Q:

Photography concepts using SX20

I've bought a Powershot sx20 half a year ago and I'm quite happy with it although I know it doesn't compare to a DSLR. 
I've learning photography on my own: bought a 58mm ring and some filters (cpl, nd8, macro, star cross). I've got chdk to save in RAW so later with Photoshop I can reduce the noise of the pictures (and also use post-processing techinques to enhance them), and I'm using chdk's histogram to check if the picture has a good range.
But there are some concepts I'm not fully understanding:
1) Aperture priority: even in the widest range mode, where it has from 2.8 to 8, I don't see any difference in the picture. I know that AP is used for DOF, and that P&S have huge DOF because of the sensor size, but then why is this mode here? What's supposed to do in a P&S?
2) Manual Focus: the slider goes from 0cm to infinity (Depending on the zoom) and even you have the option to see a zoomed zone in the center of the picture for a refined focus, I don't see much difference between little steps. May it be because it's an electronic viewfinder instead of optical?
3) Exposure: I've read that you should get a proper exposure. There is an indicator that warns me if the picture is going to be too dark or too bright, but the only options I have to change the exposure are the shutter speed and ISO, which I always try to be at 100 in order to avoid noise (of course during daylight or in a well-lit room). How can I know if it's really proper? Right now I have a post processing plugin in photoshop which does exposure compensation so I'm checking how much does the picture change.
Thanks, I hope these questions are clear enough.

A:

1) Aperture priority: even in the
  widest range mode, where it has from
  2.8 to 8, I don't see any difference in the picture. I know that AP is used
  for DOF, and that P&S have huge DOF
  because of the sensor size, but then
  why is this mode here? What's supposed
  to do in a P&S?

You are correct in saying that a P&S has a huge DOF, however on these types of cameras the Av mode is often used as an alternative for Macro (little flower icon) mode. Having the aperture wide open is a common characteristic of Macro photographs.

2) Manual Focus: the slider goes from
  0cm to infinity (Depending on the
  zoom) and even you have the option to
  see a zoomed zone in the center of the
  picture for a refined focus, I don't
  see much difference between little
  steps. May it be because it's an
  electronic viewfinder instead of
  optical?

I'd definitely agree that it's far more difficult to see focus adjustments on an LCD (the electronic viewfinder) rather than an optical eyepiece. Again, it's not an entirely useless feature, but rather a more advanced version of a possibly pre-existing function. Setting focus in manual on your P&S would be useful when shooting landscapes (the mountains or hills icon, normally) or again if shooting Macro (little flower).

3) Exposure: I've read that you should
  get a proper exposure. There is an
  indicator that warns me if the picture
  is going to be too dark or too bright,
  but the only options I have to change
  the exposure are the shutter speed and
  ISO, which I always try to be at 100
  in order to avoid noise (of course
  during daylight or in a well-lit
  room). How can I know if it's really
  proper? Right now I have a post
  processing plugin in photoshop which
  does exposure compensation so I'm
  checking how much does the picture
  change.

I dare say the comments above handled this nicely: read up, research. You'll understand it, it's something all photographers have to wrap their heads around.

