Being able to control shutter speed and aperture is more than very useful in photography. It is part of the core principles you need to produce the photos you want.
Exposure is how long the shutter is open for ( shutter speed ), how wide the opening that lets light into the lens is ( aperture ) and the ISO setting ( what used to be film speed ).
When taking a photo on a camera with manual controls you have three core options.
Av mode - Aperture Priority Mode
Tv mode - Shutter Priority Mode
M mode - Manual Mode
In Av mode you control the aperture setting and the camera works out the rest for you.
In Tv mode you control the shutter speed and the the camera works out the rest for you.
In Manual mode you choose both Aperture and Shutter Speed and the camera does exactly what you tell it to ( which is why it is so useful ).
In many cases you can get what you want using Av or Tv, but in tricky lighting conditions it is better to use Manual mode.
In Av or Tv you can apply Exposure Compensation to force the camera to adjust it's choices one way or another a little. Exposure compensation is like a hint to the camera about what you want.
In Manual mode you do not have exposure compensation because it has no meaning. You just adjust the exposure settings yourself.
The basic rule that you need to remember for exposures is the Sunny 16 rule.
Simply put on a bright sunny day a shot at aperture f16 and 1/125 th sec shutter speed using ISO 100 gets a perfect exposure.
How to choose another aperture you simply use the idea of F-Stops ( or just stops ).
Every time you half or double shutter speed you go down or up a stop respectively. You can start from 8 secs, and you get this sequence by halving :
8, 4, 2, 1, 1/2, 1/4, 1/16, 1/32, 1/64, 1/128, 1/256
And the sequence your camera will offer is :
8, 4, 2, 1, 1/2, 1/4, 1/16, 1/32, 1/64, 1/125, 1/250 and so on. The rounding is a convenience.
Every time you half or double ISO you go down a stop or up a stop. So that's simply 100, 200, 400, 800, 1600, 3200.
The aperture numbers also work like this, but are not as convenient as halving and doubling.
However there is an easy way to remember them :
Start with 1 and 1.4
Double them to get 2 and 2.8
Double them to get 4 and 5.6
Double them to get 8 and 11.2
Double them to get 16 and 22.4
Now the aperture sequence is stop order is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 and so on.
As you can see it's as simple as remembering 1 and 1.4 and being able to multiply by two.
Don't worry about why these are the number - it's not worth the effort for most people.
So if you go from f1 to f1.4 you half the amount of light you let in ( down a stop ). Going from 1.4 to 2 halves it again.
So to adjust exposure from the Sunny 16 rule you can just adjust the aperture, shutter speed and ISO in stops as long as you end up with the a net change of zero.
So you can double you shutter speed if you half your aperture, for example. One stop either way. No net change.
On darker days or in poor light you adjust the Sunny 16 rule to let in enough light. So you might use f11 instead of f16 on a slightly overcast day.
