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A fundamental question is - if threshold skills are that important, why we don't structure courses around them (instead of stuffing courses with other less important material) & whether teaching effort is appropriately proportioned to make sure students "get it"
David
I usually encounter threshold concepts in medical education where one example is recognizing shock. Recognizing shock is a firm metric. Another threshold that is far more difficult is recognizing behavior that reflects internal medical changes. I'd call the later a soft, but critical metric. Learning it requires immersive experience and critique.
In a sense, then, we see two things: the firm skill and the very difficult - the later being difficult because the medic must look to the patient's behavior with a careful balance regarding his (the medic's) personal point-of-view.
Corresponding thresholds in photography can have similar contrasts: the craft side of photography, and the art (and where the two meet or conflict for the sake of craft or art.) It is fairly easy to teach the technical part which I take to be the hard skills (not difficult, but certain.)
Concerning the art, well the history of photography as an art can be taught, but art cannot be taught. Understanding the art requires immersion and critique, and critique is almost impossible to teach, let along practice - the discourse of criticism is a study unto itself, and also because most art is taught by artists and artists tend to have entirely self-centered points-of-view. (If MDs behaved as artists, we would be in dire trouble.)
I'd like to recommend a challenging book:
Why Art Cannot Be Taught - A Handbook for Art Students by James Elkins. Elkins takes a provocative approach.