(Total time)