Resolution of Film - Part 1
What is the resolution of film? I had a YouTube video from last year that mentioned that the resolution of ISO 400 film is around four megapixels.
This hit a nerve.
There are a lot of people that think that film is higher resolution, there are a lot of people (like me) who think that film is lower resolution than modern sensor technology.
So, who is right?
Well, it’s a complicated situation. It’s hard to compare a purely digital medium, like modern sensors, to something more from the analog realm.
Now before I go further, I want to say that I personally love film; it’s a great medium that I quite enjoy using.
But is film purely analog? Again, that’s a complicated question. Certainly, film isn’t constrained by a concept of pixels. At the same time, however, it’s also a quantized concept. Talking about black & white film, black is simply metallic silver in the emulsion, while the rest is simply the absence of silver. You have something that’s either black or not—there is no gray to speak of.
Then you get clumps of that silver, that’s what we call “grain.”
On top of all that light isn’t an analog thing either. It’s composed of individual photons that interact with the silver halide crystals to start the process.
Looking at some of the data sheets about film, we can quickly determine that film has a comparatively low resolution in terms of line-pairs per mm (lp/mm), which is how things like this are measured. For ISO 400 film it’s going to be around 50 lp/mm. You can be tempted to call that a proxy for “pixels” or something, but then you’d still be wrong. The problem with a simplification like that is that while the resolution isn’t super high, the recording capability isn’t constrained to pixels either. What, on digital, would be recorded on one pixel could be scootched over by a few microns and be recorded as such. This is a way for film to record micro-contrast that a digital sensor could miss. In perfect conditions, 100-speed film has a resolution of greater than 200 lp/mm.
But you have to keep digging even deeper than that as well. Lenses are also analog creatures. Even exceptional glass has a limit in terms of the resolution it can pass. The perfect conditions quite simply don’t exist. If you have an exceptional lens, like a prime (single-focal-length) lens from a good company, the center can reach above 1000 lp/mm in the best cases. But even with that, the resolution of the film, on the typical lower-contrast images, is around 50 lp/mm. Further, if you get a normal kit lens, chances are the resolution of the film or sensor are way higher than that of the lens that how you're recording the image doesn't really mean much.
While increasing sensor or film resolution can bring the best out of the limited capability of lenses, it still can’t make information that’s just not there.
So, what is it?
If you’re strictly looking at the overall image information being recorded modern digital will win every time. 50 lp/mm using the flawed and over-simplified method will yield around 8.64 megapixels. If you would try to use 100 lp/mm it would go to 35 megapixels. At 200 lp/mm we would be reaching the stratospheric resolution of 138 megapixels. But that will need to be tempered with the fact that lenses, even good ones, don’t resolve anything close to that. You also have to account for the grain that I mentioned earlier. A digital sensor, running at or near its base ISO will give you a very smooth result, there will be little to no noise in the resultant file. Film, on the other hand—even slow film—will always have grain. It adds noise to the signal which must be accounted for when looking at the overall image content of the picture that it encodes since the image is being degraded by the noise.
Like I said, this is a complicated topic. For almost all cases, modern digital will win in terms of overall image data that are recorded when you take a picture. In some limited cases you can extract a tiny bit more micro-contrast from an image by using film, but overall the rendering of the total image will be better with digital.