You didn't say what kind of camera you have. If it's a Canon DSLR I'm not sure how you'd get more precision by having the camera control the Arduino/slider instead of Arduino controlling the camera. When you press the shutter button it releases within 100ms. You need more precision than this? You can't take it any faster. If you want the slider and camera to sync within 1ms you simply anticipate this delay. I don't see why you need any communication from the camera. You have to assume it's focused when you half-press. You can assume it takes the picture when you full-press. Please explain what you're trying to accomplish. I participated in ML development, so I can answer any questions you have about that. I have created a slider and an interface to Bescor MP101.