Is film analog?

A discussion (rant?)

Many members of the film photography community refer to it as "analog" photography. This description is certainly quite convenient, as "analog" is nearly the exact opposite of the now-common "digital" photography that has overtaken film in many (if not all) respects. However, film isn't necessarily analog, and digital imaging may very well be "more analog" than film.

Unless otherwise noted, the "digital" sensors I am referring to are the APS/CCD type sensors that are commonly used in nearly all digital cameras today.

The "analog-ness" of digital photography

If you look up "analog" in the dictionary, you'll find something along the lines of "continuously variable quantity". For example, the ambient temperature is analog to some extent, as is the output voltage of, say, an amplifier.

Using this definition, image sensors are analog (with some qualification). Why is that? Nearly every image sensor from the humble Vidicon to a modern CCD works due to the photoelectric effect - electrons are "knocked loose" by photons striking a material. The electrons are collected in some manner, then the number at each pixel location is measured. The resulting measurement is a potential at a pin on the sensor that is amplified and then digitized. One can then see where the "digital camera" moniker comes from - the actual image data is stored in a digital format. However, the image sensors themselves are analog as the analog-digital conversion circuitry is usually external to the sensor.

image depicting the photoelectric effect
The photoelectric effect.


There is a certain "but" to keep in mind. Electrons are discretized and thus the output of the measurement is also discretized, so this doesn't fit particularly well with the definition of "analog". Thus, we must be a bit forgiving with our definition. Perhaps it's better to say "practically analog"; the full well capacity of CCDs is usually in the tens of thousands of electrons, so it's effectively analog as the wells can have any number from a handful to the capacity.

Onto film...

Now, why isn't film analog? Nearly every modern film uses some form of silver-halide imaging process. The light-sensitive "elements" of such a process are crystals of silver halide (usually silver bromide, silver iodide, and silver chloride mixed) scattered in a medium such as gelatin. When a photon strikes a crystal, it releases an electron that combines with ionic silver to form an atom of metallic silver.

e- + Ag+  ⟶   Ag

Once 4 of these atoms have collected, the site is stable, and the crystal is rendered developable. The development process reduces the crystals which the requisite number of atoms have collected into clumps of metallic silver. This is a digital process as once the crystal is rendered developable, the entire crystal is converted into metallic silver (assuming development is allowed to proceed fully), regardless of the number of photons that hit it. The only thing analog about film is that the location of these crystals relative to the gelatin matrix is random. In color film, the dye clouds work in a similar manner - either the dye cloud is formed or it is not. There are no "half-density" dye clouds.

Concluding remarks

While "analog photography" is certainly a convenient description, it is in most cases completely wrong. Slight aside - the people who claim that "analog" film has infinite resolution aren't even worth their own section because no real-world analog signal has infinite resolution and these people have evidently never heard of noise or the existence of atoms. In fact, the smallest "unit" of the structure of photographic film is either the silver clump for black and white film or the dye clouds for color, which are both on the order of micrometer size (for conventional pictorial film, I am aware of the existence of holographic film). The resolution of film is a much more complex subject and I'll address it in a future post.

Regardless of how technically wrong the description is, I find using "analog" photography makes me sound more like a hipster than I would like, so usually I stick to "photographic film" or "film photography" anyways.

Thanks for reading, and please direct any questions, comments, or corrections to my email, listed below.

Please refer to Alan Hodgson's "Silver Halide Materials: General Emulsion Properties" in The Focal Encyclopedia of Photography, page 641 for a source, but a general description of the image formation mechanism of silver-halide materials is available in many books relating to the subject.

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