Resolution of Film
How many pixels does it take to describe all the detail we can get from film?
Fuji Velvia 50 resolves 160 lppm [ line pairs per millimeter ], Fuji Provia 100F Professional as well as Kodak EliteChrome resolve around 140 lppm. This is the finest level of detail it can resolve, at which point its MTF [ Modulation Transfer Function ] almost hits zero.
Each line pair will require one light and one dark pixel, or two pixels. So you will need about 320 ppm [ pixels per millimeter ] to represent what a Velvia 50 can resolve, or 280 ppm for a Provia or EliteChrome.
Now let’s do the math:
320 pixels x 320 pixels is 102.400 pixels or ~ 0.1MP per mm² .
35mm film format delivers an image size of 24 x 36mm or 864 mm².
To scan most of the detail of a 35mm slide, you need about 864 x 0.1 = 87 MegaPixels.
Agreed so far?
OK, but here is the catch: each film pixel represents true R, G and B data, and not the ‘Bayer pattern’ interpolated data from digital camera sensors. A single chip 87 MP digital camera still can not see details as fine as a 35mm film slide.
Since the ‘lie factor’ from digital cameras is factor 2 [ because you actually need two dedicated pixels to resolve one black and one white square ], we would need a digital camera of 87 x 2 = 175 MP to see every last detail compared to film. However, even ‘Full Frame’ DSLR’s are limited to 24 MP as per today [ 2010-10-07 ].
That’s 35mm film. Now do the math for a 6×9 medium format film. Because medium and large format lenses resolve less detail, I’ll use 120 lppm as a factor.
120×2=240, 240 pixels x 240 pixels is 57.600 pixels or ~ 0.057 MP per mm².
6×9 cm equals 60×90 mm and delivers an image size of 56 x 86 mm or 4.816 mm².
To scan most of the detail of a 6×9 slide, you need about 4.816 x 0.057 = 274 MegaPixels.
To be able to resolve the resolution and detail a modern film emulsion will render, you need a scanner that resolves up to 8.000 ppi or spi [ Samples Per Inch, which is identical to ppi ].
The Nikon Coolscan LS 5000 ED and LS 9000 ED scan slides @ 4.000 ppi. Now let’s see if this will be sufficient for us and convert 4.000 ppi [ pixel per inch ] to pixel per millimeter:
1 inch = 25,4 mm
4.000 ppm / 25,4 mm ~ 157,5 ppm [ pixel per mm ]
A 35mm slide scanned with this resolution will result in an image size of 24 x 157,5 ppm x 36 x 157,5 pm or 3.780 x 5.670 pixels = 21.432.600 pixels or 21.4 MegaPixel.
A 6×9 slide scanned with this resolution will result in an image size of 56 x 157,5 ppm x 86 x 157,5 ppm or 8.820 x 13.545 pixels = 119.466.900 pixels or 119.5 MegaPixel.
Pretty impressive up to this point. However, now let us break the boundaries of the Nikon scanners and turn to a drum scanner.
To resolve full detail of a 35 mm slide, we just have to multiply 36 mm (the long side) x 320 pixels which results in a required scanner resolution of 11.520 pixel for the whole slide.
Now let us convert this into ppi [ pixel per inch ]:
36mm / 25,4 = 1,417
11.520 / 1,417 = 8.130 ppi which is a tad higher than the resolution power of a high end drum scanner.
Don’t try to calculate it for medium format – a scanner to resolve that image area doesn’t exist.
BTW, just compare the camera below to digital cameras. It is almost the same size as a Canon 1 DS MK III or Nikon D3x, but the area of the recording medium is 5,57x larger. Compared to a new medium format digital back the recording medium still is 1,78x larger. Do you really want me to talk about 4×5″, 5×7″ or even 8×10″ cameras?
When it comes to real world resolution in photography, film can’t be beat until today. I guess many digital shooters who propagate the ‘fantastic resolution’ of their cameras now wish they could buy a film based camera.
OK, I already hear them screaming: “But the lens resolution [ MTF ] of medium and large format lenses is considerably lower than you have taken into account here!” All I can say at this point: please, just do the math again with lower resolving powers by applying the formulas posted above.
Film will prove its superiority in any aspect, even if the resolving power of the lenses will be lower.
[ One of my Fuji 6x9 cameras with the fantastic, razor sharp EBC Fujinon 3.5/90mm lens - a true beast in terms of resolving power ]
– photograph © 1999-2010 by jens g.r. benthien –
Additional information: Resolution and MTF curves in film and lenses