There are 3 settings that determine the amount of light on the sensor.
• F-stop
• Shutter speed
• ISO number ~ sensor sensitivity
To avoid confusion, I have only listed the whole numbers from analogue photography.
F-stops:
In a lens there is a diaphragm or iris ~ ring consisting of some blades, that can open more or less. In that way there will be a variable amount of light to hit the sensor. The number scale below for the F-stops is a bit illogical – the lowest number means the biggest opening.
f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11.0, f/16.0, f/22.0, f/32.0
For every step you go up you’re halving the light.
Bright sunny days call out for small apertures, and dark skies or indoor photography calls for wider apertures or f-numbers.
Large apertures are great for shallow-focus portrait photography where the foreground subject is sharp, and the background blurred. This is sometimes called the “bokeh” effect.
Shutter speed:
1s, 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s.
The shutter in front of the sensor is popular speaking a column opening, that can be transported slower or faster horizontally across the sensor. In that way more or less light is hitting the sensor. Things are more natural here. Every time you go one step up, the incoming light is halved.
ISO numbers: Are an expression for the sensitivity your sensor is set to.
The numbers are 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800
Every time you go one step up you double the sensitivity of the sensor.
But the sensor has only one sensitivity, and that is the lowest number. The numbers above are digital settings that more and more reduce the image quality with grain and noise. In editing software, you can remove or reduce these side effects and still use a high ISO number.
If you change one setting you must correct one of the two others to get the same exposure.
Till now I have been trying to give you an understanding of how things are linked together. Personally, I almost only shoot automatic. RAW images give me possibilities for corrections. It happens in between that I correct the light meter in the camera by raising or lowering the exposure. The meter is set to middle grey, and if you are taking an image of a person against a white wall, you must increase the exposure by 1/3 or 2/3 stop.