**Figure 6. **Errors of determining the amplification constant introduced by simplified parabolic fitting of the initial part of the response.
The experimental responses are normalized in a routine way, that is, to the dark current, which consists of the CNG channel
current plus the exchanger current.

**A**:

*Carassius* cone. Noisy curve: experimental response of a cone to 2 ms flash applied at time zero; red line 1: the model best fitted
to the entire response; line 2: least-square fit to the initial 40 ms part of the response with the routinely used Pugh and
Lamb [

15,

45,

46] parabola (Equation (2)); line 3: the response of the model in which all turnoff processes and Ca

^{2+} feedback are disabled, and the model response is normalized to the dark CNG-channel current alone (excluding the exchanger
current); line 4: same as line 3, but in addition, the cell time constant is set to zero. Curve 4 represents the “real” parabola
behind the experimental response; it yields an amplification value that is four times higher than would be derived from routine
fitting (curve 2).

**B**: Same as A, but for a frog rod. Curves 3 and 4 coincide because the effect of membrane filtering in rods is negligible. Routine
parabolic fitting results in a 1.6-fold underestimate of amplification.