Although this is the most basic of basics, a lot of people do not adhere to this rule and press the shutter button completely right in the first go itself and that is when a lot of things go wrong. However, here’s what happens when you half-press the shutter button:
In order to avoid camera shake, follow these basic steps:
- It locks AF. When using the face-detection AF mode, it fixes on a specific face and allows you to rearrange the frame
- It sets exposure i.e. it decides the required shutter speed and aperture for accurately reproducing the image, as well as readies all selected images effects such as saturation
- The lens stabilizes the image – i.e. if you’re using Image Stabilization (IS)
- It activates modes such as Self-timer and flash. And if the flash is not charged, it will indicate the same and not allow you to click.
- Finally, pressing the shutter button half-way also activates the DC charge going to the Compact Flash card in order for them to start recording the image.
Source page: https://canonedge.com/tipsdetails.aspx?id=1