Originally posted by BigMackCam TAv is probably my most used mode, unless I'm shooting in bright daylight (in which case Av or M make more sense).
I'm happy to let ISO float as high as the camera wants, on the basis that shutter speed and aperture are my priorities creatively. And if I see that ISO has gone too high, I can always compromise on one or the other...
Actually, my Sony gear offers the same capability, but it's less logical in the way it's implemented. Instead, you go to (M)anual mode and set ISO to Auto... Otherwise, though, it's the same. But I like that Pentax has a specific mode for it
In ye olde days of film, the four settings of the PASM dial were good enough because ISO/ASA was set by the film and the AE system only controlled 2 dimensions of shutter time and lens aperture. But with digital and especially with newer high-DR/high-ISO sensors, the sensitivity become a third variable.
In theory, the PASM mode dial should expand to 8 possible settings:
P: nothing manual / automated time, aperture, and sensitivity
Tv: manual time / automated aperture and sensitivity
Av: manual aperture / automated time and sensitivity
Sv: manual sensitivity / automated time and aperture
TAv: manual time & aperture / automated sensitivity
TSv: manual time & sensitivity / automated aperture
ASv: manual aperture & sensitivity / automated time
M: manual time, aperture, and sensitivity / nothing automated