Bialek is an amazing explainer, you'll be left convinced it's all so simple and straightforward. He wrote a book and taught a graduate level class around it, you'll find a draft of the book available at [1]

Chapter 1 cover photon counting:

``` 1. Photon counting in vision (Lectures W 8 Feb through W 22 Feb 2012)

In this Chapter, we will see that humans (and other animals) can detect the arrival of individual photons at the retina. Tracing through the many steps from photon arrival to perception we will see a sampling of the physics problems posed by biological systems, ranging from the dynamics of single molecules through amplification and adaptation in biochemical reaction networks, coding and computation in neural networks, all the way to learning and cognition. For photon counting some of these problems are solved, but even in this well studied case many problems are open and ripe for new theoretical and experimental work. The problem of photon counting also introduces us to methods and concepts of much broader applicability. We begin by exploring the phenomenology, aiming at the formulation of the key physics problems. By the end of the Chapter I hope to have formulated an approach to the exploration of biological systems more generally, and identified some of the larger questions that will occupy us in Chapters to come.

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[1] http://www.princeton.edu/~wbialek/PHY562.html