I know how to get a correct exposure, but I'm trying to learn which of the correct exposures given a situation would work best for me and why.
Matching ISO to shutter speed to aperture is easy its knowing the differences, why a certain aperture is better for different needs.
Yes, there are two aspects to it - light metering and exposure level, and then the business of
creative exposure setting - which goes like this.
Aperture controls depth of field (the zone of sharpness in front and behind the subject) along with magnification, which is how big it appears in the frame. Magnification includes distance and focal length, and also format size which you can't change because it's fixed by the sensor. Good on-line calculator and general DoF guide here
www.dofmaster.com
High f/numbers like f/16 deliver lots of DoF, but allow very little light through. Low f/numbers like f/2 give shallow DoF but allow lots of light to pass. And the bigger the subject appears in the picture, either by fitting a longer lens or moving closer, the shallower the DoF. The two cancel out, so it you fit a long lens but then also move back so that the subject remains the same size, DoF remains the same too (but perspective and field of view will change).
When you combine extremes of both aperture and magnification, DoF varies enormously. Shoot close, with a longer lens and a low f/number, DoF will be just a few mms. With a distant scene shot with a wide angle at a high f/number, DoF will extend from your feet to the horizon.
Shutter speeds control movement, and the first consideration is to ensure there's no blurring from camera shake. Rule of thumb for hand holding is that your shutter speed should not be longer than you 'effective' focal length (as defined by full frame camera equivalents) so if you have a 100mm lens on a Canon crop format camera, your effective focal length is 100 x 1.6 crop factor = 1/160sec or faster.
If you have image stabilisation, that will help a lot here and you can run longer speeds, but IS doesn't prevent subject movement. As a rough guide there, with people doing normal stuff, if you go below 1/60sec you will get blurring of hands for example. To freeze fast action, 1/1000sec will stop most things. Check out panning technique which is a great way to tackle subjects like motor racing and sport with longer shutter speeds, which makes life easier and also looks better.
ISO is the sensitivity of the sensor to light, and higher numbers means more sensitivity but the downside of high ISO is noise so lower is better in terms of image quality. ISO is usually used to help balance and juggle aperture and shutter speed to get the combination you want in different lighting situations.
Those are the basics of it, but you can go much deeper into every aspect if you want. Some people manage to string it out into a whole book
