There are two green triangle shaped sliders on the waveform. When you click and hold on the slider, you can move it through the waveform. Move these sliders from either end to limit the amount of the waveform that gets FFT applied. As you move the sliders and reduce the number of samples, you can see the reduction in the clarity of the spectrum.

See the attached video for a short demonstration.

Related Articles:

How many lines of resolution will I get from my recording?