For example, when shooting Rollei IR400, I use all three options, in completely manual mode. I set the ISO to 25 (that ISO gives me the results I want with the process I use and the IR filter I have in full sun). I compose, focus (hyperfocal distance and f/16 usually), and manipulate shutter speed and aperture until the camera tells me it's correctly exposed for an ISO 25 film. Then I screw on the IR filter and release the shutter. If I'm shooting any other film, I usually pick an aperture and let the camera pick the shutter speed in Av mode, though occasionally I do it in Tv mode. About the only time I use a Programmed AE mode is if I'm using flash indoors.