[girvo]

Funny you say that about photographers. I learnt from trial and error, and after a few years was taking some photos that I'm still proud of today. Then, a few years ago I decided to learn the concepts and apply them to what I'd already learnt, and now I feel I'm even better. My best photos now are as good as (or a bit better) my older best photos, but I can capture them more consistently. It's really fun.

As an aside, the best thing I did was force myself to use a 1963 Minolta rangefinder, with one film type and speed only, for an entire year. Manual everything. This allowed me to really learn those concepts properly, but it was still steeped in trial and error; reading the concepts, then coming up with them afterwards from first principles is easily the best way for myself to learn a topic :)

[pfranz]

I really wish I could have thought of some specifics to make my point better. Most professional photographers do know basics of exposure really well, but may not know things like the inverse-square law when applying to flash photography (especially with the newer flash systems out there). Instead of using a light meter to take careful measurements, they fall back on abstractions, trial and error, and rules of thumb--like sunny 16 rule, their experience, and looking at the back of the LCD.

The only example I can think of is that I've heard Paul McCartney can't read or write sheet music.

Some people (like myself), feel the need to understand the underlying technologies in order to become competent. I don't feel like that's the only way to competency.