Is it true that '80s 35mm photofilm had quality corresponding to 24 megapixels? Somebody told me so today, and I find it surprising.
I have been using various Kodak E100 series slide films (100 ISO) in a Leica, with good optics, and the detailing that my Nikon Coolscan V gets out of these is absolutely absurd. I'd say about 20 megapixels' worth of detail, give or take - easily as good as my 16.7 mp 1Ds II anyway. Given a good exposure and focusing in the first place, of course. How this compares with 80s films I do not know. And once you go to 200, 400, 800 ISO (negative) films digital wipes the floor with them, no question about it.
The answer is "it depends". With some films, the limitation was the lens for all practical purposes, just as it is with the extreme-resolution DSLRs today. Kodak Technical Pan shot at ISO 16 (yes, 16) could easily resolve 150 line pairs per millimetre, which would give it a Nyquist equivalent of just under 80 MP in the 135 (35mm) format. Kodak's Ektar 25 colour print film (shot at 20) and Kodachrome 25 slide film (shot at 32 or 40) would easily beat a 24MP sensor if all else were equal -- but all else isn't equal.
Note that those amazing, digital-ain't-there-yet resolutions are all coming from films that are, well, slow as molasses in January. And that every single one of those films has been discontinued. At ISO 100, even with the best, most-lauded films, it's a coin toss -- you can get a better tonal scale with film (analog response does that for you) but sharpness is already going to the digital camera (as long as the antialiasing filter, if present, isn't too strong). At speeds over 100 (for an equivalent-sized sensor/film cell), things are already beginning to go firmly in the direction of digital. 400-speed (colour) film was something a pro would use to get a grainy, romantic, sentimental look. ISO 400 is just another setting on a DSLR, with noise just beginning to be noticeable to people who are specifically looking for it (and that noise can be removed rather cleanly from the images most current-model cameras produce). And you don't need to spend five grand on a Fuji enlarger lens to print a decent 11x14 -- an Epson or Canon printer costing around 300 bucks will probably do a better job.
So yeah, digital isn't quite up to the best that film could do yet. But then film is no longer up to the best that film could do either. If ultimate resolution and smooth tonality are that important to you, you can always sell your house, move into a trailer and buy a Phase One IQ180 (it's only 50 grand, but you'll probably want a camera body and at least one lens to go with it).
Conventional wisdom says about 6 MP at around IS0 200 when looking at overall image quality.
Although film works very differently then digital images, so some people point out that the finest film grain is much smaller than a pixel on a 6 megapixels DSLR. I suspect this is why someone told you 24. While it is can be true, one pixel captures way more information than one grain of film. Film grains on the same frame also are not all the same size, unlike most pixels.
As soon as you boost ISO, then most comparisons become moot. Today's DSLRs wipe the floor with most 35mm film when it comes to the ability to resolve details in low-light. ISO 6400 is extremely usable on many recent DSLRs. That can be hardly said of ANY 35mm film, regardless of its age.
There are probably similar measurements with regards to dynamic range, with probably one way saying that film is better and vice-versa. Although given a whopping 14 stops of DR on modern DSLRs, I doubt film can compete in practical terms.
Well, it's four years now and as we all know, digital sensors are being packed with way much more photosites than ever.
With today's technology, in a given format size, a digital sensor is far much better than film in terms of noise and resolution. Please notice the "given format size."
I have scanned film negatives and I find that my Nikon D5500 (24MP) easily outperform my good old 35m film by a huge margin even though my D5500 is not a full frame (35m) camera.
The noise is handled way much better on digital cameras than analog cameras. Even at 400 ISO, the film has plenty of noise while my D5500 showed little noise. At higher ISO settings, film just gets really noisy and I am not liking it.
Remember APS film intended to replace 35m among consumers? It never become as good as 35m but yet most of our digital DSLR cameras are APS-sized and they outperform film APS by a huge margin in terms of resolution and noise.
Remember the disc cameras? They are one of the worst and the images were very noisy and the details murky. Our iPhone cameras have smaller sensors yet they outperform those "disc" images by a HUGE margin. Details are much clearer and noise is much better handled.
Simply put - in a given sensor size, digital beats film easily with an exception of highlights being handled - film handles them - they are rolled off nicely.
All the discussions about how film seem to be more "saturated" can be done in RAW editing. All films were "doped" to have certain colors. Velvia and Kodak 200 look totally different even though they use film - its just that they adjust the chemicals differently. An analogy is the JPG output of digital cameras - they are "baked" with their own style. RAW editing lets us bypass that "baked styles" and lets us manipulate different ways to edit images.
We know that it's EXTREMELY expensive to scale up a digital sensor. The costs increases exponentially as we increase the size and it just makes no economic sense to scale up at this point.
So, yeah, today's digital sensors outperform film in a given sensor size by a huge margin.
Perhaps some technically brilliant photographers could make images with 35mm film to rival those of todays 16.7+ Mp cameras. That is hardly relevant. Most people will get better images overall with any digital camera of 4Mp or more, than they could ever get with film. Heck ... most people get better photos from the tiny sensors on their smartphones than they could ever have achieved with film.
And besides, not only do some pixels carry much more information than others (I am thinking of Foveon X3 v. Bayer array) but also there is a lot more to image quality than Megapixel count.
And don't forget that digital files do not accumulate dust, hairs and scratches!
@Olin Lathrop gives a great answer in this Question (marked as a Duplicate): https://photo.stackexchange.com/a/30755/37074 of 5400x3600 or 19.4MP (derived from an imperfect Formula using LPM, but forgetting Bayer and other considerations like MTF).
So, 20MP is a fair ballpark and multiples of that would account for whatever wasn't considered; 60 allowing reproduction of the Grain itself, beyond what is necessary.