A faint spot or circle that stays in the same place from shot to shot is most likely dust or a hair on the camera’s sensor, not inside the lens. Debris in the lens is usually too far from the plane of focus to show up clearly in photos.

Start with the camera’s built-in sensor cleaning function. If the mark remains, use a hand blower to try to dislodge the debris. If that still doesn’t fix it, the sensor may need manual cleaning with a proper sensor-cleaning kit, or you can have a camera shop do it for you.

Be careful with manual cleaning: use tools meant for sensor cleaning and avoid touching the sensor with ordinary cloths or anything abrasive.