BT-Settl spectrum of a T = 1700 K companion at two different spectral resolutions: R = 7000 (top) and R = 17 000 (bottom). Each red rectangle delimits the observable spectral bands by keeping the same number of pixels for both resolutions. In each case, the corresponding α quantities and the S/N, both normalized to the top panel, are indicated. We note that some areas have more absorption lines, and thus benefit more than others from an increased resolution (e.g., there is a forest of lines corresponding to the presence of CO around 2.3 µm). Both α and the photon noise decrease as the resolution increases and the bandwidth decreases; comparing the two variations may lead to a lower (left rectangle) or higher (right rectangle) resulting S/N.