Question: I have purchased my first DSLR (a Canon 500D) and have been playing around quite a lot within the aperture priority mode, with auto shutter speed and ISO.

Anyways as I would like to become a better photographer I would like to start taking *all* my photos in Full Manual mode.

Are there any general rules that I should use to help me select a shutter speed and ISO combination? Currently I am using trial and error and while I get there in the end, it often takes a while. I understand that I will learn this with experience, but anything to get me started will help.

Further to this, do most 'Professional' photographers set these manually? The auto settings work so well for an average shot, and it seems too time consuming to adjust these for every angle and light variation.

Answer: 
Shooting manual mode doesn't make you a better photographer, understanding what all the settings effectively do will. Your camera has three basic settings:

* **Aperture**: Use this to control depth of field (DoF). This is usually the most important setting to most photographers, as it influences both subject matter and composition. You're not going to be taking pictures of things if they're too big to fit into focus.
* **Shutter speed**: Use this to control in-frame movement and blur. While very important, there's really not much of a difference in a typical print between a normal, semi-still image shot at 1/250 and 1/8000. As a general rule, if you're trying to keep motion frozen you want this above 1/125.
* **ISO**: This determines the "image quality" of an image. Keep this as low as possible.

The most common strategy is to first set the aperture, make sure that you have enough DoF to cover your entire subject (or whatever you're trying to achieve). Have your ISO set to the lowest possible value (without going into 'extended ISO' modes) and check your shutter speed. Is it at least 1/60 or 1/125? No? Bump the ISO up a stop.

Your 500D has a exposure level reading that you can see the bottom of [its viewfinder](http://www.bobatkins.com/photography/digital/500d/t1i_viewfinder.jpg). Use that to determine where your exposure needs to be. For example, if you're dialed in at f/5.6 and 1/500 @ 100 ISO, but the meter reads -2, you know you're going to underexpose with those settings. You can change your shutter speed to 1/125 or increase your ISO to 400 to get to the 'correct' exposure. If it reads +1, increase your shutter speed or lower your ISO to compensate.

All that said... experiment. Play around with it. You're shooting digital, who cares if some of your shots are blurry?

You can also try using the Sunny 16 rule, it might help you 'get' exposure. The Sunny 16 rule says: on a bright, sunny day, you can set your aperture to f/16 and your shutter to the same speed as your ISO value (so 1/100 for ISO 100, 1/400 for ISO 400). On a sunny, but slightly less so day, try f/11 or f/5.6 on cloudy day. 

Manual mode's number 1 use is for consistency. When you take a series of photos in non-changing light conditions, you want your known color values to stay as close to each other as possible. With constant metering, especially in-camera metering, you can't hope for that kind of consistency. Point your camera at a white wall, look at the readings, then put a piece of black paper on that wall and see how wildly the meter reading changes.

There are also situations where in-camera metering doesn't do you any good, mainly in the studio. I don't know of any camera that meters for strobe/flash light effectively. 4
In general you want to use as low an ISO setting as you can to avoid noise. If the lowest ISO means shooting at a low shutter speed, say 1/15th then you may bump up the ISO a bit so that you can shoot at a faster shutter speed. 

If the light isn't changing, setting everything manually means that you get consistent exposures. Think of a sports or events photographer. If you leave in Aperture priority, then as your background changes (more sky in the photo for example) the camera may adjust the exposure and you end up with under or overexposed images.

Nothing wrong with using Aperture or Shutter priority if it suits you. Full manual yields more consistent results when you have time (and remember you're in manual!) and especially when you'll do a lot of shots in the same light.