If a machine or device does something automatically, it does it independently, without human control.

The camera adjusts the shutter speed automatically. 这架照相机可自动调节快门速度。

If you do something automatically, you do it without thinking about it.

I automatically put my hand out to catch it. 我不由自主地伸手去抓它。

If something happens automatically, it happens as part of the normal process or system.

Employees who steal are dismissed automatically. 盗窃公司财物的雇员理所当然会被开除。