you've been told incorrectly. It is a very much digital process wherein a photon strikes a photoreceptor which yields a cascade of events that reverses membrane potentials.
You seem to be suggesting that because a photon either strikes, or it doesn't strike, makes it a digital process. If that (probability/statistics) is your train of reasoning, then there would be no analog reality.
Yet, the arrival time of photons is random (Poisson distributed probability), the effect of a photon striking may be inhibited (e.g. lateral inhibition) by other chemicals, and neurotransmitters use electrotonic conduction which produces a constant flow of electric current
(see "Graded Response and Release of Neurotransmitters") along the membrane. This turns a variable stream of discrete photons into a very analog process.
Credible references abound;
Maybe it is the interpretation of that information that is a bit 'debatable'?
http://www.vetmed.vt.edu/education/curriculum/vm8054/eye/rhodopsn.htm - a straightforward review of the chemistry, initiated by a photon. Not an analog process, a conformational change (cis-trans) that is again a 0 or 1. It is cis or trans, not cistrans.
That's your interpretation. What I read is a "The disintegration of rhodopsin into retinal and scotopsin is progressive
", and "The eventual
result is release", and "metarhodopsin II, is the agent that ultimately effects the change
in the rod membrane's charge
". Also, ""Under conditions of impinging light, [...],
the flow of sodium ions
into the rod outer segment is slowed
or stopped. Also "One photon, the minimum quantity of light possible, will cause the movement of millions of sodium ions
, because of the catalytic nature of the enzymes
and the large surface area provided for them to work".
Not a very binary process at all. Sure, one photon makes a difference (in a Rod), but not always exactly the same, because the result is an analog process flow
http://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Retina/VisualCycle.html - an expansion demonstrating the fact that different colors are sensed by the energy of the given photon
Another gradual transition process "the activation of rhodopsin during phototransduction
isomerizes 11-cis-retinal to the all-trans form, which dissociates from the opsin in a series of steps called "bleaching"
". Dark adaptation also causes a continuously variable sensitivity, not digital at all.
http://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Retina/Photoreceptors.html - which further expands upon sensitivity in terms of how many photons it takes to cause a transduced signal to be produced.
Indeed, Rods have more photopigment and have a high (single photon) sensitivity but lower temporal resolution, more signal integration. Cones have lower sensitivity, higher temporal resolution and less signal integration. Both are not digital at all.
All digital processes.
Really? It seems to be quite the opposite.
Not you personally, but where is this insistence that light is somehow analog coming from?
Light is discrete packets of energy who's speed determines wavelength.
Actually, the speed of light is constant (in vacuum), Photons can be considered to exhibit both wavelength, and energy characteristics. That's known as the wave-particle duality
of light. Waves are not digital, and even energy particles fluctuate as (valence) electrons are knocked in and out of the (outer) shell of atoms.