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Possible Duplicate:
What megapixel value is equivalent to which ISO film?
Is it true that '80s 35mm photofilm had quality corresponding to 24 megapixels?

I know this is a little bit like comparing apples with oranges but I am trying to find the point where digital photography caught up with conventional film photography.

Today's high-end full-frame digital cameras have 35mm sensors which is the size of a conventional 35mm film. These two technologies (digital and analog) are fundamentally different but at the end of the day, the images produced are consumed alike.

Now, consider an 35mm analog image recorded on top-quality film with top quality equipment. What would be the resolution (megapixels) and color-depth of an equivalent digital image.

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marked as duplicate by mattdm, Nick Miners, Matt Grum, John Cavan, Imre Dec 24 '12 at 15:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
This is such an apples-to-oranges that I can't tell that the OP is really asking. When we enlarged 35mm film, we often got grain. It was considered artistic. It was, in fact, just noise, or the limitation of recording ability of the film. –  Pat Farrell Dec 24 '12 at 4:43
    
It depends on the film –  Fake Name Dec 24 '12 at 8:20

2 Answers 2

That depends entierly on what you mean by the question.

If you mean the point where there is about the same amount of detail recorded, perhaps 5 MP, 8 bpp.

If you mean the point where you can make an enlargement the same size without feeling that the quality is lacking, perhaps 20 MP, 12 bpp.

If you mean the resolution that you can scan a negative and get out all the information, perhaps 50 MP, 16 bbp.

Those are of course just ballpark figures, and varies depending on what film you are using, how you measure detail, what you find acceptable, et.c. It should however give you an idea of what you could expect.

As you say, it's like comparing apples and oranges. We can blow up a negative to a size where you clearly see every grain, without seeing that as a lack of quality. When you blow up a digital image so that you see the pixels, most will feel that the quality is lacking. Generally a digital image needs a higher quality to be perceived as equivalent.

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Instead of picking arbitrary numbers out of the air, do the math to get some comparisons.

A "35mm" frame is 36x24mm in size. Look at the resolution spec for some films and lenses. Some films were rated at nearly 200 lines/mm, but some much less. There was a tradeoff between sensitivity and grain size. That added noise and lowered spacial resolution of more sensitive films. Lenses also cover a range. Let's say roughly 50 lines/mm would be "good", and 100 lines/mm astonishingly superb. Of course that's only at the optimum f-stop and camera mounted and held very still.

You said top equipment, so let's see what 75 lines/mm comes out to as a starting point. A "line" is actually one complete light-dark cycle, so you have to allow for at least 2 pixels per line width. So the 75 lines/mm becomes 150 pixels/mm, which means a full 35mm frame would have 5400 x 3600 pixels = 19.4 Mpix.

However before you run off and declare that the answer, look at all the judgement calls that went into ultimately getting that number, and that film resolution and digital resolution are in some ways a apples to puppy dog comparison. Film not only has some maximum resolution, but also grain. This is effectively noise added to the image. Pixels have some noise on them too, but this noise is random and occurs on the pixel grid pattern.

Film and lens resolution is a "soft" thing. Neither reproduces frequencies up to the limit perfectly fine, then suddenly mushes everything to the average after that. The contrast falls off with frequency, so the resolution spec is some arbitrary point along that curve. Usually the -3dB point is used. In contrast, what one pixel does is pretty much independent of its neighbors. The resolution is fixed and finite due to the pixel pitch, but that also introduces aliasing which is something completely foreign to the analog film process.

Before I switched to a digital camera, I used to scan negatives at about 9 Mpix. That wasn't a deliberate choice, just happened to be the limit of my scanner. However, at that resolution the grain was clearly evident. I now get effectively 12.1 Mpix from the same image area with my digital camera. I can tell you that subjectively the pictures from the digital camera look better than the scanned negatives. The pixel-level noise is much lower, mostly because the grain noise is gone. The digital sensor is also significantly more sensitive such that 9 Mpix would be a joke with "high speed" film to match the sensor.

The digital sensor also has more dynamic range. The camera has a 14 bit A/D internally. Of course you don't get 16k useable levels from every pixel, but you do get a lot more than with film. This opens up a lot more options when post processing. Not only could many of those things not be done with optical processes, but the original dynamic range captured on film simply wasn't there. I usually used color negative film because it had better dynamic range than slide film, but that was still well below what a good sensor can do today.

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