I'm really frustrated because many of my nicely framed photos are of poor technical quality, mainly because of the wrong depth of field or subject being out of focus. I always thought I know theory but I'm having hard time to put that into practice. How do you learn to set the right focus and depth of field in practice?
Here are some details of how I find hard putting theory in practice.
Playing with aperture. In theory I know that depth of field depends on the matrix size, the focal length, aperture and a distance to a photographed subject, lots of calculators online can compute depth of field for me. However, when I'm outside photographing friends and expect to have them sharp and the wall just behind them blurred I cannot really stop for too long, measure a distance to them, and do calculations. There's no time for that. I tried with 50mm lens doing on f/1.4 to take a photo of a couple standing 2m from me. Only first person, a woman, closer to me is in focus, while the other is blurred, just because, as I calculated later, depth of field was around 9cm. How, the hell, should I know that in real time?
Playing with preview function. Yes, there's a magic button in Canons to preview a frame with actually selected aperture but what I see really small and dark for larger apertures. That doesn't seem very practical to me.
Playing with focus points. Canon body let you choose focus points manually: central or side points. Another option is to chose central point. But when a situation happens I often have little time to update the focus point. The best solution I think is to keep a central focus point and freeze focus and then move lens.