Summary

Exposure determines the brightness of your image and is a result of three parameters:

  • exposure time = the time during which light falls onto the image sensor
  • aperture = the size of the opening through which light falls onto the image sensor
  • ISO sensitivity = how sensitive the image sensor reacts to light

You can adjust the brightness of your photos pragmatically and without paying attention to these three numbers by taking a photo, checking it on the camera’s monitor (ideally using the histogram) and if needed take another one with exposure compensation until it fits.

But it is worth to pay attention to these values and to get familiar with the related numbers because each of them has visible impact on your images:

exposure time increasing risk of blurs from camera or subject motion when exposure time gets longer
aperture increasing depth-of-field with smaller apertures (=bigger f-numbers)

best possible sharpness for subject in focus usually at f8 or f11 (valid for digital SLR cameras without full frame sensor)

ISO sensitivity increasing noise at higher sensitivities

These three parameters are dependent on each other; if you change one of them, one or both of the others have to be adjusted too to keep exposure and the brightness of your image unchanged.

Which amount of light should be used for the exposure is a result of exposure metering and an additional exposure compensation that you may have set.
Different methods of exposure metering differ in where they measure the brightness of the subject and how different levels of brightness are combined.

The exposure lock allows to measure the exposure with a different frame than your later photo and to keep it unchanged over a number of shots.