Methods, apparatuses, and systems for imaging biological/chemical samples are provided. A calibrated imaging system can allow a user to obtain an optimal focus setting (position) for any effective distance (e.g. a zoom setting). The optimal focus can be determined from a functional approximation that defines a relationship between effective distance and focus setting. A user can input a size, and an imaging system can determine the appropriate effective distance and focus. An imaging system can also determine a size based on any effective distance. A flat-field correction can also be determined for any effective distance or size.