On the longitudinal axis of image after FFT

After the FFT if the longitudinal axis is the value after abs, and for the bilateral image, then the longitudinal axis is the value of the amplitude of the signal at this frequency *N/2, that is, if there is a sine signal, the amplitude of 1, if the FFT of 50 points, then the amplitude of this signal frequency is 1*50/2=25.

After the FFT if the longitudinal axis is a db, in fact, the value of 10*LOG10 (ABS), the value itself is meaningless, but the difference of two values is meaningful, such as the amplitude of a signal a=20db, another amplitude b=23db two subtract is 3db, then what does it mean? Suppose the original signal amplitude is a, B. Then there is a=10*log10 (A*50/2) b=10log10 (B*50/2). B-A=10*LOG10 (b/a) =3db, then b/a=2 that means B is twice times the amplitude of a.

On the longitudinal axis of image after FFT