I have not yet determined the resolution of some of the excellent films currently available from Kodak, Fuji and others. The analysis done here is based on experience and data available from data sheets available online as of December 2010. Please check the Film Photography Resources Page for information about some of the high resolution films available today.

Variable Names Used Here

Variable Names
Symbol Description Units
X,Y Image plane horizontal and vertical dimensions Millimeters
AspectR Ratio of X size to Y size, presumed larger than 1 Number
NY Number of pixels in Y direction Number
LPmm Number of line pairs per millimeter 1/mm
MPX Number of megapixels Number
D Aperture width Millimeters
 λ Wavelength of light; green is 550 nanometers Millimeters
f Focal length of lens Millimeters
θA Angular resolution of lens according to Airy disk Radians
SDIST Distance to the subject from the camera Meters
IPpitch Image plane distance between pixels  μm

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Film and Megapixels

A Common Ground: Number of pixels in the vertical direction

How many megapixels are in a film image? This is an important question for comparison of quality between film cameras and digital cameras and raises itself in the choice of resolution in digitizing negatives and slides.

The answer is not an exact thing, for many reasons, including, but not limited to, the following:

In order to eliminate aspect ratio so that we are talking about the same number for different cameras, different image plane sizes, and different print sizes and aspect ratios, I will present results in terms of the number of pixels across the image in the smallest direction. For example, for an image of 5616 by 3744, this number is the smaller of the two, 3744. We will call this number NY, the number of pixels in the Y direction, or vertically; we presume a landscape orientation for this purpose.

As an exercise for the reader, I will leave it to you to show the following relationships:





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These equations make me crazy

What are the resolutions of 35mm films?

Browse the enthusiast literature or the web and you will find many claims, some seemingly very credible, about the resolution capabilities of 35mm films. Numbers as high as 200 lines per mm can be seen if you look for them; these may be double what we are calling line pairs per mm, but even so this is a high figure, particularly for color film. The truth is a bit unsettling. I've looked at Fuji Film data sheets as of November 2010 and found their plots of image contrast versus line pairs (or cycles, in their data sheet) per millimeter, estimated values from them and entered them in a spreadsheet. I then plotted the curves to approximate the curves given in the Fuji data sheets. For their ISO 50 and 100 films, the result is here:

FujiFilm Resolutions

Data sheets of others such as Kodak and Agfa are similar. Slower films are better than faster films because high ISO films have thicker emulsions to capture more of the light, which is why the slowest films are shown here, and why Ektar 25 was sold for a time. The truth of the matter is that none of the color films exceeds about 65 line pairs per millimeter. Kodachrome, while it lasts, is a little better because the emulsion is thinner than that of any E6 or C41 process film.

I don't have access to similar data for B/W films but will plot that data when I have it. My estimate is that the best of these films (special copy or process films, Kodak Panatomic X and other low ISO fine grain films) will achieve up to about 100 line pairs per millimeter at best.

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Lines per Millimeter on the Film Plane

Using the equations given above with 65 lines per millimeter, this gives us 3120 for NY, or about 13 megapixels for a 4:3 aspect ratio or 14.6 megapixels for a 3:2 aspect ratio.

Using a more realistic 50 line pairs per millimeter, we have a value of 2400 for NY, or about 7.68 megapixels for a 4:3 aspect ratio or 8.64 megapixels for a 3:2 aspect ratio.

The camera magazines once tested and printed actual resolutions for film as measured in their laboratories. I recall a figure of 40 line pairs per millimeter for a very popular slide film, Kodak High Speed Ektachrome Daylight, ISO 160. For this resolution, NY is 1920, and image resolutions are 4.9 and 5.5 megapixels.

So, we have anywhere from about 5 megapixels to an ultimate maximum of about 15 megapixels as the range of effective image resolutions for 35mm film. And, you can scan your slides and negatives at 2000 DPI to 3000 DPI and get all there is on the image. Going farther and resolving the grain may be psychologically better but you won't get better prints from the files, and processing them to print well may be more difficult. It may be better to let the scanner optics and software do the averaging than to struggle with it yourself in the post-processing software.

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What About the Optics?

The resolution of an lens in terms of angle is usually given in terms of the peak-to-null distance of the Airy disk, the diffraction-limited focus point of a circular disk. This is, in terms of the subtended angle from the lens,


This is an enormously important equation because it tells us the resolution of the optics out in the world. If the distance to the subject is R, the resolvable distance on the subject SD is


These two simple equations tell you the resolvable elements — the pixels — on the subject! Wait — you really need to have your information in terms of the f-stop and the distance? Well, the f-stop is


and the wavelengths of visible light range from 0.4 μm to 0.7 μm, with the center of this range being green light with a wavelength of 0.55 μm, so we have the angular resolution in the world, for green light, as


To get the distance between the center of resolvable points on the image plane, the pitch or distance between resolvable pixels on the image plane IPPitch, we multiply the angular resolution by the focal length f; thus


in microns (SI unit μm) for green light; a micron is 10-6 meters, or 1/1000 millimeter. Note we have assumed that the object distance is many focal lengths away from the camera. What we have is the minimum resolvable distance on the image plane is proportional to the f-stop and the wavelength.

Since the optical limit of resolution on the image plane is independent of the focal length, this means that the ultimate number of resolvable pixels attainable is proportional to the area of the focal plane. Note that the number of pixels in the Y direction is limited, unless we want to oversample a diffraction-limited image, by

NP < Y/IPPitch

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Give me a break

We have enough to show the diffraction limitations on lines per millimeter on the focal plane versus the f-stop:

Line Pairs per mm vs. f-Stop

Note that this curve is idealized and that essentially no lenses achieve diffraction limited performance with apertures faster than about f/4. The range is about f/2.8 for very expensive lenses and only at the center of the field, to a more typical f/8 or smaller for affordable lenses. This curve also tells us why lenses for digital cameras are more expensive than the old film camera lenses, why point-and-shoot cameras with small focal planes don't stop down below f/8, and why professionals never go below f/11 even with macrophotography when good sharpness is mandatory.

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Taking Both Film and Optics Into Account

This is really very simple.

Bottom line?

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