The camera’s computer will always come up with settings that it thinks will provide the most balanced exposure possible in any given scene. But as you may already know, that may or may not necessarily be the one to give the results that you want.
On the other hand, going from Auto to Manual may not be the best way to go if you’re not really sure what you’re doing either. It may not even be the best way to go, period.
Therefore, what I suggest is to first understand what all these modes mean. These are perhaps the simplest way that I can explain them:
Auto – the camera dictates everything
Program (P) mode – the camera dictates shutter speed and aperture, you decide what to focus on and whether or not to use the flash.
Shutter priority (S or Tv) mode – on top of what P mode allows, you also set the shutter speed, while the camera adapts by setting the aperture to what itthinks is the correct exposure.
Aperture (A or Av) mode – on top of what P mode allows, you also set the aperture, while the camera adapts by setting the shutter speed to what it thinks is the correct exposure.
Manual (M) mode – you set everything yourself.
ISO is controlled separately. While I personally prefer to set this manually and staying between 100-400, there’s nothing necessarily wrong about setting it automatically either.
Unless you know the exposure wheel by heart, chances are, your camera knows better than you when it comes to getting a balanced exposure. Or at the very least it has a more consistent basis than you. Do not hesitate or feel ashamed to take your cue and learn from your own camera. Fighting your camera wastes time. And when you spend more time worrying about your camera settings than actually shooting, you’ve already lost half the battle.
My suggestion is to start with P mode when in doubt or if you don’t want to fuss over the settings. All you need to do is set your point of focus and decide whether or not to use a flash. If you are not satisfied with the result, then at least you have a solid reference point to start with. It’s definitely much more efficient than going straight to Manual and figure things out through trial and error.
Under absolutely no circumstances are you expected to master or even use the M mode. Learning to go Manual may seem like a noble goal. It’s use has a puristic or even macho charm to it. But the truth of the matter is that more often than not, it’s hardly a practical mode of choice. Personally, I only shoot in Manual as a last resort. I would also rather you concentrate on composing your picture properly rather than constantly guesstimating exposure settings in your head.
The camera’s M mode is for its exposure settings. Manual lens focusing is set separately.