In spite of the common use of digital cameras, I have at least three, photographic film IS still available.
With film available, the question is: how do you then expose your artwork to the film? It's a chicken-egg problem.
In PCB industry, photographic film was used, because they used laser photoplotters that exposed the digital artwork on the film. Those photoplotters did not have enough power (or suitable wavelength) to do Direct Laser Exposure, that's a later thing; so film was used as an intermediate. But few of us mere mortals have those photoplotters at our homes.
And have you ever tried to use ordinary film for a PCB negative or positive? I have. It didn't work.
Not only I have, without any problems, I have also measured the densities. I don't know how you exposed your film, and why did you use the film as a medium at all, but probably you
1) exposed it for too little time,
2) developed for too little time or using spent or aged developer,
3) used aged film which had fogging
4) had stray light etc. that reduced your contrast by causing "fogging", exposure of supposedly non-exposed areas
For example take a look at Kodak T-MAX 100 datasheet:
https://imaging.kodakalaris.com/sites/default/files/files/resources/f4016_TMax_100.pdf , one can get density difference log D = 2.5, which is far better than any laser printer printout I ever measured (which usually varied between maybe 1.5 to 2.0). With a high-contrast print film you'd get D = 3.0, and you would get that more easily from narrower exposure (log H) range, that's for sure, but that does not mean one could not use a standard negative film if that's what you have, and for some strange reason would like to use photographic film as an intermediate step.
But quite frankly, I can't figure out why anyone would do that in year 2023. Time is better spent looking for a printer that does good job directly on transparencies (laser or inkjet), or if you want to DIY, do a Direct Laser Exposure thing, using a powerful UV laser to raster scan the photoresist-coated PCB. These have been a big thing in the industry during last decade or so.
And, for PCB exposure you do need something that is almost completely opaque.
"completely opaque" is meaningless, please put it in numbers. In my experience, something like logD = 2.0 is well enough. The more you have, the more forgiving your process will be against variations (specifically exposure intensity/time). Around D=1.5..2.0 is what many laser printers and most inkjet printers produce with their default settings. My experience just is that in laser printer settings, the toner density adjustment does nothing, while inkjet printers respond better to the adjustments. It's just you need to use Windows and the official manufacturer's driver to get access to these settings.
Where to measure with laser prints is a big question. I noticed with practically all laser printers they have edge effects: larger black areas are less dense in the middle. With negative process, this sometimes meant you got fair isolation next to the traces, and thin traces were OK, but larger polygons or ground pours were crappy in the middle. Never saw anything like this with inkjets; the density is more even (unless the nozzles are clogged or ink running out, so that stripes appear. Yes, inkjets are pain in the ass and expensive to maintain, price per print skyrockets if you only need to print rarely, but always need good prints!)
As I said, I tried it and it just didn't work.
You failed to analyze the reasons why the test failed. This is the most difficult part in failing, and it's where all the learning happens. For me, things never ever "just don't work". There has to be a reason.
When I struggled with this question (how much density is needed), I just used density test strips (e.g.
https://ndtsupply.com/film-viewers-densitometers/density-strips/ndt-density-step-tablets/ ). Expose through the test strips and simply see what is the minimum density difference between a properly developed and properly undeveloped area. Note that if you get for example D=1.0, then it means if you use D=1.0 films in production, your exposure time and process need to be 100% perfectly tuned each time. That's why you actually want at least D=1.6.
I also tried ink jet transparencies. They are even worse than ordinary photographic film. They let even more through.
It's not about the transparencies, it's about the
ink. Specifically, you need an inkjet printer which allows adjustment of print quality / ink density. I have tried something like 5-6 different inkjet printers and only 1 of them was unusable due to way too little density, and no functional adjustment in the driver. Do note though that budget inks (not approved by the manufacturer) usually ruin the whole thing, never had good results with those, even if they look OK-ish when printed on paper.
But discussing this is useless as we both have just anecdotal data over small number of printers. My point is, I have
never seen an excellent laser print; the density is usually "just barely OK", causing trouble with gauging the correct exposure time. With inkjets, I have seen both "just barely OK" but also excellent prints.