For those of us practiced in traditional silver halide photography, it’s obvious that b&w images (are there really any other kind?) made with film are different than digital b&w. Run your RAW files through any b&w film emulation program you desire and, at bottom, they come out looking different than a native black and white negative. It’s true: silver halide film is capable of certain aesthetic qualities that digital capture simply cannot match. It may be subtle at times, but it’s there, and unlike what some think, It’s not just film grain or lack thereof that constitutes the difference.
It has to do with the differing ways film and digital sensors react to the same given amount of light. Digital sensors are linear. The actual output voltage from each cell of a digital sensor is directly proportional to the amount of light that strikes it during the exposure. As such, you can use three light sources and what you get will be the exact sum of the three. You can then subtract one of them and be left with the exact sum of the remaining two. Etc Etc.
This isn’t possible when using film because film’s response to light isn’t linear. Black and white film records light as degrees of tone. The more light that hits the film, the more grains of silver chloride are converted to darkened silver. However, unlike digital sensors, film does not transform light into tone in a linear 1= 1 fashion. All silver halide films capture tones in a characteristic S curve, the S representing the relationship between a given amount of light and the tone actually produced by a film. Film’s unique qualities have as much or more to do with film’s characteristic tonal response curve — the “toe” at the dark end, and the “shoulder” at the light end. The toe of a film’s curve represents the decreasing proportional differences in negative densities when exposed to lower amounts of light; the shoulder represents the decreasing proportional differences when exposed to higher amounts of light. Digital, being linear, does not produce decreasing proportional differences either in the lowest or highest light.
The practical result of the differences in the light sensitivity of digital versus film is this: Negative film is less tolerant to under-exposure in the shadows while digital sensors are less tolerant to over-exposure in the highlights. A film negative will give you deeper, less defined blacks at the bottom end. Conversely, film will give richer, more nuanced highlights at the top end than will a digital file of the same scene.
Digiphiles will claim that given a good enough digital original, you should be able replicate film qualities by adjusting the tonal curve of a RAW file, applying a fairly steep S curve to its exposure histogram in Lightroom or Photoshop. What this misses is the fact that the RAW digital file you’ll be applying the curve to, unlike the spectral response of film, captured its exposure values linearly i.e. It didn’t compress brightness values at the light end and darkness values at the dark end. This means that you’ll be applying a post-process S curve to a digital file that has much different native information than does a film negative of the same scene – particularly with respect to highlight detail (or the lack thereof). Where film, with its shoulder, compresses and thus saves highlights, digital’s linear response to highlights is more apt to record bright highlights as pure white, devoid of detail, what we call “clipping” of the highlights, while at the same time giving more shadow detail and thus less deep blacks. For highlight detail, this means that you’re often starting with a RAW file with lots of native shadow detail but whose highlights have been rendered as pure white i.e no detail. Applying a film-like S curve to such a file will mimic film’s deep blacks (by simply compressing out existing details) but cannot mimic film’s highlight detail because the emulation software, no matter how clever, can’t turn pure white into highlight detail.
RAW converters, both as post-processing software and in camera as jpgs, will apply some form of tone curve to the image to give it a more “natural” film-like output. Once your RAW file has been converted, you can then use a dedicated b&w emulation software like Silver Efex to apply appropriate tonal curves and grain characteristics to mimic specific film stocks. What’s interesting here is that you’ll be applying two different curves digitally – the initial curve from RAW to TIFF or Jpg by your RAW converter, and then a secondary curve applied to the (already curved) file by the emulation software. When using film, there’s only one embedded curve – a function of the film and developer combination.
This is why film gives better, more pleasing b&w highlights and more detail than does a digital file run through film emulation software. Details in highlights require native information to work with; they can’t be reconstituted after the fact by software when the capturing medium has clipped them altogether. A RAW file, being the result of linear capture, will contain less native information in the highlights, and no amount of RAW conversion or secondary film emulation will reconstitute highlight detail that isn’t there to begin with. Another way to put it is this: the information needed for nuanced highlights is either there to begin with (film), or it isn’t (digital), and no amount of post-process tweaking can add highlight details when highlights have been “blown” or “clipped” i.e. are recorded as pure white.