I'm involved in many different online photography groups - some are for learning new skills or new techniques, and some are for providing advice and support for less experienced photographers. One of the beginner questions which comes up frequently (not far behind 'Which camera should I buy?') is the subject of how to move 'off auto' and learn the basics of aperture, shutter speed and ISO.
The lazy answer, given by many, is to 'learn the exposure triangle' - but I take a very dim view of that, not just because it's unhelpful, but because this tool is far from perfect - see my article on 'Deconstructing the exposure triangle' to find out why.
More importantly though, the beginner is usually bombarded with a mix of views and a subsequent debate between respondees as to whether they should jump straight to full manual, or opt for one of the priority modes instead. The 'old school' approach does seem to lean very much towards manual, and even those who recommend priority modes often position it as a 'halfway house' on the road to full manual. By why is that?
Let's go back a step (for any beginners reading this especially!) - in manual (M on the top dial), you choose the aperture and shutter speed, depending on the exposure, and the artistic effect, you want. In Aperture Priority (A, or Av if you're using a Canon), you choose the aperture and the camera chooses what it thinks is the best shutter speed, based on getting an accurate exposure. In Shutter Priority (S, or TV if you're using a Canon) you choose the shutter speed, and the camera chooses the aperture in the same way.
So, at first glance it would seem like this is a 'semi-automatic' mode, and is somehow inferior to full manual. But those who think so are forgetting one vital tool - which is Exposure Compensation. When using a priority mode, every camera (that has full manual override) has the ability to adjust the exposure that the camera is suggesting, simply by turning whichever wheel or dial is designated to that function. On the button itself, and sometimes on the LCD screen or in the viewfinder, this is displayed by a +/- symbol - which then changes to a sliding scale that moves left or right, depending on whether you choose to over or under expose the shot.
So, in effect, you still have full manual control of your camera, but the priority mode is giving you a 'start point' to work from - rather than you having to guess what that might be. Something which is especially useful for beginners.
All you then have to do is take the shot - or look at your proposed shot in the viewfinder, in the case of those using cameras with an electronic viewfinder (EVF) - and then decide if you think the camera got it right. If you'd like it a little brighter, to capture more shadow detail, then simply adjust the Exposure Compensation dial to brighten the image slightly. Or if you think the sky is a bit bright for example, and you're risking the whitest clouds 'burning out', then adjust the Exposure Compensation dial to darken the image slightly. This gives you complete manual control over your exposure, but takes away some of the initial guesswork!
How the camera achieves this change will depend on whether you have your ISO set to manual or auto, but that's a whole different conversation . . . In short though, there are two ways for your camera to increase or decrease the amount of light which hits the sensor. Firstly by changing the aperture or shutter speed (depending on which priority mode you're in) and secondly by increasing the ISO to make the sensor more sensitive - but it can only do this if you have your ISO set to auto.
One last thing to mention - while both priority modes are equally valid, when I'm teaching, I always recommend Aperture Priority. This is because you always need to make a decision as to what aperture you need - based on the artistic effect you want (for example a wide aperture for shallow depth of field) but shutter speed only really matters if you're trying to capture a moving subject, and as a general rule, this happens far less often. Which is why Aperture Priority is always my first choice.
Before I end this post, I'll qualify all of the above by saying that of course there are some situations where you do have to use manual - astro-photography for example, or long exposure shots with filters where you need to use 'bulb' mode and time a one-minute-plus exposure - but for everyday use, especially for beginners, I believe what I'm saying is fair.
So there you have it - a controversial viewpoint possibly, but as far as I'm concerned priority mode + exposure compensation = manual. Just with an easier place to start! So why wouldn't you?