Projection device and auto focus method

A projection device, configured to project an image beam to a projection surface, the projection device comprising:

• a projection module, a projection lens, a distance sensor, a focus adjustment module, and a control unit, wherein
• the projection module is configured to generate the image beam;
• the projection lens is disposed on a transmission path of the image beam from the projection module;
• the focus adjustment module is connected to the projection lens, and the control unit is electrically connected to the distance sensor and the focus adjustment module, wherein the control unit is configured to:
• enable the projection device to enter a focus mode;
• enable the projection module to project the image beam to the projection surface through the projection lens to generate a projection image;
• enable the distance sensor to sense a projection distance between the projection device and the projection surface;
• calculate a focus adjustment parameter by using an auto focus transfer function according to the projection distance from the distance sensor;
• and
• control the focus adjustment module to adjust a focal length of the projection lens according to the focus adjustment parameter to perform auto focus.