The difference between film and digital is in the ASA/ISO business. With film, each roll was a given ISO. If you wanted a different ISO, you had to remove the roll or develop it differently knowing you were developing some as desired and hope for the best with the rest.

With digital, each shot can have its own ISO. Judging from you last comment, just pick an ISO and leave it. Now, you can shoot in an automatic mode, shutter-priority, aperture-priority or manual just as with film. Take ISO out of the equation. You can always add it back later.

I say ASA in deference to those of us who pioneered photography way back when. At least, I did not say DIN. As for shooting at 100, my Canon 60D will go to 400 with no perceivable increase in noise. Those two stops give me a bit more flexibility that I greatly appreciate. In fact, today's 6400 gives me better "grain" and contrast than Tri-X and T-Max did at 400. And I shot 2 1/4 square.

As for beginners sticking to 100, I agree with you but would have no problem with their going to 400 to reduce motion blur and depth of field issues.
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So everything is as easy as ABC1234 and I agree with you too! It's been a long time now since the name change and just one more thing for a beginner to learn. Personally, if it's bright...sun, summer, sky ...or winter snow...I go down to ISO 64 as outdoor base. Anyone can then choose, after making the ISO basic decision for the situation, to shoot a manual or semi-auto exposure. Happy shooting to you too.
Happy shooting.

It seems to me that this is only approximately correct if one is using an auto exposure mode, say shutter priority or aperture priority and one relies entirely on the camera's metering system to determine "correct" exposure.

If I use manual mode, I can select all three parameters. This may or may not result in an acceptable exposure. For example, if the subject is on a field of snow, and I select an ISO of 200 and an aperture of f/4, auto metering may give me a high shutter speed (making the snow gray and my subject too dark. I can decide to select a slower shutter speed, so the snow is brighter and my subject is also brighter, leaving the other parameters as previously selected.

In the auto modes, one could use exposure compensation to achieve the same ends.

Where have I gone wrong?

The exposure triangle is a familiar concept and presents the three key variables for exposure: shutter speed, aperture and ISO. You need all three to make a picture. However, most discussions either ignore or gloss over the fact that the photographer can pick only two of the three variables at a time. Once he or she does, then the amount of light present determines the third. This is the technical reality and artistic considerations may override it.

Another problem with the exposure triangle is that it does not clearly show how one variable affects another. For example, how does changing the shutter speed affect the aperture or ISO and what do you gain or not gain for it. For experienced photographers, this is trivial. However, for beginners, this can be baffling.

Therefore, to help beginners, I transformed the exposure triangle into a scheme called "Pick Any Two". Any two of the three exposure variables gives three combinations of shutter speed, aperture and ISO. The pictures below show each combination, how the two independent variables affect each other, and the various trade-offs. The values shown are only examples and should not be taken literally.

I look forward to comments on how to improve this. If you would like a PDF, please send me a private message.

SS, in this age of built-in meters, why would you use sunny 16? Is a sunny day in my neighborhood as sunny as in yours? Or is a sunny day in January the same as on in June? Or is 9 AM as sunny as 4 PM. Might be in July but how about December?

As for cutting corners, what happens if you cut a f/stop in half or double it? f/4 becomes f/2 or f/8. Right? Oops, now we have four times the light or a fourth. Oops is right.

I know you know better. Thanks for all your enlightening posts.