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Is there some kind of equivalency table or formula that expresses what kind of pixels you need in a digital camera to have roughly the same quality as a particular ISO graded film? What other variables would influence this (focal-length, exposure time, etc)?

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Megapixels and film ISO are unrelated. Film noise is even different than digital noise. Where are you going with this question? –  Rezlaj Jul 15 '10 at 20:43
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I think it's a valid question. Film ISO affects information capacity, as do megapixels. However, your underlying sentiment may be, megapixels basically no longer matter, and that I agree with. –  Reid Jul 15 '10 at 21:00
    
Sorry but I too think these are too unrelated. I don't think you can come some vague correspondence either unless you specify the camera too... megapixel quality not uniform among cameras at all. –  Itai Nov 10 '10 at 18:58
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6 Answers 6

up vote 6 down vote accepted

I remember seeing a figure of 22MP was "as good" as 35mm resolution (of course, with film it isn't just the ISO, but the manufacturer and age of the film, skill of developer etc.)

Higher ISO film tended to have more grain; and higher ISO digital shots exhibit more noise - a similar cause, but the visual appearance is different.

Digital ISO noise is related to the size of each pixel, as the noise is per-pixel (so the more pixels you have, the less obvious noise is when viewed the same size). One analogy I've used in the past to demonstrate this is to ask several people to time with a stopwatch how long it takes a car to drive around a car park, and then to time how long a person takes to do the same journey - because the person is slower, the margin of error is a smaller in proportion to the overall figure, even though different people will give timings to within a few seconds of each other.

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This doesn't make sense...1800x1200 is a shade under 2.2MP, not 22MP –  davr Jul 22 '10 at 19:15
    
Ditto on what davr stated..22mp is high enough to print an unscaled 13x19 print, or a much larger scaled one. Generally speaking, digital noise is less apparent when a sensor is "less dense", which is usually the case with larger full-frame sensors, even though they often have higher MP counts. When the photosites of a sensor are smaller and more densely packed, the signal bleed from each photosite has a greater chance of affecting the neighbor sites, hence the higher apparent noise levels on smaller, denser sensors (i.e. APS-C vs. Full-Frame, the FF will usually exhibit lower signal noise.) –  jrista Jul 23 '10 at 16:04
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There are more problems with comparing the resolution of film to that of a digital sensor.

One problem is that the grain that you see is in fact not the image forming element but a form of noise. The actual elements are much smaller.

Also, it matters if you look at B/W film or color film; the image in B/W film is composed of silver particles, while most color processes use dyes coupled to the silver particles and then removing the actual particles; this result in "clouds"of color that are more or less centered on the position of the (now removed) particle, but are much bigger.

A good source on the nature of grain and film resolution is:

http://vitaleartconservation.com/PDF/film_grain_resolution_and_perception_v24.pdf

Pretty technical and I won't pretend I understand it all but if you want to know about he subject, it will certainly help you a lot.

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I think Film vs Digital article by Roger N. Clark answers exactly this question. Let me quote the chart from its summary:

Megapixel equivalent vs Film ISO

The main point is that digital sensors have fixed resolutions and variable sensitivity, while films have fixed sensitivity and varied resolution. Overall, at high ISO (> 400) most of the modern sensors provide higher resolution, and to match Velvia 50 you need at least 16 Megapixel.

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+1 - Nice find :) –  ltn100 Nov 11 '10 at 15:49
    
@Evan Krall (correctly) comments over at [photo.stackexchange.com/q/10158/] that this 2002 chart (updated in 2008) is rapidly getting out of date as sensor technology improves. –  mattdm Jan 5 '12 at 5:14
    
Even though the chart is outdated, it visually illustrates the major difference between film and digital: fixed sensitivity with variable resolution vs. fixed resolution with variable sensitivity. –  Michael Clark Feb 7 '13 at 7:08
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No. Because different films with the same ISO can have different quality aspects, and digital cameras with the same megapixel count can have different quality aspects. There are also many potential variables in processing/development and printing for both film and digital that will effect image quality.

You can discuss very specific examples. For instance how do these compare: a 12"x18" silver gelatin print from a Pentax Spotmatic w/ Super Takumar 50mm f/1.4 loaded with 35mm Kodak TX-400 rated at ISO 1250 and developed in Diafine, 2 bath, compensating developer and printed with a contrast filter grade of 3.5 using a condenser enlarger compare to a 12"x18" ink jet print from a file taken with a Canon 5D w/ EOS L 24-70 f/2.8 shooting raw at ISO 1600, and processed in Adobe Camera Raw with {specific processing settings} and printed using an Epson printer {specific model, specific inks, specific paper, printer software, etc...}.

In my opinion the finished image quality is going to have a lot more to do with the photographer's experience and skill with the tools, materials, and process they choose. When I got my first 8mp APS-C DSLR I couldn't match the image quality I was used to getting from the BW 35mm film I was developing in my darkroom. Several years of raw processing and digital printing practice later there was little doubt I was surpassing the quality level I was used to getting from 35mm film with the same 8mp camera. Now when looking at large prints from my Hasselblad 500c/m hanging next to large prints from my 5DII I think it's easily seen by most that I'm surpassing the technical quality I was getting from medium format film. Yet there are obviously folks who are not reaching this level of quality with the same DSLR and processing software.

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Excellent answer, and I think it is critically important to bring the skill of the photographer with the processing tools they use into the mix. –  jrista Mar 25 '11 at 20:25
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Film when scanned and viewed through the 'filter' of digital media (meaning, film pixelized and viewed on a monitory) will always be at a disadvantage compared with a first generation digital file. A true comparison would be to a slide viewed in optimal conditions next to a suitably graphics-centric monitor (high end). For color work, you would see the additional gamut of film, the deeper colors, and the superior sharpness (all things being equal, camera, lens, technique). So, in speaking of equivalents, we have to come up with 'information' equivalences for film and digital, recognizing that with different films, scanning hardware, techniques, etc., the results can vary widely.

My experience? Working with films that scan well (Fuji NPH, Kodak Portra, chromogenic BW films) on a high end desktop scanner (Minolta Multi Pro - 4800 PPI), film can easily surpass digital in the following ways: smoothness of tonality, sharpness (NOT acutance, which is digital with unsharp masking applied, which simply increase contrast of adjacent pixel edges), and color fidelity with certain subjects such as skin and landscape subjects (not necessarily accuracy across all colors!).

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It's not easy to compare ISO grade with sensor resolution because they're not related. What's more related is noise ratio versus film grain, but it's not that simple.

Film grain behaves differently that sensor noise. Where sensor noise makes you lose detail is where the noise limits the ability to perceive detail. Film has grains of different sizes and sometimes more detail can be found in the smaller grains of the lighter areas (in negative film, for instance) than in the larger grains of darker areas.

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