How does flexible ISO make shooting digital different from film?

I was thinking at the differences between SLR and DLSR (in Manual mode). In both cases you can change aperture and shutter speed as it suits you. But with SLR you are stuck with the ISO of the film which you happen to have in the camera at the moment, while with DSLR you can vary ISO as you wish, too.

Now maybe the question is naive, but how is this handled in practice?

I imagine that it requires more planning: you want to shoot today under this particular light and you choose a specific film roll rather than another. What if you find a nice shot but there is a shadow which changes the light: you can recover the exposure by changing the other parameters but this is not side-effects free obviously.

What are the effects of this "freedom" afforded by DSLRs? Is it making me (a complete beginner) lazy because I am not forced to think about it? Is it freeing me of a burden by removing an unnecessary constraint?

Answer

With film, you generally don't think of sensitivity as a free variable. Often your favorite emulsion is only available in one or two ISOs. So you have to reach proper exposure by adjusting aperture, shutter speed and/or lighting. Also, negative film is generally considered forgiving on some under- and overexposure.

In digital, you have more freedom to choose other variables, but you should still remember the simple truth that having more light on sensor will give you less noise in photo, so that is what you should consider first before pumping up ISO. But this does not mean that you should never use higher ISO values - noise would still be a far easier problem than excess blur or underexposure.

Actually, with film, you can change ISO by swapping film - take a note which frame you had the film on, rewind and when re-using take as many frames (plus one or two to be sure) with lens cap on. Some advanced bodies (such as Pentax MZ-S) also offer winding film to chosen frame. In medium format, many cameras employ interchangeable film backs. Large format film is handled as sheets, not roll film, so you can always change it between frames.