There are so many opinions on how film and digital photography compares, from everything such as image quality in general, workflow and process, dynamic range, tonality, highlight rolloff, color science to of course, resolution. Opinions on the matter of how film compares to digital with image resolution is all over the place - but one thing is typically a constant throughout this entire topic - equating megapixels to resolution and using megapixels as a benchmark to define what medium offers more resolution over the other.
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| The resolution of film, according to Dall-E 3 |
First things first, we need to understand what resolution actually means - and what it doesn't.
Megapixels don't matter.
Megapixels do not measure resolution.
Pixel count - or Megapixels - are needed to store data digitally, the data that embeds the fine detail information resolved in an image through the optical system.
Looking at a megapixel count only, you will not know how many details are actually stored within these megapixels - you only know what the upper ceiling is for the amount of details that can be stored within a certain megapixel count. Confused yet? We'll get back to that.
If not with megapixels, how can we measure resolution?
By measuring the detail resolution - such as measuring how many distinct lines from a scene an optical system can resolve and record as distinct lines. When we say distinct lines, what we really mean is line pairs. A pair of two lines, one distinct from the other, rendered so that the human eye can recognize them as distinct lines, not a singular line. The higher the contrast between two lines in a pair is, the higher the perceived sharpness to the human eye.
Measuring optical resolution in lines per mm (also, line pairs per mm) is a common way to quantify the detail resolution an optical system can capture. lp/mm is a factor used in Modulation Transfer Function (MTF) charts lens manufacturers provide for their lenses. Film emulsion manufacturers sometimes include a lp/mm measurement in a film's specification sheet to denote what amount of detail a film can capture (under the best/controlled circumstances).
Knowing how to measure details resolved by counting distinguishable line pairs per millimeter on an analog medium such as film, we can now translate from information that is stored through the magic of chemistry on an emulsion to the magic of information science & electronics on a digital storage medium as a file, using megapixels.
Translating resolution from analog to digital
The smallest unit of information in a digital image file is an individual pixel. To store the information representing a line pair - the shortest possible line pair - we need at least two pixels, one with the color/luminance value for one line, the other with the color/luminance value for the other line in the line pair. For a square millimeter of surface on a film emulsion, we hence need four pixels per line pair per mm a film can resolve on that surface.
This is important: we are now only looking at the chemical storage medium - the film itself. The film inside a camera will only be able to capture the amount of detail the entire optical system in front of it can capture. And as you will see, the film itself is not the limiting factor in the overall amount of detail an analog optical system is able to capture. In this sense, film is no different from digital.
Fuji Velvia 50
Let's start with a film known to reproduce fine details very well: Fuji Velvia 50, a color transparency film. Velvia 50 may be the highest resolving color film known. Fuji specifies that Velvia 50 can resolve 160 lines/mm (At Contrast 1000:1).
Per mm of Fuji Velvia in one dimension, we need 320 pixels to store the information of the 160 line pairs the film can resolve. Per square millimeter, we need 320 x 320 pixels = 102,400 pixels. To cover 35mm/135 format film, we need to account for a surface of 24mm x 36mm = 864 square millimeters. Multiplying 864 square millimeters with the 102,400 pixels we need to cover 1 square millimeter of Fuji Velvia 50, we end up with 88.4736 megapixels of distinct, uncompressed information.
The maximum amount of information Velvia can chemically store according to Fuji's specification sheet, requires almost 90 megapixels just to cover the small frame format of 135 for what today is known as "full frame".
Accounting for the smallest analog medium format frame size of 6x4.5, we need 276.48MP. 6x6 on 120 requires 368.64MP to store the maximum amount of detail Velvia can possibly hold. 6x9 on 120 requires 552.96MP. And for large format, 4x5 we require 1167.36MP and for 8x10 we require 4630.528MP respectively.
Mind-boggling.
Digital/Analog Equivalence?
So, have we just proven that Fuji Velvia's specified resolution is equal to that of a 90MP sensor in a modern digital "full frame" camera?
No, we haven't.
Unlike film, most modern digital cameras do not capture details - including color information - such as line pairs directly to the sensor with two pixels per line pair per mm. Most modern digital cameras do not just capture luminance information per pixel, they apply a color interpolation algorithm through a color array filter on top of capturing luminance per pixel, because most digital cameras cannot capture the true color value per pixel directly on the sensor. This is either done via a Bayer algorithm or X-Trans algorithm on the color array filter. A color array filter greatly degrades the efficiency of how much detail information the sensor can actually store with the nominal pixel count it has, because it needs to leverage some of the pixels to "guess" color values it needs to fill in from its color array filter. The few digital sensors not affected by this issue are monochromatic digital sensors without any color array filter such as Leica's Monochrome cameras and camera sensors which mimic how film captures color information by allowing red, green and blue light each to be directly captured on each pixel on the sensor. The only cameras in market to provide this are select Sigma cameras with a Foveon sensor that captures three layers of pixels in silicon on top of each other, with each nominal pixel in the resulting image file having a distinct color value captured for red, green and blue independently - a color array and color interpolation is not necessary.
The cost of applying a color filter array on top of the luminance layer of a sensor is severe, sensors without a color array filter capture at least twice the amount of detail present. So, in other words, you would need a 180MP Bayer full frame sensor with a color array to resolve the 160 lp/mm Fuji Velvia 50 captures on full frame - or a monochrome sensor with 90MP, sacrificing all those wonderful Velvia colors.
And that's just 135 format. To match what Velvia 50 can resolve in 4x5 large format, a digital camera with a Bayer color array filter would have to provide a nominal pixel count higher than 2,000 megapixels.
You may work your way backwards from a digital camera system somewhat.
Let's take Sigma's Merrill cameras, acclaimed for their fine detail resolution. They are the closest thing to capturing color information chemically on film. The Merrill cameras have a 45MP Foveon sensor that captures three layers of pixels on top of each other, 15MP of blue, 15MP of green and 15MP of red. The sensor size is APS-C, which translates to a surface area of 25.1mm x 16.7mm = 419.17 square mm. 15MP are spread across 419.17 square mm for the final image file coming out of these cameras, so per square mm, this comes out to be 35,785 pixels per square mm. The square root of that is ~189 pixels per mm in one dimension. Knowing that we need 2 pixels to resolve a line pair per mm, the Foveon Merrill sensor can resolve a maximum of about ~94 line pairs per mm.
Let's take another resolution monster, the Leica M11 with its 60MP full frame sensor. This translates to ~372 pixels per mm in one dimension, but accounting for the color array filter and its hurtful impact on resolution able to be stored, we need to apply a factor of 0.5 and we end up with resolving a maximum of ~93 line pairs per mm. In theory, without the color array filter, the Leica M11 Monochrome's sensor should be able to resolve up to ~186 line pairs per mm on the sensor in grayscale.
What about other films? How realistic is Fuji's specification sheet?
So, what about other films? Is Velvia 50 the norm or the extreme? It's difficult to find reliable sources on other color films - Kodak does not specify resolution in lp/mm in their spec sheets, but it seems that ~100lp/mm is a sound assumption for the majority of professional brand film emulsions in C41 and B&W. What does 100lp/mm translate to? On 135 "full frame", this results in ~34MP needed to store all that information, requiring about ~60MP on a Bayer CMOS "full frame" sensor to capture the same amount of detail.
A noteworthy specialty film is Adox CMS 20 II, an ultra-high-resolution black&white film. It's marketed to have a resolution of up to 800lp/mm. Let that sink in. With proper exposure - and a lens that can resolve that much detail, on 135 "full frame", you would need 272MP to store that information when using a digital sensor without a color filter array. Make it ~500MP on a regular Bayer sensor. Adox CMS 20 II is a very demanding film though, it needs to be exposed at ISO16 or 20 depending on the scene contrast, it has very low dynamic range and it's easy to either blow highlights or drown shadows, making it unsuitable for high contrast scenes that cannot be managed with a graduated neutral density filter.
Are these specification sheet numbers realistic? It all depends. There is no doubt the film medium can store that amount of detail when exposed properly under best/controlled conditions as specified by the manufacturer. The same applies to a digital sensor - the Leica M11's color sensor should be able to store the information of ~93lp/mm within its 60MP sensor - if that amount of detail hits the sensor.
That if is really the key in this entire comparison. Sensor or medium resolution really is a theoretical comparison. Many lenses do not resolve fine detail to the degree a sensor or medium can capture anything in between ~90lp/mm to ~160lp/mm. To get the best detail resolution out of a lens for a subject in a scene, the lens probably needs to be stopped down - either to hit the lens' sharpness sweet spot and/or to provide the depth of field to have enough of the scene in focus to show detail in the first place. You don't need megapixels to capture bokeh after all. With high resolution digital sensors, diffraction limits will then also play into the equation, especially when stopping down the lens too far. While film speed is fixed - and very slow compared to digital - a digital photographer also needs to watch his ISO and/or shutter speed, carefully avoiding long shutter speeds which blur details of moving objects, either because the object is moving or by camera shake. Digital cameras clearly have a technological advantage over film because they can produce a clean, sharp image at ISO sensitivities many stops higher than any film, allowing the digital photographer to resolve details at higher shutter speeds in spite of high ISO. And then there is image stabilization - sorry Leica M11, not you! - which also contributes to sharper images, with more lines per mm actually reaching the sensor. It's easier to get sharp images - and hence more resolution - using a modern, digital camera.
Does it matter?
All of that being said, resolution doesn't really matter that much. There is more than enough resolution in a 24mm by 36mm negative of a properly exposed frame to print large for a demanding audience - certainly compared to any digital camera released up to now in 2024.
Even if you assume that the real world average of most films' resolution is closer to 25lp/mm than 100lp/mm or higher, you would still end up with a respectable 8.64MP for 135 format, only matched by a CMOS Bayer sensor with ~16MP. Old movies, shot on 35mm film in Super 35 are being remastered into 4K by re-scanning the old movie master rolls in 4K. The original Star Wars trilogy from the 1970s and 1980s is available in true 4K because the original film material was remastered in 4K. These remastered movies shot on film look better than the newer Star Wars prequels shot on digital only, when cinema cameras then maxed out at ~2K. Go figure.