I asked the same question here but I never got a response.
According to the Principle of Univariance, the input to a photoreceptor differs along two dimensions (wavelength and intensity) but the output can only vary along one dimension (the cell's graded potential). This means that there is inherent ambiguity in the output of the photoreceptor.
What I do not understand then is how it is possible for a rod (or cone) to distinguish between intensities (or wavelengths) when each photopigment can absorb a range of wavelengths at different probabilities.
For example, say a rod cell absorbs 50% of all photons at wavelength x and 25% at wavelength y. If 100 photons reach the rod cell at wavelength x we can roughly assume that 100*0.5=50 isomerizations will occur. Equally, if 200 photons reach the rod cell at wavelength y then we can roughly assume that 200*0.25=50 isomerizations will also occur. From the perspective of the cell's synaptic activity, the two responses will be the same despite the large difference in intensity.
Since there are an infinite number of permutations to this basic pattern it is unclear to me how the visual system is able to obtain any information about the intensity of light in rod cells. The same issue also appears present to me in cone cells (the usual explanation is that the relative amount of excitation/inhibition can be compared between cone cells, but this does not seem to solve my overall dilemma).