>>3918609>Well, what I really care about is how large an error I'm introducing by halving or quartering the amount of light per second, and doubling or quadrupling the exposure time.Practically speaking, you only have to "worry" if the change is so big that the new exposure (before accounting for reciprocity) is in the minutes, while the previous one was 5-10 seconds. This usually only happens when you have to print very big.
In practice, it's not much of an issue (and hence not much discussed) because you do an exposure strip test at the magnification and aperture your final print will be. There's little reason to do that in any other aperture.
The only reason I can think of, that you'd need a calculated (and not striptest) time, is if you want to skip the striptest altogether due to time concerns or increased volume.
For those niche cases, darkroom exposure meters exist. Sometimes even attachments for normal handheld lightmeters, so they can be used for prints - I know for a fact Gossen made some for their "pro" models (Luna Pro SBC etc.), others too.
Or, you run some tests and keep extensive notes, which you then use for the higher volumes without having to run them every time.
But I'm guessing for most papers you can find a datasheet with reciprocity. I know for sure that they state ISO (also under different temp lightsources), but I honestly never bothered to even look if reciprocity is also stated (probably is).
