How to calculate the frequency of 2D FFT Output image?

Hi,

I have a surface image of 15x15 microns which is characterized by
512x512 matrix. (you can see the pic in http://img363.imageshack.us/img363/7671/pic1tk8.jpg)

Now I want to take a 2D FFT of that image. So in matlab I used "FFT =
fftshift(fft2(InputImageData))" to get the FFT, which looks like in
the pic http://img363.imageshack.us/img363/3346/pic2ne5.jpg

But I am not able to judge how to represent the frequency in the FFT
image. I think the plot represents the magnitude or amplitude in the z-
direction and the frequency is represented in the x and y direction.
But I want to know how to calculate those frequency (I think it
involves the sampling frequency data from input image size and pixel
rate but I am not sure).

Can you explain me how to calculate this?

Thanks,
Joel

>Hi,
>
>I have a surface image of 15x15 microns which is characterized by
>512x512 matrix. (you can see the pic in
http://img363.imageshack.us/img363/7671/pic1tk8.jpg)
>
>Now I want to take a 2D FFT of that image. So in matlab I used "FFT =
>fftshift(fft2(InputImageData))" to get the FFT, which looks like in
>the pic http://img363.imageshack.us/img363/3346/pic2ne5.jpg
>
>But I am not able to judge how to represent the frequency in the FFT
>image. I think the plot represents the magnitude or amplitude in the z-
>direction and the frequency is represented in the x and y direction.
>But I want to know how to calculate those frequency (I think it
>involves the sampling frequency data from input image size and pixel
>rate but I am not sure).
>
>Can you explain me how to calculate this?
>
>Thanks,
>Joel

Hi Joel,
Here's a link on the topic.
Regards,
Steve

http://www.dspguide.com/ch24/5.htm

>